Text Book of Biology Part 4

{Lines from First Edition only.} -Its position is indicated by a heavy black line in 4, and it is called, the vomer bone (vo.).-

{Lines from Second Edition only.} [In the frog it is represented by two laterally situated bones.

This is the vomer bone (vo.).]

The nasal pa.s.sages are partially blocked by foliated bony outgrowths, from the inner aspect of their walls, which in life are covered with mucous membrane, and increase the surface sensitive to smell. The ethmoid ends in the ethmo-turbinal (e.t.); the nasal, the naso-turbinal (n.t.); and the maxilla, the maxillo-turbinal (m.t.). In the anterior corner of the orbit there is a bone, the lachrymal (lc. Figure 1), which is hidden by the maxilla in the side view of the skull.

Section 91. The lower jaw (mandible) is one continuous bone in the mammal. Three incisors bite against the three of the upper jaw. Then comes a canine, four premolars, and three molars, the first of which is blade-like (sectorial tooth), and bites against the similar sectorial tooth (last premolar) of the upper jaw. The third molar is small. The arrangement of tooth is indicated in the following dental formula:-- I. 3.3/3.3, C. 1.1/1.1, P.M. 4.4/4.4, M. 2.2/3.3

Section 92. Attached just behind the bulla above, and pa.s.sing round on either side of the throat to meet at the base of the tongue, is the hyoid apparatus (Figure 6). The stylohyal (s.h.), epihyal (e.h.), and ceratohyal (c.h.) form the anterior cornu of the hyoid. The body of the hyoid (b.h.) forms a basis for the tongue. The posterior coruna (t.h.) of the hyoid are also called the thyrohyals.

Section 93. The following table presents these bones in something like their relative positions. A closer approximation to the state of the case will be reached if the student will imagine the maxilla raised up so as to overlie and hide the palatine and presphenoid, the squamosal similarly overlying the periotic bone, and the jugal reaching between them. Membrane bones are distinguished by capital letters.

-Cranium_

-Nasal_ (paired), Ethmoid Bone (median), -Vomer_ -Frontal_ (paired), -Lachrymal_ (paired), Orbito-sphenoid (paired), Pre-sphenoid (median), Eye -Parietal_ (Paired), Ali-sphenoid (paired), Basi-sphenoid (median)*, Periotic Bone (paired) -Bulla_ (paired) Supra-occipital (median), Ex-occipital (paired), Basi-occipital (median)

-Upper Jaw_

-Pre-Maxilla_ (paired) Palatine (paired) Pterygoid (paired)

-Lower Jaw_

-Maxilla_ (paired) -Jugal_ (paired) -Squamosal_ (paired)

*In this table the small bones of the ear are simply indicated by an asterisk.

Section 94. Hidden by the bulla, and just external to the periotic bone, are the auditory ossicles, the incus, malleus, os...o...b..culare, and stapes. These will be more explicitly treated when we discuss the ear.

Section 95. When we come to the study of the nerves, we shall revert to the skull, and treat of its perforations. The student should not fail, before proceeding, to copy and recopy our figures, and to make himself quite familiar with them, and he should also obtain and handle an actual skull. For all practical purposes the skull of a sheep or cat will be almost as useful as that of the dog.

6. _Muscle and Nerve_

Section 96. We have, in the skeleton, a complicated apparatus of parts hinged and movable upon one another; the agent moving these parts is the same agent that we find in the heart walls propelling the blood through the circulation, in the alimentary ca.n.a.l squeezing the food along its course, and universally in the body where motion occurs, except in the case of the creeping phagocytes, and the ciliary waving of ciliated epithelium. This agent is muscle. We have, in muscular tissue, a very wide departure from the structure of the primordial cell; to use a common biological expression, a very great amount of modification (= differentiation). Sheet 7 represents the simpler kind of muscular tissue, unstriated muscle, in which the cell character is still fairly obvious. The cells are fusiform (spindle-shaped), have a distinct nucleus and faint longitudinal striations (striations along their length), but no transverse striations.

Section 97. In striated muscle extensive modifications mask the cell character. Under a 1/4 inch objective, transverse striations of the fibres are also distinctly visible, and under a much higher power we discern in a fibre (Sheet 7) transverse columns of rod-like sarcous elements (s.e.), the columns separated by lines of dots, the membranes of Krause (k.m.), and nuclei (n.), flattened and separated into portions, and lying, in some cases, close to the sarcolemma (sc.) the connective tissue enclosing the fibre, in others scattered throughout the substance of the fibre. The figure shows the fibre ruptured, in order to display the sarcolemma; e.p. is the end plate of a nerve (n.v.), and fb. are the fibrillae into which a fibre may be teased.

Section 98. In the heart we have an intermediate kind of muscle cardiac muscle (Figure 2), in which the muscle fibres branch; there is apparently no sarcolemma, and the undivided nuclei lie in the centre of the cell.

Section 99. Unstriated muscle is sometimes called involuntary, and striated, voluntary muscle; but there is really not the connexion with the will that these terms suggest. We have just mentioned that the heart-muscle is striated, but who can alter the beating of the heart by force of will? And the striated muscles of the limbs perform, endless involuntary acts. It would seem that unstriated muscle contracts slowly, and we find it especially among the viscera; in the intestine for instance, where it controls that "peristaltic" movement which pushes the food forward. Voluntary muscle, on the other hand, has a sharp contraction. The muscle of the slow-moving snails, slugs, and mussels is unstriated; all the muscle of the active insects and crustacea (crabs, lobsters, and crayfish) is striated. Still if the student bears the exception of the heart in mind, and considers muscles as "voluntary" that his will can reach, the terms voluntary and involuntary will serve to give him an idea of the distribution of these two types of muscle in his own body, and in that of the rabbit.

Section 100. Muscular contraction, and generally all activity in the body is accompanied by kataboly. The medium by which these katabolic changes are set going and controlled is the nervous system. The nervous system holds the whole body together in one harmonious whole; it is the governing organization of the multicellular community (Section 55), and the supreme head of the government resides in the brain, and is called the mind. But just as in a political state only the most important and most exceptional duties are performed by the imperial body, and minor matters and questions of routine are referred to boards and local authorities, so the mind takes cognisance only of a few of the higher concerns of the animal, and a large amount of the work of the nervous system goes on insensibly, in a perfectly automatic way-- even much that occurs in the brain.

Section 101. The primary elements in the tissue of the nervous system are three; nerve fibres, which are simply conducting threads, telegraph wires; ganglion cells, which are the officials of the system; and neuroglia, a fine variety of connective tissue which holds these other elements together, and may also possibly exercise a function in affecting impressions. A message along a nerve to a ganglion cell is an afferent impression, from a cell to a muscle or other external end is an efferent impression. The pa.s.sage of an impression may be defined as a flash of kataboly along the nerve, and so every feeling, thought, and determination involves the formation of a certain quant.i.ty of katastases, and the necessity for air and nutrition.

Section 102. Unlike telegraph wires, to which they are often compared, nervous fibres usually convey impressions only in one direction, either centrally (afferent or sensory nerve fibres), or outwardly (efferent or motor nerve fibres). But the so-called motor nerve fibres include not only those that set muscles in motion, but those that excite secretion, check impulsive movements, and govern nutrition.

Section 103. Figure 7, Sheet 8, shows the typical structure of nervous tissues. The nerve fibres there figured are bound together by endoneurium into small ropes, the nerves, encased in perineurium.

There is always a grey axis cylinder (a.c.), which may (in medullated nerves), or may not (in non-medullated or grey nerves) have a medullary sheath (s.S.) interrupted at intervals by the nodes of Ranvier (n.R.). Nuclei (n.) at intervals under the sheath indicate the cells from which nerve fibres are derived by a process of elongation.

The nerves of invertebrata, where they possess nerves, are mostly grey, and so are those of the sympathetic system of vertebrata, to be presently described, g.c., g.c. are ganglion cells; they may have many hair-like processes, usually running into continuity with the axis cylinders of nerve fibres, in which case they are called multi-polar cells, or they may be uni- or bi-polar.

Section 104. The simplest example of the action of the nervous system is reflex action. For instance, when the foot of a frog, or the hand of a soundly sleeping person, is tickled very gently, the limb is moved away from the irritation, without any mental action, and entirely without will being exercised. And when we go from light into darkness, the pupil of the eye enlarges, without any direct consciousness of the change of its shape on our part. Similarly, the presence or food in the pharynx initiates a series of movements-- swallowing, the digestive movements, and so on-- which in health are entirely beyond our mental scope.

Section 105. A vast amount of our activities are reflex, and in such action an efferent stimulus follows an afferent promptly and quite mechanically. It is only where efferent stimuli do not immediately become entirely trans.m.u.ted into outwardly moving impulses that mental action comes in and an animal feels. There appears to be a direct relation between sensation and motion. For instance, the shrieks and other instinctive violent motions produced by pain, "shunt off" a certain amount of nervous impression that would otherwise register itself as additional painful sensation. Similarly most women and children understand the comfort of a "good cry," and its benefit in shifting off a disagreeable mental state.

Section 106. The mind receives and stores impressions, and these acc.u.mulated experiences are the basis of memory, comparison, imagination, thought, and apparently spontaneous will. Voluntary actions differ from reflex by the interposition of this previously stored factor. For instance, when a frog sees a small object in front of him, that may or may not be an edible insect, the direct visual impression does not directly determine his subsequent action. It revives a number of previous experiences, an image already stored of similar insects and a.s.sociated with painful or pleasurable gustatory experiences.

With these arise an emotional effect of desire or repulsion which, pa.s.ses into action of capture or the reverse.

Section 107. Voluntary actions may, by constant repet.i.tion, become quasi-reflex in character. The intellectual phase is abbreviated away.

Habits are once voluntary and deliberated actions becoming mechanical in this way, and slipping out of the sphere of mind. For instance, many of the detailed movements of writing and walking are performed without any attention to the details. An excessive concentration of the attention upon one thing leads to absent-mindedness, and to its consequent absurdities of inappropriate, because imperfectly acquired, reflexes.

Section 108. This fluctuating scope of mind should be remembered, more especially when we are considering the probable mental states of the lower animals. An habitual or reflex action may have all the outward appearance of deliberate adjustment. We cannot tell in any particular case how far the mental comes in, or whether it comes in at all. Seeing that in our own case consciousness does not enter into our commonest and most necessary actions, into breathing and digestion, for instance, and scarcely at all in the details of such acts as walking and talking we might infer that nature was economical in its use, and that in the case of such an animal as the Rabbit, which follows a very limited routine, and in which scarcely any versatility in emergencies is evident, it must be relatively inconsiderable. Perhaps after all, pain is not scattered so needlessly and lavishly throughout the world as the enemies of the vivisectionist would have us believe.

7. _The Nervous System_

Section 109. A little more attention must now be given to the detailed anatomy of the peripheral and central nerve ends. A nerve, as we have pointed out, terminates centrally in some ganglion cell, either in a ganglion or in the spinal cord or brain; peripherally there is a much greater variety of ending. We may have tactile (touch) ends of various kinds, and the similar olfactory and gustatory end organs; or the nerve may conduct efferent impressions, and terminate in a gland which it excites to secretion, in a muscle end-plate, or in fact, anywhere, where kataboly can be set going and energy disengaged.

We may now briefly advert to the receptive nerve ends.

Section 110. Many sensory nerves, doubtless, terminate in fine ends among the tissues. There are also special touch corpuscles, ovoid bodies, around which a nerve twines, or within which it terminates.

Section 111. The eye (Figure 8) has a tough, dense, outer coat, the sclerotic (sc.), within which is a highly vascular and internally pigmented layer, the choroid, upon which the percipient nervous layer, the retina (r.) rests. The chief chamber of the eye is filled with a transparent jelly, the vitreous humour (v.h.). In front of the eye, the white sclerotic pa.s.ses into the transparent cornea (c.). The epidermis is continued over the outer face of this as a thin, transparent epithelium. The choroid coat is continued in front by a ring-shaped muscle, the iris (ir.) the coloured portion of the eyes. This iris enlarges or contracts its central aperture (the black pupil) by reflex action, as the amount of light diminishes or increases.

Immediately behind this curtain is the crystalline lens (l.), the curvature of the anterior face or which is controlled by the ciliary muscle (c.m.). In front of the lens is the aqueous humour (a.h.). The description of the action of this apparatus involves the explanation of several of the elementary principles of optics, and will be found by the student in any text-book of that subject. Here it would have no very instructive bearing, either on general physiological considerations or upon anatomical fact.

Section 112. The structure of the retina demands fuller notice. Figure 9 shows an enlarged, diagram of a small portion of this, the percipient part of the eye. The optic nerve (o.n. in Figure 8) enters the eye at a spot called the blind spot (B.S.), and the nerve fibres spread thence over the inner retinal surface. From this layer of nerve fibres (o.n. in Figure 9) threads run outward, through certain clear and granular layers, to an outermost stratum of little rods (r.) and fusiform bodies called cones (c.), lying side by side. The whole of the retina consists of quite transparent matter, and it is this outermost layer of rods and cones (r. and c.) that receives and records the visual impression. This turning of the recipient ends away from the light is hardly what one would at first expect-- it seems such a roundabout arrangement-- but it obtains in all vertebrata, and it is a striking point of comparison with the ordinary invertebrate eye.