Disease and Its Causes Part 6

The disease is caused by a small bacillus discovered in 1894 which forms no spores and is easily destroyed by sunlight, but in the dark is capable of living with undiminished virulence for an indefinite time. The disease in man appears in two forms, the most common known as bubonic plague, from the great enlargement of the lymph nodes, those of the groin being most frequently affected. The more fatal form is known as pneumonic plague, and in this the lungs are the seat of the disease.

In the old descriptions of the disease it was frequently mentioned that large numbers of dead rats were found when it was prevalent, and the most striking fact of the recent investigations is the demonstration that the infection in man is due to transference of the bacillus from infected rats. There are endemic foci of the disease where it exists in animals, the present epidemic having started from such a focus in Northern China, in which region the _Tarabagan_, a small fur-bearing animal of the squirrel species, was infected. Rats are easily infected, the close social habits of the animal, the vermin which they harbor, and the habits of devouring their dead fellows favor the extension of infection. The disease extends from the rat to man chiefly by means of the fleas which contain the bacilli, and in cases of pneumonic plague from man to man by means of sputum infection. The disease once established in animals tends to remain, the virus being kept alive by transmission from animal to animal, and the persistence of the infection is favored by mild and chronic cases.

CHAPTER IX

DISEASE CARRIERS.--THE RELATION BETWEEN SPORADIC CASES OF INFECTIOUS DISEASE AND EPIDEMICS.--SMALLPOX.--CEREBRO-SPINAL MENINGITIS.--POLIOMYELITIS.--VARIATION IN THE SUSCEPTIBILITY OF INDIVIDUALS.--CONDITIONS WHICH MAY INFLUENCE SUSCEPTIBILITY.--RACIAL SUSCEPTIBILITY.--INFLUENCE OF AGE AND s.e.x.--OCCUPATION AND ENVIRONMENT.--THE AGE PERIOD OF INFECTIOUS DISEASES.

We have seen that insects serve as carriers of disease in two ways: in one, by becoming contaminated with organisms they serve as pa.s.sive carriers, and in the other they undergo infection and form a link in the disease. The more recent investigations of modes of transmission of infectious diseases have shown that man, in addition to serving while sick as a source of infection, may serve as a pa.s.sive carrier in two ways. For infection to take place not only must the pathogenic organism be present, but it must be able to overcome the pa.s.sive and active defences of the body and produce injury. Pathogenic organisms may find conditions favorable for growth on the surfaces of the body, and may live there, but be unable to produce infection, and the individual who simply harbors the organisms can transmit them to others. Such an individual may be a greater source of infection than one with the disease, because there is no suspicion of danger. The organisms which thus grow on the surfaces have in some cases been shown to be of diminished virulence, but in others have full pathogenic power. Such pa.s.sive carriers of infection have been found for a number of diseases, as cerebro-spinal meningitis, diphtheria, poliomyelitis and cholera. In all these cases the organisms are most frequently found in those individuals who have been exposed to infection as members of a family in which there have been cases of disease. The other sort of carrier has had and overcome the disease, but mutual relations have been established with the organism which continues to live in the body cavity. Diphtheria bacilli usually linger in the throat after convalescence is established, and until they have disappeared the individual is more dangerous than one actually sick with the disease. Health officers have recognized this in continuing the quarantine against the disease until the organism disappears. In typhoid fever bacilli may remain in the body for a long time and be continually discharged, as in the well-known case of "typhoid Mary."[1]

Single cases of certain infectious diseases may appear in a community year after year, and at intervals the cases become so numerous that the disease is said to be epidemic. Such a disease is smallpox. This is a highly infectious disease, towards which all mankind is susceptible. Complete protection against the disease can be conferred by Jenner's discovery of vaccination. The disease becomes modified when transferred to cattle, producing what is known as cowpox, in which vesicles similar to those of smallpox appear on the skin. The inoculation of man with the contents of such a vesicle produces a mild form of disease known as vaccinia, which protects the individual from smallpox. This protection is fully as adequate as that produced by an attack of smallpox, and we are warranted in saying that if thorough vaccination, or the inoculation with vaccinia, were carried out smallpox would disappear. There are great difficulties in the way of carrying out effective vaccination of the whole population, which are accentuated by the active opposition of people who are ignorant and wilfully remain so. There exists in every state a number of people unprotected by vaccination, and among these single cases of smallpox appear. The unprotected individuals gradually increase in number, forming an inflammable material awaiting the spark or infection which produces a conflagration in the one case and an epidemic in the other.

Cerebro-spinal meningitis is another example of a disease which exists in sporadic and epidemic form. This disease is caused by a small micrococcus, the organisms joined in pairs. The seat of the disease is in the meninges or membranes around the brain and spinal cord. The micrococci enter the body from the throat and nose, and either pa.s.s directly from here into the meninges, or they enter into the blood and are carried by this into the meninges. The organisms are easily destroyed and cannot long survive the conditions outside the body, so that for infection to take place the transmission must be very direct.

Carriers who have the organisms in the throat, but who do not have the disease, are the princ.i.p.al agents in dissemination. The mortality is high, and even in recovery permanent damage is often done to the brain or to the organs of special sense. Sporadic cases constantly occur in small numbers, and it is difficult or impossible to trace any connection between these cases. At varying intervals, often twenty years intervening, an epidemic appears which sometimes remains local in a city or state, sometimes extends to adjoining cities or states, and may even extend over a very large area. In the epidemics the mortality is much higher than in the sporadic cases. The same explanation given for smallpox cannot apply here, for there is not a similar acc.u.mulation of susceptible material. We know there is a great deal of variation in the virulence of the different pathogenic organisms, and the virulence can be artificially increased and diminished. In epidemics of meningitis the virulence of the organisms is increased, as is shown by the greater mortality. It is highly probable that such epidemics are due to changes which arise in the organisms from causes we do not know and which increase their capacity for harm. It is possible that such a change would convert a carrier into a case of disease, the organism acquiring greater powers of invasion. Such a strain of organisms arising in one place and producing an epidemic could be transported to another locality and exert the same action, or similar changes in the organisms could arise simultaneously in a number of places. a.n.a.logies to such conditions are given in plants. In certain plants it has been shown that from unknown causes there appears a tendency to the production of variations. A very beautiful herbaceous peony known as "Bridesmaid" after having grown for a number of years in single form, in one year wherever grown suddenly became double. The peculiar thing with the lower unicellular organisms is that the changes which so arise do not tend to become permanent, the organism reverts to its usual character, the disease to its sporadic type.

A very fatal form of poliomyelitis has for a number of years prevailed in Sweden. In the United States there have been continually a number of single cases of the disease, and it is not impossible that a more pathogenic strain of the organism has developed in Sweden and has been imported into this country, giving rise to the much greater extension of the disease in a number of places.

The most cursory study of the infectious diseases shows that there is great variation in the susceptibility of individuals. Even in the most severe epidemics all are not equally affected, some escape the infection, others have the disease lightly, others severely, some die.

Chance enters into this, but plays a small part, for the same varying individual susceptibility is shown experimentally. If a given number of animals of the same species, age and weight, even those from the same litter, be inoculated with a given number of bacteria shown to be pathogenic for that species, the results differ. If the dose be necessarily fatal, death will take place at intervals; if a dose smaller than the fatal be used, some animals will die, others will recover. The defences of the organism being centred in the activity of the living tissue, any condition which depresses cell activity may have an effect in increasing susceptibility to infection. Animals which ordinarily are not susceptible to infection with a certain organism may be made so by prolonged hunger, or fatigue, by the influence of narcotics, by reduction of the body temperature, by loss of blood. In man prolonged fatigue, cold, the use of alcohol to excess and even psychic depression increases susceptibility. It has been shown that such conditions are accompanied by a diminution in the power of the blood to destroy bacteria.

There is variation in the susceptibility to infection in the different races of man. If a race be confined to one habitat with close intercourse between the people, such a race may acquire a high degree of immunity to local diseases by a gradual weeding out of the individuals who are most susceptible. A degree of comparative harmony may be gradually established between host and parasite, as is the case in wild animals. These have few diseases, the weak die, the resistant breed; they harbor, it is true, large numbers of parasites, but there is mutual adjustment between parasite and host. Diseases in animals greatly increase under the artificial conditions of domestication.

Certain highly specialized breeds of cattle, as the Alderneys, are much more susceptible to tuberculosis than the less specialized. The high development of the variation which consists in a marked ability to produce milk fat is probably combined with other qualities, shown in diminished resistance to disease, and under natural conditions the variation would not have persisted. The introduction of a new disease into an isolated people has often been attended with dire consequences. It is much the same thing with the introduction of disease of plants. In Europe the brown-tail moth and the gypsy moth produce continuously a certain amount of damage to the trees, but their parasitic enemies have developed with them and check their increase. These pests were brought to this country in which there were no conditions r.e.t.a.r.ding their increase and have produced great damage.

It is very difficult to estimate the degree of racial susceptibility.

The negro race seems to be more susceptible to certain diseases, such as tuberculosis and smallpox, less so to others, as yellow fever, malaria and uncinariasis. What are apparently differences in susceptibility may be explained by racial customs. A statistical inquiry into death in India from poisonous snakes might be interpreted as showing a marked resistance on the part of the white to the action of the venom, but it is merely a question of the boots of the whites and the naked feet and legs of the natives. The relatively greater frequency of smallpox in the blacks is due to the greater difficulties in carrying out vaccination measures among them and the greater opportunity for infection which results from their less hygienic life.

It has always been noted that when plague prevails in Oriental cities, the natives are more frequently attacked than are Europeans. This does not depend upon differences in susceptibility, but on the better hygienic conditions of the whites which prevent the close relation to rats and vermin by which infection is extended. There would be but little extension of the hookworm disease in a community where shoes were worn and the habits were cleanly.

It is by no means improbable that the formation of the habits of civilization was influenced by infection. Most of these habits, such as personal cleanliness, the avoidance of close contact, the demand for individual utensils for eating and drinking, are all of distinct advantage in opposing infection. Certain habits, on the other hand, such as kissing, which probably represents the extension of a habit of s.e.xual origin, are disadvantageous and infection is often transmitted in this way. In syphilitic infection the mouth forms one of the most common localizations of the disease and may contain the causal organisms in great numbers. This, the _spirochaeta pallida_, is an organism of great virulence, and man is the most susceptible animal.

The disease, like gonorrhoea, is essentially a s.e.xual disease, the primary location is in the s.e.xual organs, and it is transmitted chiefly by s.e.xual contact. Of all the infectious diseases, it is the one most frequently transmitted to the unborn child; in certain cases the disease is transmitted, in others the developing foetus may be so injured by the toxic products of the disease that various imperfections of development result, as is shown in deformities, or in conditions which render the entire organism or individual organs, particularly the nervous system, more susceptible to injury. Following the primary localization of the acquired form of the disease, there is usually secondary localization in the mucous membrane of the mouth, and the disease may be transmitted by kissing or by the use of contaminated utensils. The habit of indiscriminate kissing is one which might with great benefit be given up.

There is definite relation between age and the infectious diseases. In general, susceptibility is increased in the young; young animals can be successfully inoculated with diseases to which the adults of the species are immune, and certain human diseases, such as scarlet fever, measles and whooping cough, seem to be the prerogatives of the child.

It must be remembered, however, that one attack of these diseases confers a strong and lasting immunity and children represent a raw material unprotected by previous disease. Where measles has been introduced into an island population for the first time, all ages seem equally susceptible. All ages are equally susceptible to smallpox, and yet in the general prevalence of the disease in the prevaccination period it was almost confined to children, the adults being protected by a previous attack. The habits and environment at different ages have an influence on the opportunities for infection. There is comparatively little opportunity for infection during the first year, in which period the infant is nursed and has a narrow environment within which infection is easily controlled. With increasing years the opportunities for infection increase. When the child begins to move and crawl on hands and knees the hands become contaminated, and the habit of putting objects handled into the mouth makes infection by this route possible. Food also becomes more varied, milk forms an important part of the diet, and we are now appreciating the possibilities of raw milk in conveying infection. With the enlarging environment, with the school age bringing greater contact of the child with others, there come greater opportunities for infection which are partly offset by the increase in cleanliness. The dangers of infection in the school period are now greatly lessened by medical inspection and care of the school children. In the small epidemic of smallpox which prevailed in Boston from 1881 to 1883, there was a sharp decline in the incidence of the disease in children as soon as the school age was reached, this being due to the demand of vaccination as a condition for entrance into the schools. Many of the infectious diseases are much milder in children than in adults. This is the case in typhoid fever, malaria and yellow fever. The comparative immunity of the natives to yellow fever in regions where this prevails seems to be due to their having acquired the disease in infancy in so mild a form that it was not recognized as such.

The infectious diseases are preeminently the diseases of the first third of life. After the age of forty man represents a select material. He has acquired immunity to many infections by having experienced them. Habits of life have become fixed and there is a general adjustment to environment. The only infectious disease which shows no abatement in its incidence is pneumonia, and the mortality in this increases with age. Between thirty-five and fifty-five man stands on a tolerably firm foundation regarding health; after this the age atrophies begin, the effects of previous damage begin to be apparent, and the tumor incidence increases.

FOOTNOTE: [1] This was the case of a woman, by occupation a cook, whose numerous exchanges of service were accompanied by the appearance of cases of typhoid fever in the families. This became so marked that an examination was made and she was found to be a typhoid carrier and as such constantly discharging typhoid bacilli. She is now isolated.

CHAPTER X

INHERITANCE AS A FACTOR IN DISEASE.--THE PROCESS OF CELL MULTIPLICATION.--THE s.e.xUAL CELLS DIFFER FROM THE OTHER CELLS OF THE BODY.--INFECTION OF THE OVUM.--INTRA-UTERINE INFECTION.--THE PLACENTA AS A BARRIER TO INFECTION.--VARIATIONS AND MUTATIONS.--THE INHERITANCE OF SUSCEPTIBILITY TO DISEASE.--THE INFLUENCE OF ALCOHOLISM IN THE PARENTS ON THE DESCENDANTS.--THE HEREDITY OF NERVOUS DISEASES.--TRANSMISSION OF DISEASE BY THE FEMALE ONLY.--HEMOPHILIA.-- THE INHERITANCE OF MALFORMATIONS.--THE CAUSES OF MALFORMATIONS.--MATERNAL IMPRESSIONS HAVE NO INFLUENCE.--EUGENICS.

The question of inheritance of disease is closely a.s.sociated with the study of infection, and the general subject of heredity in its bearing on disease can be considered here. By heredity is understood the transference of similar characteristics from one generation of organisms to another. The formation of the s.e.xual cells is a much more complex process than that of the formation of single differentiated cells, for the properties of all the cells of the body are represented in the s.e.xual cells, to the union of which the heredity transmission of the qualities of the parents is due. In the nucleus of all the cells in the body there is a material called _chromatin_, which in the process of cell division forms a convoluted thread; this afterwards divides into a number of loops called _chromosomes_, the number of which are constant for each animal species. In cell division these loops divide longitudinally, one-half of each going to the two new cells which result from the division; each new cell has one-half of all the chromatin contained in the old and also one-half of the cytoplasm or the cell material outside of the nucleus. The process of s.e.xual fertilization consists in the union of the male and female s.e.x cells and an equal blending of the chromatin contained in each (Fig. 22). In the process of formation of the s.e.xual cells a diminution of the number of chromosomes contained in them takes place, but this is preceded by such an intimate intermingling of the chromatin that the s.e.xual cells contain part of all the chromosomes of the undifferentiated cells from which they were formed. The new cell which is formed by the union of the male and female s.e.xual cells and which const.i.tutes a new organism, contains the number of chromosomes characteristic of the species and parts of all the chromatin of the undifferentiated cells of male and female ancestors. As a result of this the most complicated mechanism in nature, it is evident that in a strict sense there can be no heredity of a disease because heredity in the mammal is solely a matter of the chromosomes and these could not convey a parasite. The new organism can, however, quickly become diseased and, by the transference of disease to it and by either parent, there is the appearance of hereditary transmission of disease, though in reality it is not such. The ovum itself can become the site of infection; this, which was first discovered by Pasteur in the eggs of silkworms, takes place not infrequently in the infection of insects with protozoa. In Texas fever the ticks which transmit the disease, after filling with the infected blood, drop off and lay eggs which contain the parasites, and the disease is propagated by the young ticks in whom the parasites have multiplied. The same thing is true in regard to the African relapsing or tick fever, which is also transferred by a tick. In the white diarrhoea of chickens the eggs become infected before they are laid and the young chick is infected before it emerges from the sh.e.l.l. It is highly improbable, and there is no certain evidence for it, that the extremely small amount of material contributed by the male can become infected and bring infection to the new organism. In the cases in which disease of the male parent is transferred to the offspring, it is either by an infection of the female by the male, with transference of the infection from her to the developing organism, or with the male s.e.xual cells there may be a transference to the female of the infectious material and the new organism may be directly infected. No other disease in man is so easily and directly transferred from either parent to offspring as is syphilis, and the disease is extremely malignant for the foetus, usually causing death before the normal period of intra-uterine development is reached.

[Ill.u.s.tration: FIG. 21.--DIAGRAM SHOWING THE RELATION OF THE s.e.xUAL CELLS TO THE SOMATIC CELLS OR THOSE OF THE GENERAL BODY. The s.e.xual cells are represented to the left of the line at the bottom of diagram and are black. From the fertilized ovum at the top there is a continuous cell development, with differentiation represented in the cell groups of the bottom row. It is seen that the s.e.xual cells are formed directly from the germ cell and contain no admixture from the cells of the body.]

The mother gives the protection of a narrow and unchanging environment and food to the new organism which develops within the uterus, and there is always a membranous separation between them. Disease of the mother may affect the foetus in a number of ways. In most cases the membrane of separation is an efficient guard preventing pathogenic organisms reaching the foetus from the mother. In certain cases, however, the guard can be pa.s.sed. In smallpox, not infrequently, the disease extends from the mother to the foetus, and the child may die of the infection or be born at term with the scars resulting from the disease upon it. Syphilis in the mother in an active stage is practically always extended to the foetus. We have said that in an infectious disease substances of an injurious character are produced by bacteria, and such substances being in solution in the blood of the infected mother can pa.s.s through the membranous barrier and may destroy the foetus although the mother recovers from the infection.

[Ill.u.s.tration: FIG. 22.--DIAGRAMMATIC REPRESENTATION OF THE PROCESS OF FERTILIZATION. (Boveri.) In the first cell (_a_) the ovum is shown in process of fertilization by the entering spermatozoon or male s.e.xual element. In the following cells there is shown the increase in amount of the male material and the final intimate commingling in _g_ which precedes the first segmentation. _g_ represents a new organism formed by the union of the male or female cell but differing from either of them.]

Living matter is always individual, and this individuality is expressed in slight structural variations from the type of the species as shown in an average of measurements, and also in slight variations in function or the reactions which living tissue shows towards the conditions acting upon it. The anatomical variations are more striking because they can be demonstrated by weight and measure, but the functional variations are equally numerous. Thus, no two brains react in exactly the same way to the impressions received by the sense organs; there are differences in muscular action, differences in digestion; these variations in function are due to variations in the structure of living material which are too minute for our comparatively coa.r.s.e methods of detection. In the enormous complexity of living matter it is impossible that there should not be minute differences in molecular arrangement and to this such functional variations may be due. Chemistry gives us a number of examples of variations in the reaction of substances which with the same composition differ in the molecular arrangement. Even in so simple a mechanism as a watch there are slight differences in structure which gives to each watch certain individual characteristics, but the type as an instrument constructed for recording time remains. In the fusion of the chromosomes of the male and female s.e.xual cells, to which the hereditary transmission of the ancestral qualities to the new offspring is due, there are differences in the qualities of each, for the individuality of the parents is expressed in the germ cells, and the varying way in which these may fuse gives to the new cell qualities of its own in addition to qualities which come from each ancestor, and from remote ancestors through these. The qualities with which the new organism starts are those which it has received from its ancestors plus its individuality. The fact that the s.e.xual cells are formed from the early formed cells of the new organism which represent all of the qualities of the fertilized ovum or primordial cell, renders it unlikely that the new offspring will contain qualities which the parents have acquired. The question of the inheritance of characteristics which the parents have acquired as the result of the action of environment upon them is one which is still actively investigated by the students of heredity, but the weight of evidence is opposed to this belief.

In the new organism the type of the species is preserved and the variations from the mean to which individuality is due are slight. We are accustomed to regard as variations somewhat greater departures from the species type than is represented in individuality, but there is no sharp dividing line between them.

Very much wider departures from the species type are known as mutations. Such variations and mutations, like individuality, may be expressed in qualities which can be weighed and measured, or in function, and all these can be inherited; certain of them known as dominant characteristics more readily than others, which are known as recessive. If these variations from the type are advantageous, they may be preserved and become the property of the species, and it is in this way that the characteristics of the different races have arisen.

Certain of the variations are unfavorable to the race. The varying predisposition to infection which undoubtedly exists and may be inherited represents such a variation. Tuberculosis is an instance of this; for, while the cause of the disease is the tubercle bacillus, there is enormous difference in the resistance of the body to its action in different individuals. The disease is to a considerable extent one of families, but while this is true the degree of the influence exerted by heredity can be greatly overestimated. The disease is so common that in tracing the ancestry of tuberculous patients it is rare to find the disease not represented in the ancestors. A further difficulty is that the environment is also inherited. The child of a tuberculous parent has much better opportunity to acquire the infection than a child without such an environment [page 167]. Other diseases than the infectious seem to be inherited, of which gout is an example. In gout there is an unusual action of the cells of the body which leads to the formation and the retention in the body of substances which are injurious. Here it is not the disease which is inherited, but the variation in structure to which the unusual and injurious action of the cells is due.

While tuberculosis and gout represent instances in which, although the disease itself is not inherited yet the presence of the disease in the ascendants so affects the germinal material that the offspring is more susceptible to these particular diseases, much more common are the cases in which disease in the parents produces a defective offspring, the defect consisting in a general loss of resistance manifested in a variety of ways, but not necessarily repeating the diseased condition of the parent. In these cases the disease in the parents affects all the cells of the body including the germinal cells, and the defective qualities in the germ cells will affect the cells of the offspring which are derived from these. There is a tendency in these cases to the repet.i.tion in the offspring of the disease of the parents, because the particular form of the parental disease may have been due to or influenced by variation of structure. One of the best examples of affection of the offspring by diseased conditions of the parents produced by a toxic agent which directly or indirectly affects all the cells of the body is afforded by alcohol when used in excess. Since drunkenness has become a medical rather than a moral question, a great deal of reliable data has acc.u.mulated in regard to it as a factor in the heredity of disease. Grotjahn gives the following examples: Six families were investigated in which there were thirty-one children. In all these families the father and grandfather on the father's side were chronic alcoholics, and in certain of the families drunkenness prevailed in the more remote ancestors. The following was the fate of the children: eight died shortly after birth of general weakness, seven died of convulsions in the first month, three were malformed, three were idiotic, three were feeble-minded, three were dwarfs, three were epileptics, two were normal. In a second group of three families there were twenty children. The fathers were drunkards, but their immediate ancestors were free: four children died of general weakness, three of convulsions in the first month, two were feeble-minded, one was a dwarf, one was an epileptic, seven were normal. In a family in which both father and mother and their ancestors were drunkards there were six children: three died of convulsions within six months, one was an idiot, one a dwarf, and one an epileptic. For comparison there were taken from the same station in life ten families in which there was no drunkenness: three children died from general weakness, three from intestinal troubles, two of nervous affection, two were feeble-minded, two were malformed, fifty were normal. Legrain has studied on a larger scale the descendants of two hundred and fifteen families of drunkards in which there were eight hundred and nineteen children. One hundred and forty-five of these were insane, sixty-two were criminals, and one hundred and ninety-seven drunkards. Of course all this cannot be attributed to alcohol alone. There is first to be considered a probable variation in the nervous system which is expressed in the alcoholic habit; second, the environment consisting in poverty, bad a.s.sociates, etc., which the alcoholic habit brings; third, the alcohol alone. That defective inheritance so frequently takes the form of alcoholism is largely due to the environment. There has never been the opportunity to study on a large scale the effect of the complete deprivation of alcohol from a people living in the environment of modern civilization. There is a possibility, and even probability, that the defective nervous organization which predisposes to alcoholism would seek satisfaction in the use of some other sedative drug. So complex are all the interrelations of the social system that it would be possible to regard alcohol as an agent useful in removing the defective, were it not for its long-enduring action and its effects on the descendants, procreation not being affected by its use.

Diseases of the nervous system are particularly apt to affect the offspring, and often the inherited condition repeats that of the parents. This is due to the fact that most of the nervous diseases depend both upon intrinsic factors which consist in some defective condition of the nervous system representing a variation, and extrinsic factors due to environment or occupation which make the basal condition operative. The definite relation between alcoholism and insanity is due to alcohol acting not as an intrinsic but an extrinsic factor, bringing into effectiveness the hereditary weakness of the nervous system. The influence of heredity in producing insanity is variously estimated at from twenty-six per cent to sixty per cent of all cases. This great difference in the estimation of the hereditary influence is due to the personal equation of the statistician, and the care with which other factors are eliminated. In the more severe form of the hereditary degeneration the same pathological conditions are repeated in the descendants. In certain cases the severity of the condition increases from generation to generation. According to Morel there may be merely what is recognized as a nervous temperament often a.s.sociated with moral depravity and various excesses in the first generation; in the second, severe neuroses, a tendency to apoplexy and alcoholism; in the third, psychic disturbances, suicidal tendencies and intellectual incapacity; and in the fourth, congenital idiocy, malformations and arrests of development. There are some very definite data with regard to inheritance in the nervous disease known as epilepsy. The essential condition in this consists in attacks of unconsciousness, usually accompanied by a discharge of nerve force shown in convulsions, the attack being often preceded by peculiar sensations of some sort known as the aura. In the most marked forms of the affection heredity plays but little part, owing to the early supervention of imbecility and helplessness, and it is a greater factor in the better cla.s.ses of society than in the proletariat. In the better cla.s.ses, owing to the greater care of the cases and the avoidance of exciting causes of the attacks, the disease is better controlled and rarely advances to the extent that it does among the poor. The a.s.sociation of epilepsy and alcoholism is especially dangerous, for a slight amount of alcohol may greatly accentuate the disease. In five hundred and thirty-five children in whose parentage there were sixty-two male and seventy-four female epileptics, twenty-two were born dead, one hundred and ninety-five died from convulsions in infancy, twenty-seven died in infancy from other causes, seventy-eight were epileptics, eleven were insane, thirty-nine were paralyzed, forty-five were hysterical, six had St. Vitus's dance, one hundred and five were ordinarily healthy.

That variations in the nervous system which produce more or less unusual mental peculiarities and which do not take the form of nervous disease are inherited, the most superficial consideration shows. A child in its mental characteristics is said to take after one or the other of its parents, certain habits and mental traits are the same, often even the handwriting of a child resembles that of a parent.

In certain cases the inheritance is transmitted by the female alone.

This is the case in the haemophilia, the unfortunate subjects of which are known as bleeders. There is in this a marked tendency to haemorrhage which depends upon an alteration in the character of the blood which prevents clotting. This, the natural means of stopping bleeding from small wounds, being in abeyance, fatal haemorrhage may result from pulling a tooth or from an insignificant wound. There is a seeming injustice in the inheritance, for the females do not suffer from the disease although they transmit it, while the males who have the disease cannot even create additional sympathy by transmitting it.

The most obvious inheritance is seen in the case of malformations.

These represent wide departures from the type of the species as represented in the form. There is no hard and fast line separating the slight departures from the normal type known as variations and mutations, from the malformations. Certain of the malformations known as monstrosities hardly represent the human type. These are the cases in which the foetus is represented in a formless ma.s.s of tissue, or there is absence of development of important parts such as the nervous system or there is more or less extensive duplication of the body.

There has always been a great deal of popular interest attached to the malformations owing to the part which maternal impressions are supposed to play in their production. In this, some striking impression made on the pregnant woman is supposed to affect in a definite way the structure of the child. The cases, for instance, in which a woman sees an accident involving a wound or a loss of an arm and the child at birth shows a malformation involving the same part.

There is no a.s.sociation between maternal impressions and malformations, although there have been many striking coincidences.

All malformations arise during the first six weeks of pregnancy known as the embryonic period, in which the development of the form of the child is taking place, and during which time there is little consciousness of pregnancy. Maternal impressions are usually received at a later period, when the form of the child is complete and it is merely growing. It must be remembered also that there is neither nervous nor vascular connection between the child in the uterus and the mother, the child being from the period of conception an independent ent.i.ty to which the mother gives nutriment merely. Of course, as has been said, the mother may transmit to the child substances which are injurious, and in certain cases parasites may pa.s.s from the mother to the foetus. The same types of malformations which occur in man are also seen in birds, and it would require a more vigorous imagination than is usual to believe that a brooding hen could transmit an impression to an egg and that a headless chick could result from witnessing the sacrifice of an a.s.sociate. The idea of the importance of maternal impressions in influencing the character of the offspring is a very old one, a well-known instance being the sharp practice of Jacob's using peeled wands to influence the color of his cattle. In regard to coincidences the great number of cases in which strong impressions made on the mind of the pregnant mother without result on the offspring are forgotten. The belief has been productive of great anxiety and even unhappiness during a period which is necessarily a trying one, and should be dismissed as being both theoretically impossible and unsupported by fact.

The malformations are divided anatomically into those characterized, first, by excess formation, second, by deficient formation, third, by abnormal displacement of parts. They are due to intrinsic causes which are in the germ, and which may be due to some unusual conditions in either the male or female germ cell or an imperfect commingling of the germinal material, and to extrinsic causes which physically, as in the nature of a shock or chemically as by the action of a poison, may affect the embryo through the mother. Malformations are made more numerous in chickens by shaking the eggs before brooding. A number of malformations are produced by accidental conditions arising in the environment; for instance, the vascular cord connecting mother and child may become wound around parts constricting them or even cutting them off, and the membrane around the child may become adherent to certain parts and prevent the development of these. The extrinsic causes are more operative the more unfavorable is the environment of the mother. Malformations are more common in illegitimate children than in legitimate and more common in alcoholic mothers; there is an unfavorable environment of poverty in both cases, added to in the latter and usually in the former by the injurious action of the alcohol.

The more extensive malformations have no effect on heredity, because the subjects of them are incapable of procreation. The malformations which arise from the accidents of pregnancy and which are compatible with a perfectly normal germ are in the nature of acquired characteristics and are not inherited. Those malformations, however, which are due to qualities in the germinal material itself are inherited, and certain of them with remarkable persistence. There are instances in which the slight malformation consisting in an excess of fingers or toes has persisted through many generations. It may occasionally lapse in a generation to reappear later. In certain cases, notably in the bleeders, the inheritance is transmitted by the female alone, in other cases by the s.e.xes equally, but there are no cases of transmission by the male line only. It is evident that when the same malformation affects both the male and the female line the hereditary influence is much stronger. A case has been related to me in which most of the inhabitants in a remote mountain valley in Virginia where there has been much intermarriage have one of the joints of the fingers missing. There is a very prevalent idea that in close intermarriage in families variations and malformations often unfortunate for the individual are more common. All experimental evidence obtained by interbreeding of animals shows that close interbreeding is not productive of variation, but that variations existing in the breed become accentuated. Variations either advantageous or disadvantageous for the race or individual may either of them become more prevalent by close intermarriage. It seems, however, to have been shown by the customs of the human race that very close intermarriage is disadvantageous.

Eugenics, which signifies an attempt at the betterment of the race by the avoidance of bad heredity, has within recent years attracted much attention and is of importance. Some of its advocates have become so enthusiastic as to believe that it will be possible to breed men as cattle and ultimately to produce a race ideally perfect. It is true that by careful selection and regulation of marriage certain variations, whether relating to coa.r.s.e bodily form or to the less obvious changes denoted by function, can be perpetuated and strengthened. That the Semitic race excels in commerce is probably due to the fact that the variation of the brain which affected favorably the mental action conducive to this form of activity, was favorable for the race in the hostile environment in which it was usually placed and transmitted and strengthened by close intermarriage. It is impossible, however, to form a conception of what may be regarded as an ideal type of the human species. The type which might be ideal in a certain environment might not be ideal in another, and environment is probably of equal importance with the material. The eugenics movement has enormously stimulated research into heredity by the methods both of animal experimentation and observation, and study of heredity in man. As in all of the beginning sciences there is not the close inter-relation of observed facts and theory, but there is excess of theory and dearth of facts. Certain considerations, however, seem to be evident. It would seem to be evident that individuals should be healthy and enabled to maintain themselves in the environment in which they are placed, but the qualities which may enable an individual successfully to adapt himself to factory life, or life in the crowds and strong compet.i.tion of the city, may not be, and probably are not, qualities which are good for the race in general or for his immediate descendants. At present our attempts to influence heredity should be limited to the heredity of disease only. We can certainly say that intermarriage between persons who have tuberculosis or in whose families the disease has prevailed is disadvantageous for the offspring; the same holds true for insanity and for nervous diseases of all sorts, for forms of criminality, for alcoholism, and for those diseases which are long enduring and transmitted by s.e.xual contact such as syphilis and gonorrhoea. It is of importance that the facts bearing on the hereditary transmission of disease should become of general knowledge, in order that the dangers may be known and voluntarily avoided. No measures of preventive medicine are successful which are not supported by a public educated to appreciate their importance, and the same holds true of eugenics. How successful will be public measures leading to the prevention of offspring in the obviously unfit by sterilization of both males and females is uncertain. It is doubtful whether public sentiment at the present time will allow the measure to be thoroughly carried out. Some results in preventing unfit heredity may be attained by the greater extension of asylum life, but the additional burden of this upon the labor of the people would be difficult to bear. At best such measures would only be carried out in the lower cla.s.s of society.

CHAPTER XI

CHRONIC DISEASES.--DISEASE OF THE HEART AS AN EXAMPLE.--THE STRUCTURE AND FUNCTION OF THE HEART.--THE ACTION OF THE VALVES.--THE PRODUCTION OF HEART DISEASE BY INFECTION.--THE CONDITIONS PRODUCED IN THE VALVES.--THE MANNER IN WHICH DISEASE OF THE VALVES INTERFERES WITH THEIR FUNCTION.--THE COMPENSATION OF INJURY BY INCREASED ACTION OF HEART.--THE ENLARGEMENT OF THE HEART.--THE RESULT OF IMPERFECT WORK OF THE HEART.--VENOUS CONGESTION.--DROPSY.--CHRONIC DISEASE OF THE NERVOUS SYSTEM.--INSANITY.--RELATION BETWEEN INSANITY AND CRIMINALITY.--ALCOHOLISM AND SYPHILIS FREQUENT CAUSES OF INSANITY.--THE DIRECT AND INDIRECT CAUSES OF NERVOUS DISEASES.--THE RELATION BETWEEN SOCIAL LIFE AND NERVOUS DISEASES.--FUNCTIONAL AND ORGANIC DISEASE.--NEURASTHENIA.

Chronic diseases are diseases of long duration and which do not tend to result in complete recovery; in certain cases a cause of disease persists in the body producing constant damage, or in the course of disease some organ or organs of the body are damaged beyond the capacity of repair, and the imperfect action of such damaged organs interferes with the harmonious inter-relation of organs and the general well-being of the body. The effect of damage in producing chronic disease may not appear at once, for the great power of adaptation of organs and the exercise of reserve force may for a time render the damage imperceptible; when, however, age or the supervention of further injury diminishes the power of adaptation the condition of disease becomes evident. Chronic disease may be caused by parasites when the relation between host and parasite is not in high degree inimical, as in tuberculosis, gonorrhoea, syphilis, most of the trypanosome diseases and the diseases produced by the higher parasites. In certain cases the chronic disease represents really a series of acute onsets; thus in the case of the parasites there may be periods of complete quiescence of infection but not recovery, the parasites remaining in the body and attacking when the defences of the body are in some way weakened. In such cases there may be temporary immunity produced by each excursion of the disease, but the immunity is not permanent nor is the parasite destroyed. There is a further connection between chronic disease and infection in that the damage to the organs, which is the great factor underlying chronic disease, is so often the result of an infection.

The infectious diseases are those of early life; chronic disease, on the other hand, is most common in the latter third of life. This is due to the fact that in consequence of the general wear of the body this becomes less resistant, less capable of adaptation, and organic injury, which in the younger individual would be in some way compensated for, becomes operative. The territory of chronic disease is so vast that not even a superficial review of the diseases coming under this category can be attempted in the limits of this book, and it will be best to give single examples only, for the same general principles apply to all. One of the best examples is given in chronic disease of the heart.

The heart is a hollow organ forming a part of the blood vascular system and serving to give motion to the blood within the vessels by the contraction of its strong muscular walls. It is essentially a pump, and, as in a pump, the direction which the fluid takes when forced out of its cavity by the contraction of the walls diminishing or closing the cavity s.p.a.ce, is determined by valves. The contraction of the heart, which takes place seventy to eighty times in a minute, is automatic and is due to the essential quality of the muscle which composes it. The character, frequency and force of contraction, however, can be influenced by the nervous system and by the direct action of substances upon the heart muscle. The heart is divided by a longitudinal part.i.tion into a right and left cavity, and these cavities are divided by transverse septa, with openings in them controlled by valves, each into two chambers termed _auricle_ and _ventricle_. The auricle and ventricle on each side are completely separated.

The circulation of the blood through the heart is as follows: The blood, which in the veins of the body is flowing towards the heart, pa.s.ses by two channels, which respectively receive the blood from the upper and lower part of the body, into the right auricle. When this becomes distended it contracts, forcing the blood into the right ventricle; the ventricle then contracts and sends the blood into the arteries of the lungs, the pa.s.sage of blood into the auricle being prevented by valves which close the opening between auricle and ventricle when the latter contracts upon its contents. When the ventricle empties by its contraction the wall relaxes and the back flow from the artery is prevented by crescentic-shaped valves placed where the artery joins the ventricle. A similar arrangement of valves is on the left side of the heart. The pressure given the blood by the contraction of the right ventricle sends it through the lungs; from these, after it has been oxygenated, it pa.s.ses into the left auricle, then into the left ventricle and from this into the great artery of the body, the aorta, which gives off branches supplying the capillaries of all parts of the body. Both of the auricles and both of the ventricles contract at the same, time, the ventricular contraction following closely upon the contraction of the auricles. Contraction or systole is followed by a pause or diastole during which the blood flows from the veins into the auricles. The work which the right ventricle accomplishes is very much less than that of the left, and the right ventricle has a correspondingly thinner wall. The size of the heart is influenced by the size and the occupation of the individual being larger in the large individual than in the small, and larger in the active and vigorous than in the inactive. Generally speaking, the heart is about as large as the closed fist of its possessor.