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 canal 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 ismuscle. 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 ofmodification(= differentiation).Sheet 7represents 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 theirlength), but no transverse striations.
Section 97. Instriated muscleextensive modifications mask the cell character. Under a 1/4 inch objective,transverse striationsof the fibres are also distinctly visible, and under amuch higher powerwe 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 musclecardiac muscle(Figure 2), in which the muscle fibresbranch; there is apparently no sarcolemma, and the undivided nuclei lie in the centre of the cell.
Section 99. Unstriated muscle is sometimes calledinvoluntary, and striated,voluntarymuscle; 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 thenervous 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. Theprimary elementsin 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; andneuroglia, 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 anafferent impression, from a cell to a muscle or other external end is anefferent impression. The passage 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 quantity 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 (afferentorsensorynerve fibres), or outwardly (efferentormotornerve 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 byendoneuriuminto small ropes, the nerves, encased inperineurium. There is always a grey axis cylinder (a.c.), which may (inmedullatednerves), or may not (innon-medullatedorgreynerves) 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 calledmulti-polarcells, or they may beuni- orbi-polar.
Section 104. The simplest example of the action of the nervous system isreflex 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 transmuted 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 otherwiseregisteritself 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 accumulated experiences are the basis of memory, comparison, imagination, thought, and apparently spontaneous will.Voluntary actionsdiffer 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 associated with painful or pleasurable gustatory experiences. With these arise an emotional effect of desire or repulsion which, passes into action of capture or the reverse.
Section 107. Voluntary actions may, by constant repetition, becomequasi-reflexin character. The intellectual phase is abbreviated away.Habitsare 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 thedetailsof 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.
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 specialtouch corpuscles, ovoid bodies, around which a nerve twines, or within which it terminates.
Section 111. Theeye(Figure 8) has a tough, dense, outer coat, thesclerotic(sc.), within which is a highly vascular and internally pigmented layer, thechoroid, upon which the percipient nervous layer, theretina(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 passes 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 blackpupil) by reflex action, as the amount of light diminishes or increases. Immediately behind this curtain is thecrystalline lens(l.), the curvature of the anterior face or which is controlled by theciliary 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 theretinademands fuller notice.Figure 9shows an enlarged, diagram of a small portion of this, the percipient part of the eye. The optic nerve (o.n. inFigure 8) enters the eye at a spot called theblind spot(B.S.), and the nerve fibres spread thence over the inner retinal surface. From this layer of nerve fibres (o.n. inFigure 9) threads run outward, through certain clear and granular layers, to an outermost stratum of littlerods(r.) and fusiform bodies calledcones(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.
Section 113. We may pause to call the student's attention to a little point in the physiology of nerves, very happily illustrated here. The function of a nervefibreis the conduction of impressions pure and simple; the light radiates through the fibrous layer of the retina without producing the slightest impression, and at the blind spot, where the rods and cones are absent, and the nerve fibres are gathered together, no visual impressions are recorded. If there is any doubt as to the existence of a blind spot in the retinal picture, the proof is easy. Let the reader shut his left eye, and regard these two asterisks, fixing his gaze intently upon the left-hand one of them.
* *
* *
At a distance of three or four inches from the paper, both spots will be focussed on his retina, the left one in the centre of vision, and the right one at some spot internal to this, and he will see them both distinctly. Now, if he withdraws his head slowly, the right spot will of course appear to approach the left, and at a distance of ten or twelve inches it will, in its approach, pass over the blind spot and vanish, to reappear as he continues to move his head away from the paper. The function of nerve fibres is simply conduction, and the nature of the impressions they convey is entirely determined by the nature of their distal and proximal terminations.
Section 114. Certain small muscles in the orbit (eye-socket) move the eye, and by their action contribute to our perception of the relative position of objects. There is a leash of four muscles rising from a spot behind the exit of the optic nerve from the cranium to the upper, under, anterior, and posterior sides of the eyeball. These are thesuperior,inferior,anterior, andposterior recti. Running from the front of the orbit obliquely to the underside of the eyeball is theinferior oblique muscle. Corresponding to it above is asuperior oblique. A lachrymal gland lies in the postero-inferior angle of the orbit, and a Handerian gland in the corresponding position in front. In addition to the upper and lower eyelids of the human subject, the rabbit has a third, thenictitating lid, in the anterior corner of the eye.
Section 115. Theear(Sheet VII.) consists of an essential organ of hearing, and of certain superadded parts. The essential part is called theinternal ear, and is represented in all the true vertebrata (i.e., excluding the lancelet and its allies). In the lower forms it is a hollow membranous structure, embedded in a mass of cartilage, the otic capsule; in the mammal the latter is entirely ossified, to form theperiotic bone. The internal ear consists of a central sac, from which three semicircular canals spring. The planes of the three canals are mutually at right angles; two are vertical, the anterior and posterior (p.v.c.) vertical canals, and one is horizontal, the horizontal canal (h.c.). There are dilatations, calledampullae, at the anterior base of the anterior, and at the posterior base of the posterior and horizontal canals. Indirectly connected with the main sac is a spirally-twisted portion, resembling a snail shell in form, thecochlea. This last part is distinctive of the mammalia, but the rest of the internal ear is represented in all vertebrata, with one or two exceptions. The whole of the labyrinth is membranous, and contains a fluid, theendolymph; between the membranous wall of the labyrinth and the enclosing bone is a space containing theperilymph. Strange as it may appear at first, the entire lining of the internal ear is, at an early stage, continuous with the general epidermis of the animal. It grows in just as a gland might grow in, and is finally cut off from the exterior; but a considerable relic of this former communication remains as a thin, vertical blind tube (not shown in the figure), theductus endolymphaticus.
Section 116. The eighth nerve runs from the brain case (Cr.), into the periotic bone, and is distributed to the several portions of this labyrinth. In an ordinary fish this internal ear is the sole auditory organ we should find; the sound-waves would travel through the water to the elastic cranium and so reach and affect the nerves. But in all air-frequenting animals this original plan of an ear has to be added to, to fit it to the much fainter sound vibrations of the compressible and far less elastic air. A "receiving apparatus" is needed, and is supplied by the ear-drum, middle ear, or tympanic cavity (T.). In the mammal there is also a collecting ear trumpet (the ear commonly so-called), the external ear, andexternal auditory meatus(e.a.m.). A tightly stretched membrane, thetympanic membrane, separates this from the drum. A chain of small bones, the malleus (m.), the incus (i.), the os orbiculare (o.or.), a very small bone, and a stirrup-shapedstapes, swing across the tympanum, from the tympanic membrane to the internal ear. At two points the bony investment of this last is incomplete-- at thefenestra rotunda(f.r.), and at thefenestra ovalis, (f.o.), into which latter the end of the stapes fits, and so communicates the sound vibrations of the tympanic membrane to the endolymph. A passage, the Eustachian tube, communicates between the tympanic cavity and the pharynx (Ph.), and serves to equalize the pressure on either side of the drum-head. A comparative study of the ears of the vertebrata brings to light the fact that, as we descend in the animal scale, the four ear ossicles are replaced by large bones and cartilages connected with the jaw, and the drum and Eustachian tube by a gill slit. We have, in fact, in the ear, as the student will perceive in the sequel, an essentiallyaquaticauditory organ, added to and patched up to fit the new needs of a life out of water.
Section 117. The impressions ofsmellare conducted through the first nerve to the brain, and are first received by special hair-bearing cells in the olfactory mucous membrane of the upper part of the nasal passage. The sense oftastehas a special nerve in the ninth, the fibres of which terminate in special cells and cell aggregates in the little papillae (velvet pile-like processes) that cover the tongue.
Section 118. At an early stage in development, thebrainof a mammal consists of a linear arrangement ofthreehollow vesicles (Figure 5,Sheet VIII., 1, 2, and 3), which are the fore-, mid-, and hind-brain respectively. The cavities in these in these vesicles are continuous with a hollow running through the spinal cord. On the dorsal side of the fore-brain is a structure to be dealt with more fully later, thepineal gland(p.g.), while on its under surface is thepituitary body(pt.).
Section 119. The lowerfigureof (5) shows, in a diagrammatic manner, the derivation of the adult brain from this primitive state. From thefore-brainvesicle, a hollow outgrowth on either side gives rises to the (paired)cerebral hemisphere(c.h.), which is prolonged forward as theolfactory lobe(o.l.). From the fore-brain the retina of the eye and the optic nerve also originate as an, at first, hollow outgrowth (op.). The roof of themid-brainis also thickened, and bulges up to form two pairs of thickenings, thecorpora quadrigemina, (c.q.). Thehind-brainsends up in front a median outgrowth, which develops lateral wings, the cerebellum (cbm.), behind which the remainder of the hind-brain is called themedulla oblongata, and passes without any very definite demarcation into the spinal cord.
Section 120.Figure 1is a corresponding figure of the actual state of affairs in the adult. The brain is seen in median vertical section. (ch.) is the right cerebral hemisphere, an inflated vesicle, which, in the mammal-- but not in our lower types-- reaches back over the rest of the fore-brain, and also over the mid-brain, and hides these and the pineal gland in the dorsal view of the brain (Figure 2). The hollow of the hemisphere on either side communicates with thethird ventricle, the original cavity of thefore-brain(1 inFigure 5), by an aperture (theforamen of Monro), indicated by a black arrow (f.M.). Besides their original communication through the intermediation of the fore-brain, the hemispheres are also united above its roof by a broad bridge of fibre, thecorpus callosum(c.c.), which is distinctive of the mammalian animals. The original fore-brain vesicle has its lateral walls thickened to form theoptic thalami(o.th.), between which a middle commissure, (m.c.), absent in lower types, stretches like a great beam across the third ventricle. The original fore-brain is often called thethalamencephalon, the hemisphere, theprosencephalon, the olfactory lobes, therhinencephalon.
Section 121. The parts ofmid-brain(mesencephalon) will be easily recognised. Its cavity is in the adult mammal called theiter; its floor is differentiated into bundles of fibres, thecrura cerebri(c.cb.), figured also inFigure 4.
Section 122. The cerebellum (metencephalon) consists of a central mass, thevermis(v.cbm.), and it also haslateral lobes(l.l.), prolonged intoflocculi(f.cbm.), which last are -em-bedded in pits, [in] the periotic bone, and on that account render the extraction of the brain from the cranium far more difficult than it would otherwise be. The roof of thehind-brain, before and behind the cerebellum, consists of extremely thin plates of nervous matter. Its floor is greatly thickened to form the mass of the medulla, and in front a great transverse track of fibres is specialized, thepons Varolii(p.V.). Its cavity is called, thefourth ventricle.
Section 123.Figure 2gives a dorsal view of the rabbit's brain; a horizontal slice has been taken at the level of the corpus callosum. Thelateral ventricle(i.e., the hollows of the hemisphere) is not yet opened. A lower cut (Figure 3) exposes this (V.L.). The level of these slices is approximately indicated inFigure 1by the lines A and B. This latter figure will repay careful examination. The arrow, ar., plunges into the third ventricle, behind the greatmiddle commissure(m.c.), and the barb is supposed to lie under the roof of the mid-brain, the corpora quadrigemina (c.q.). The position of ar. is also indicated inFigure 1. Before reading on, the beginner should stop a while here; he should carefully copy or trace our figures and, putting the book aside, name the parts, and he should then recopy, on an enlarged scale, and finally draw from memory, correct, and again draw. By doing this before the brain is dissected a considerable saving of time is possible.
Section 124. Proceeding from the brain are twelve pairs ofcranial nerves. From the fore-brain spring two pairs, which differ from the rest of the cranial nerves in being, first of all,hollowoutgrowths of the brain-- the others are from the beginningsolid. Thefirstnerve is the olfactory lobe, which sends numerous filaments through the ethmoid bone to the olfactory organ. Thesecondis the optic nerve, the visual sensory nerve.
Section 125. Themid-braingives rise to only one nerve, thethird, which supplies all the small muscles of the eye (seeSection 114), except the superior oblique and external rectus.
Section 126. The remainder of the nerves spring from thehind-brain. Thefourthpair supply the superior obliques, and thesixththe external recti; so that III., IV., and VI. are alike purely motor nerves, small and distributed, to the orbit. Thefifthnerve, thetrigeminal, is a much larger and more important one; it is a mixed nerve, having three main branches, of which the first two are chiefly sensory, the third almost entirely motor; it lies deeply in the orbit. V1 (seeSheet 9) runs up over the recti behind the eyeball, it is theophthalmicbranch; V2, themaxillarybranch, runs deeply under the eyeball and emerges in front of the malar, and V3, the mandibular branch, runs down on theinnerside of the jaw-bone to the jaw muscles and tongue.
Section 127. If the student will now recur to the figures of the dog's skull (Sheet 6), he will see certain apertures indicated in the cranial wall. Of these, o.f. is theoptic foramenfor the exit of nerve II., perforating the orbito-sphenoid. Behind this there comes an irregular aperture, (f.l.a.), theforamen lacerum anterius, giving exit to III., IV., VI., and V1. V2 emerges from theforamen rotundum, and V3 from theforamen ovale, two apertures uniting behind a bony screen.* Just in front of the bulla is aforamen lacerum medium(f.l.M.), through which no nerve passes.
* In the rabbit's skull f.l. anterius, the foramen rotundum, and foramen ovale are not distinct, and there are two condylar foramina instead of one, through each of which, a moiety of XII. passes.
Section 128. Theeighthnerve (auditory) is purely sensory, the nerve of the special sense of hearing; it runs into the periotic bone, and breaks up on the labyrinth. Theseventhnerve (facial) is almost entirely motor; it passes through the periotic anterior to VIII., and emerges by thestylo-mastoid foramen(s.m.f.) behind the bulla, to run outside the great jaw muscle across the cheek immediately under the skin (Figure 1).
Section 129. Theninth(glossopharyngeal) nerve is chiefly sensory; it is the special nerve oftaste, and is distributed to the tongue. Thetenthnerve (vagus) arises by a number of roots, and passes out of the skull, together with IX and XI, by theforamen lacerum-posterium- [posterius] (f.l.p.). It is a conspicuous white nerve, and runs down the neck by the side of the common carotid artery. It sends asuperior laryngeal branch(Xa) to the larynx. The left vagus passes ventral to the aortic arch, and sends a branch (l.x.b.) under this along the trachea to the larynx-- therecurrent laryngeal nerve. The corresponding nerve on the right (r.x.b.) loops under the subclavian artery. The main vagus, after this branching, passes behind the heart to the oesophagus and along it to the stomach. XI., thespinal accessory, supplies certain of the neck nerves. XII., thehypoglossal, runs out of the skull by the condylar foramen (c.f.), is motor, crosses the roots of XI., X., and IX., passesventralto the carotid, and breaks up among the muscles of the tongue and neck.
Section 130. Of thefunctionsof the several parts of the brain there is still very considerable doubt. With disease or willful destruction of the cerebral tissue the personal initiative is affected-- the animal becomes more distinctly a mechanism; the cerebellum is probably concerned in the coordination of muscular movements; and the medulla is a centre for the higher and more complicated respiratory reflexes, yawning, coughing, and so on. The great majority of reflex actions centre, however, in the spinal cord, and do not affect the brain.
Section 131. A cross section of thespinal cordis shown in Figure 6,Sheet 8. It is a cylinder, almost bisected by a dorsal (d.f.) and a ventral (v.f.) fissure. Through its centre runs a central canal (c.c.), continuous with the brain ventricles, and lined by ciliated epithelium. The spinal cord consists of an outer portion, mainly of nervous fibres, the white matter, and of inner,ganglionated, and more highly vascular grey matter. (In the cerebrum the grey matter is external, and the white internal.) The cord, like the brain, is surrounded by a vascular fibrous investment, and protected from concussion by a serous fluid. The nerves which emerge from the vertebral column between the vertebrae, arise, unlike the cranial nerves, by two roots. The dorsal of these, thesensory root(d.n.), has a swelling upon it, the dorsal ganglion, and-- by experiments upon living animals-- has been shown to contain only afferent fibres; the ventral, themotor root, is without a ganglion, and entirely or mainly motor. The two unite outside the cord, and thereafter the spinal nerves are both sensory and motor.
Section 132. Besides the great mass of brain and spinal cord (cerebro-spinal axis), there is, on either side of the dorsal wall of the body cavity, asympatheticnervous chain. The nerve fibres of this system, like the nerve fibres of invertebrates, are non-medullated. It may be seen as a greyish thread running close by the common carotid in the neck (sym.,Figure 1); it then runs over the heads of the ribs in the thorax and close beside the dorsal aorta in the abdominal region. In the anterior region of the neck it dilates to form asuperior cervical ganglion, and opposite the first rib it forms aninferior cervical ganglion. Thence, backwards, there is a ganglion on each sympathetic chain opposite each spinal nerve, and the two exchange fibres through a thread, theramus communicans. To the sympathetic chain is delegated much of the routine work of reflex control of the bloodvessels and other viscera, which would otherwise fall upon the spinal cord.
Section 133. There are eight cervical (spinal)nerves, one in front of the atlas, and one behind each of the cervical vertebrae. The last four and the first thoracic (spinal) contribute to a leash of nerves running out to the fore limb, thebrachial plexus(plexus, literally network, but here meaning a plaited cord). The fourth cervical also sends down aphrenic nerve(p.n.,Figure 1), along by the external jugular vein and the superior caval vein to the diaphragm. The last three lumbar and the sacral nerves form asacral plexus, supplying the hind limb.
Section 134. From the sympathetic in the hinder region of the thorax a nerve, thegreat splanchnicnerve, arises, and runs, back to a ganglionated nervous network, just behind the coeliac artery, into which the vagus also enters; this is thecoeliac ganglion, and together with a similarsuperior mesenteric ganglionaround the corresponding artery, makes up a subsidiary visceral nervous network, thesolar plexus. A similar and smaller nervous tangle, bearing aninferior mesenteric ganglion, lies near the inferior mesenteric artery.
Section 135. Finally, we may note thepineal glandand thepituitary body, as remarkable appendages above and below the thalamencephalon. Their function, if they have a function, is altogether unknown. Probably, they are inherited from ancestors to whom they were of value. Such structures are called reduced orvestigial structures, and among other instances are the clavicles of the rabbit, the hair on human limbs, the little pulpy nodule in the corner of the human eye, representing the rabbit's third eyelid, and the caudal vertebrae at the end of the human spinal column. In certain lowly reptiles, in the lampreys, and especially in a peculiar New Zealand lizard, the pineal gland has the most convincing resemblance to an eye, both in its general build and in the microscopic structure of its elements; and it seems now more than probable that this little vascular pimple in our brains is a relic of a third and median eye possessed by ancestral vertebrata. The pituitary body is probably equivalent to a ciliated pit we shall describe in the lancelet (Amphioxus).
Section 136. We have now really completed our survey of the individual animal's mechanism. But no animal that was merely complete in itself would be long sanctioned by nature. For an animal species to survive, there must evidently, also, be proper provision for the production of young, and the preservation of the species as well as of the individual. Hence in an animal's physiology and psychology we meet with a vast amount ofunselfishprovision, and its structure and happiness are more essentially dependent on the good of its kind than on its narrow personal advantage. The mammalia probably owe their present dominant position in the animal kingdom to the exceptional sacrifices made by them for their young. Instead of laying eggs and abandoning them before or soon after hatching, the females retain the eggs within their bodies until the development of the young is complete, and thereafter associate with them for the purposes of nourishment, protection, and education. In the matter of the tail, for instance, already noted, the individual rabbit incurs the disadvantage of conspicuousness for the rear, in order to further the safety of the young.
Section 137. Thefemale organs of reproductionare shown inSheet 10. The essential organ is the ovary (ov.), in which theova(eggs) are formed.Figure 3gives an enlarged and still more diagrammatic rendering of the ovary. There is a supporting ground mass, orstroma, into which numerous bloodvessels and nerves enter and break up. Theovaappear first as small cells in the external substance of the ovary (as at 1), and move inward (2 and 3), surrounded by a number of sister cells, which afford them nourishment. At (4) an ovum with its surrounding group of cells is more distinct and near the centre of the ovary; a fluid is appearing within theovisacas the development proceeds. (5) is a much more matureovisacorGraafian follicle.
Section 138. The ovum (ov.), is now large, and its nucleus and nucleolus (thegerminal vesicleandspot) are very distinct. The wall of the follicle consists, in the mammal, of several layers of cells, themembrana granulosa(or "granulosa" simply); the ovum lies on its outer side embedded in a mass of cells,discus proligerus, separated from actual contact with the ovum by azona pellucida. The ripening follicle moves to the surface of the ovary and bursts, the ovum falls into the body cavity. InFigure 2, a ripe Graafian follicle (G.F.), projects upon the ovary.
Section 139. The liberated ovum is caught up by the funnel-shaped opening of the Fallopian tube, which passes without any very conspicuous demarcation into the cornu uteri (c.ut.) of its side; the two uterine cornua meeting together in the middle line form thevagina(V.), which runs out into a vestibule (vb.) opening between tumid lips to the exterior. Theurinary bladder(ur.b.) also opens into the vestibule, and receives the two ureters from the kidney.
Section 140. In themalewe find, in the position of the female uterus, auterus masculinus(u.m.). The essential sexual organ is thetestis(T.), a compact mass of coiling tubuli, which opens by a number of ducts, thevasa efferentia, into a looser and softerepididymis(ep.), which sends the sexual product onward through avas deferens(v.d.), to open at the base of the uterus masculinus. The urinary bladder and ureters correspond with those of the female, and the common urogenital duct (= vestibule), theurethra, is prolonged into an erectile penis (P.) surrounded by a fold of skin, theprepuce. A prostate gland (pr.), contributes to the male sexual fluid. The character of the essential male element, thespermatozoon, the general nature of the reproductive process, will be conveniently deferred until the chapters upon development are reached.
Section 141. The following facts of classificatory importance may now be considered, but their full force will be better appreciated after the study of other vertebrate types. They are such as come prominently forward in the comparison of the rabbit with other organisms.
Section 142. In the first place, the rabbit is a metazoon, one of the metazoa, i.e., a multicellular organism, as compared with the amoeba, which belongs to the protozoa or one-cell animals (Section 55). In the next place, it is externallybilaterally symmetrical, its parts balance, and where, in its internal anatomy, it departs from this symmetry (as in the case of the aorta, the stomach and intestines, and the kidneys), the departure has an appearance of being the results of partial reductions and distortions of an originally quite symmetrical plan. And the facts of development strengthen this idea; in the very earliest stages we havepairedaortic arches, of which, the left only remains, a straight alimentary canal, and less asymmetrical kidneys. In the vast majority of animals the same bilateral symmetry is to be seen, but in the star-fish and sea-urchins, and in the jelly-fish, corals, sea anemones, andhydra, the general form of the animal is, instead, arranged round a centre, like a star and its rays, and the symmetry is calledradial.
Section 143. We also see in various organs of the rabbit, and especially in the case of the limbs and vertebral column, what is calledmetameric segmentation, that is, a repetition of parts, one behind the other, along the axis of the body. Thus the bodies and arches of the vertebrae repeat each other, and so do the spinal nerves. The renal organ of the rabbit, some time before birth, displays a metameric arrangement of its parts; but this disappears, as development proceeds, into the compact kidney of the adult. But the metameric segmentation in the rabbit's organism is not nearly so marked as that of an earthworm, for instance, which is visibly a chain of rings. If the student wants a perfect figure of metameric segmentation he should think of a train of precisely similar carriages, or a string of beads. One bead, one carriage, one vertebra, would be ametamere.
Section 144. In contrast to metameric segmentation is theantimericrepetition of radial symmetry (Section 142), in which each ray of the star is called anantimere. It is possible to have bilateral symmetry without a metameric arrangement of parts, as in the mussel and the cuttle-fish; but metameric segmentation without complete or reduced bilateral symmetry does not occur.
Section 145. We are now in a position to appreciate the fact that the old and more popularly know division of animals into vertebrata and invertebrata scarcely represents the facts of the case, that the primary division should be into protozoa and metazoa, and that the vertebrata are one of several groups of metazoa with a fundamental bilateral symmetry and imperfect metameric segmentation.
The rabbit is one of thevertebrata, and, in common with all the other animals collected under this head, it has--
(a)A skeletal axis(the vertebral column)between its central nervous system and its body cavity. In the adult rabbit this consists of a chain of vertebrae, but in the embryo (i.e., the young rabbit before birth) it is represented by a continuous chord, thenotochord, and it remains as such in some of the lowest vertebrata throughout life. In other words, in these lower vertebrata, the vertebral axis is not metameric.(b)A dorsal and-Tubular_nervous axis. (Section 131, the central canal)(c) It has, though in the embryo only, certain slits between the throat and the exterior, like thegill slitsof a fish. Such slits are-- with one or two remarkable exceptions outside the sub-kingdom-- distinctly vertebrate features, and remain, of course, in fishes throughout life.
The presence of true cartilage and bone mark a vertebrate, but vertebrata occur in which -these tissues- [bone] -are- [is] absent.
Section 146. The rabbit shares the following features with all the vertebrata, except the true fishes, which do not possess any of them--
(a)Lungs(but many fish have a swimming bladder which answers to the lungs in its anatomical relations.)(b)Limbs which consist of a proximal joint of one bone an intermediate part of two, and a distal portion which has five digits, or is evidently a reduced form of the five-digit limb.*(c)The absence of a median fin supported by fin rays.*** The frog shows indications of a sixth digit.** The frog's tadpole has a median fin, butno fin rays.
Section 147. The rabbit shares the following features with all the vertebrata above the fishes and amphibia (= frogs, toads, newts, and etc.)--
(a)Absence of gills(notgill slits, note)at any stage in development.(b)An amnion, and(c)An allantoisin development.
The meaning of (b) and (c) we shall explain to the student in the chapters on embryology. We simply mention them here to render our table complete.
Section 148. The rabbit shares with allmammals, and differs from all other vertebrata (i.e., birds, reptiles, amphibia, and fishes), in having--
(a)Hair.(b)A diaphragm.(c) Only oneaortic arch, and that on theleft sideof the body.(d) Its young born alive. (But two very reptile-like mammals of Australia, the duck-billed platypus and theechidna, lay eggs, and certain fish and reptiles bear living young.)(e)Epiphysesto its vertebral -centre- [centra].*(f) The cerebral hemispheres covering the mid-brain.(g) Corpora quadrigemina instead of bigemina.[(h) A corpus callosum.][(i) A spirally coiled cochlea to the internal ear.][(In respect to h and i also, the echidna and platypus are scarcely mammalian.)]* But certain mammals have no such epiphyses.
Section 149. The rabbit, together with the hares and conies, rats and mice, voles, squirrels, beavers, cavies, guineapigs is included in that order of the class of mammals which is called therodentia, and is distinguished by the character of the incisor teeth from other orders of the class.