A Practical Physiology Part 28

The continued congestion of the minute structure of the kidney cuts off the needed nutrition of the organ, and forms the primary step in the series of disasters. Sometimes from this continued irritation, with the resulting inflammation, and sometimes from change of structure of the kidney by fatty degeneration, comes the failure to perform its proper function. Then, with this two-edged sword of disaster, the urea, which becomes a poisonous element, and should be removed, is retained in the system, while the alb.u.men, which is essential to healthy blood, is filtered away through the diseased kidney.

259. Alcoholic Liquors as a Cause of Bright's Disease. The unfortunate presence of alb.u.men in the urine is often a symptom of that insidious and fatal malady known as _alb.u.minuria_ or Bright's disease, often accompanied with dropsy and convulsions. One of the most constant causes of this disease is the use of intoxicants. It is not at all necessary to this fatal result that a person be a heavy drinker. Steady, moderate drinking will often accomplish the work. Kidney diseases produced by alcoholic drinks, are less responsive to medical treatment and more fatal than those arising from any other known cause.[39]

Experiment 129. Obtain a sheep's kidney in good order. Observe that its shape is something like that of a bean, and note that the concave part (hilus), when in its normal position, is turned towards the backbone. Notice that all the vessels leave and enter the kidney at the hilus. Observe a small thick-walled vessel with open mouth from which may be pressed a few drops of blood. This is the renal artery. Pa.s.s a bristle down it. With the forceps, or even with a penknife, lift from the kidney the fine membrane enclosing it. This is the kidney capsule.

Divide the kidney in halves by a section from its outer to near its inner border. Do not cut directly through the hilus. Note on the cut surfaces, on the outer side, the darker cortical portion, and on the inner side, the smooth, pale, medullary portion. Note also the pyramids of Malpighi.

Chapter X.

The Nervous System.

260. General View of the Nervous System. Thus far we have learned something of the various organs and the manner in which they do their work. Regarding our bodily structure as a kind of living machine, we have studied its various parts, and found that each is designed to perform some special work essential to the well-being of the whole. As yet we have learned of no means by which these organs are enabled to adjust their activities to the needs of other tissues and other organs. We are now prepared to study a higher, a more wonderful and complex agency,--the nervous system, the master tissue, which controls, regulates, and directs every other tissue of the human body.

The nervous system, in its properties and mode of action, is distinct from all the other systems and organs, and it shares with no other organ or tissue the power to do its special work. It is the medium through which all impressions are received. It connects all the parts of the body into an organism in which each acts in harmony with every other part for the good of the whole. It animates and governs all movements, voluntary or involuntary,--secretion, excretion, nutrition; in fact all the processes of organic life are subject to its regulating power. The different organs of the body are united by a common sympathy which regulates their action: this harmonious result is secured by means of the nervous system.

This system, in certain of its parts, receives impressions, and generates a force peculiar to itself. We shall learn that there can be no physical communication between or coordination of the various parts of organs, or harmonious acts for a desire result, without the nerves. General impressions, as in ordinary sensation, or special impressions, as in sight, smell, taste, or hearing,--every instinct, every act of the will, and every thought are possible only through the action of the nerve centers.

261. Nerve Cells. However complicated the structure of nerve tissue in man seems to be, it is found to consist of only two different elements, nerve cells and nerve fibers. These are a.s.sociated and combined in many ways. They are arranged in distinct ma.s.ses called nerve centers, or in the form of cords known as nerves. The former are made up of nerve fibers; the latter of both cells and fibers.

[Ill.u.s.tration: Fig. 110. Nerve Cells from the Spinal Cord.]

Nerve cells, which may be regarded as the central organs of the nerve fibers, consist of ma.s.ses of cell protoplasm, with a large _nucleus_ and _nucleolus_. They bear a general resemblance to other cells, but vary much in size and shape. Nerve cells grow, become active, and die, as do other cells. A number of processes branch off from them, some cells giving one or two, others many. The various kinds of nerve cells differ much in the shape and number of processes. One of the processes is a strand which becomes continuous with the axis cylinder of the nerve fibers; that is, the axis cylinders of all nerve fibers are joined in one place or another with at least one cell.

Each part of this system has its own characteristic cell. Thus we have in the spinal cord the large, irregular cells with many processes, and in the brain proper the three-sided cells with a process jutting out from each corner. So characteristic are these forms of cells, that any particular part of nerve structure may be identified by the kind of cells seen under the microscope. Nerve cells and nerve fibers are often arranged in groups, the various cells of the groups communicating with one another.

This cl.u.s.tered arrangement is called a nerve center.

262. Nerve Fibers. The nerve fibers, the essential elements of the nerves, somewhat resemble tubes filled with a clear, jelly-like substance. They consist of a rod, or central core, continuous throughout the whole length of the nerve, called the axis cylinder. This core is surrounded by the white substance of Schwann, or medullary sheath, which gives the nerve its characteristic ivory-white appearance. The whole is enclosed in a thin, delicate sheath, known as neurilemma.

[Ill.u.s.tration: Fig. 111.--Nerve Cells from the Gray Matter of the Brain.]

The axis cylinder generally pa.s.ses without any break from the nerve centers to the end of the fibers.[40] The outer sheath (neurilemma) is also continuous throughout the length of the fibers. The medullary sheath, on the other hand, is broken at intervals of about 1/25 of an inch, and at the same intervals nuclei are found along the fiber, around each of which is a minute protoplasmic ma.s.s. Between each pair of nuclei the sheath is interrupted. This point is known as the _node of Ranvier_.

Some nerve fibers have no inner sheath (medullary), the outer alone protecting the axis cylinder. These are known as the non-medullary fibers.

They are gray, while the ordinary medullary fibers are white in appearance. The white nerve fibers form the white part of the brain and of the spinal cord, and the greater part of the cerebro-spinal nerves.

The gray fibers occur chiefly in branches from the sympathetic ganglia, though found to some extent in the nerves of the cerebro-spinal system.

In a general way, the nerve fibers resemble an electric cable wire with its central rod of copper, and its outer non-conducting layer of silk or gutta percha. Like the copper rod, the axis cylinder along which the nerve impulse travels is the essential part of a nerve fiber. In a cut nerve this cylinder projects like the wick of a candle. It is really the continuation of a process of a nerve cell. Thus the nerve cells and nerve fibers are related, in that the process of one is the axis cylinder and essential part of the other.

The separate microscopic threads or fibers, bound together in cords of variable size, form the nerves. Each strand or cord is surrounded and protected by its own sheath of connective tissue, made up of nerves.

According to its size a nerve may have one or many of these strands. The whole nerve, not unlike a minute tendon in appearance, is covered by a dense sheath of fibrous tissue, in which the blood-vessels and lymphatics are distributed to the nerve fibers.

[Ill.u.s.tration: Fig. 112.--Medullated Nerve Fibers.

A, a medullated nerve fiber, showing the subdivision of the medullary sheath into cylindrical sections imbricated with their ends, a nerve corpuscle with an oval nucleus is seen between the neurilemma and the medullary sheath; B, a medullated nerve fiber at a node or constriction of Ranvier, the axis cylinder pa.s.ses uninterruptedly from one segment into the other, but the medullary sheath is interrupted.

263. The Functions of the Nerve Cells and Nerve Fibers. The nerve cells are a highly active ma.s.s of living material. They find their nourishment in the blood, which is supplied to them in abundance. The blood not only serves as nourishment, but also supplies new material, as it were, for the cells to work over for their own force or energy. Thus we may think of the nerve cells as a sort of a miniature manufactory, deriving their material from the blood, and developing from it nervous energy.

The nerve fibers, on the other hand, are conductors of nervous energy.

They furnish a pathway along which the nerve energy generated by the cells may travel. Made up as they are of living nerve substance, the fibers can also generate energy, yet it is their special function to conduct influences to and from the cells.

[Ill.u.s.tration: Fig. 113.--Non-Medullated Fibers.

Two nerve fibers, showing the nodes or constrictions of Ranvier and the axis cylinder. The medullary sheath has been dissolved away. The deeply stained oblong nuclei indicate the nerve corpuscles within the neurilemma.]

264. The Nervous System Compared to a Telegraphic System. In men and other highly organized animals, nerves are found in nearly every tissue and organ of the body. They penetrate the most minute muscular fibers; they are closely connected with the cells of the glands, and are found in the coats of even the smallest blood-vessels. They are among the chief factors of the structure of the sense organs, and ramify through the skin.

Thus the nervous system is the system of organs through the functions of which we are brought into relation with the world around us. When we hear, our ears are bringing us into relation with the outer world. So sight opens up to us another gateway of knowledge.

It will help us the better to understand the complicated functions of the nervous system, if we compare it to a telegraph line. The brain is the main office, and the mult.i.tudes of nerve fibers branching off to all parts of the body are the wires. By means of these, nerve messages are constantly being sent to the brain to inform it of what is going on in various parts of the body, and asking what is to be done in each case. The brain, on receiving the intelligence, at once sends back the required instructions. Countless messages are sent to and fro with unerring accuracy and marvelous rapidity.

Thus, when we accidentally pick up something hot, it is instantly dropped. A nerve impulse pa.s.ses from the nerves of touch in the fingers to the brain, which at once hurries off its order along another set of nerves for the hand to drop the burning object. These examples, so common in daily life, may be multiplied to any extent. Almost every voluntary act we perform is executed under the direction of the nervous system, although the time occupied is so small that it is beyond our power to estimate it.

The very frequency with which the nerves act tends to make us forget their beneficent work.

265. Divisions of the Nervous System. This system in man consists of two great divisions. The first is the great nerve center of the body, the cerebro-spinal system, which rules the organs of animal life. This includes the brain, the spinal cord, and the cerebro-spinal nerves. Nerves are given off from the brain and the cord, and form the mediums of communication between the external parts of the body, the muscles or the sense organs, and the brain.

The second part is the sympathetic system, which regulates the organic life. This consists of numerous small nerve centers arranged in oval ma.s.ses varying greatly in size, called ganglia or knots. These are either scattered irregularly through the body, or arranged in a double chain of knots lying on the front of the spine, within the chest and abdomen. From this chain large numbers of nerves are given off, which end chiefly in the organs of digestion, circulation, and respiration. The sympathetic system serves to bring all portions of the animal economy into direct sympathy with one another.

266. The Brain as a Whole. The brain is the seat of the intellect, the will, the affections, the emotions, the memory, and sensation. It has also many other and complex functions. In it are established many reflex, automatic, and coordinating centers, which are as independent of consciousness as are those of the spinal cord.

The brain is the largest and most complex ma.s.s of nerve tissue in the body, made up of an enormous collection of gray cells and nerve fibers.

This organ consists of a vast number of distinct ganglia, or separate ma.s.ses of nerve matter, each capable of performing separate functions, but united through the cerebral action into a harmonious whole.

[Ill.u.s.tration: Fig. 114.--The Upper Surface of the Cerebrum. (Showing its division into two hemispheres, and also the convolutions)]

The average weight of the adult human brain is about 50 ounces for men and 45 ounces for women. Other things being equal, the size and weight of the brain bear a general relation to the mental power of the individual. As a rule, a large, healthy brain stands for a vigorous and superior intellect.

The brains of many eminent men have been found to be 8 to 12 ounces above the average weight, but there are notable exceptions. The brains of idiots are small; indeed, any weight under a certain size, about 30 ounces, seems to be invariably a.s.sociated with an imbecile mind.

The human brain is absolutely heavier than that of any other animal, except the whale and elephant. Comparing the size of these animals with that of man, it is instructive to notice how much larger in proportion to the body is man's brain. The average proportion of the weight of the brain to the weight of the body is greater in man than in most animals, being about 1 to 36. In some small birds, in the smaller monkeys, and in some rodents, the proportional weight of the brain to that of the body is even greater than in man.

267. The Cerebrum. The three princ.i.p.al ma.s.ses which make up the brain when viewed as a whole are:

1. The cerebrum, or brain proper.

2. The cerebellum, or lesser brain.

3. The medulla oblongata.

The cerebrum comprises nearly seven-eighths of the entire ma.s.s, and fills the upper part of the skull. It consists of two halves, the right and left cerebral hemispheres. These are almost separated from each other by a deep median fissure. The hemispheres are united at the bottom of the fissure by a ma.s.s of white fibers pa.s.sing from side to side. Each of these hemispheres is subdivided into three lobes, so that the entire cerebrum is made up of six distinct lobes.

The cerebrum has a peculiar convoluted appearance, its deep folds being separated by fissures, some of them nearly an inch in depth.

It is composed of both white and gray matter. The former comprises the greater part of the ma.s.s, while the latter is spread over the surface in a layer of about ? of an inch thick. The gray matter is the portion having the highest functions, and its apparent quant.i.ty is largely increased by being formed in convolutions.

The convolutions of the cerebrum are without doubt a.s.sociated with all those higher actions which distinguish man's life; but all the convolutions are not of equal importance. Thus it is probable that only the frontal part of the brain is the intellectual region, while certain convolutions are devoted to the service of the senses.