Text Book of Biology Part 5

Section 113. We may pause to call the student's attention to a little point in the physiology of nerves, very happily ill.u.s.trated here. The function of a nerve fibre is 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, pa.s.s 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 the superior, inferior, anterior, and posterior recti. Running from the front of the orbit obliquely to the underside of the eyeball is the inferior oblique muscle. Corresponding to it above is a superior 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, the nict.i.tating lid, in the anterior corner of the eye.

Section 115. The ear (Sheet VII.) consists of an essential organ of hearing, and of certain superadded parts. The essential part is called the internal 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 ma.s.s of cartilage, the otic capsule; in the mammal the latter is entirely ossified, to form the periotic bone. The internal ear consists of a central sac, from which three semicircular ca.n.a.ls spring. The planes of the three ca.n.a.ls are mutually at right angles; two are vertical, the anterior and posterior (p.v.c.) vertical ca.n.a.ls, and one is horizontal, the horizontal ca.n.a.l (h.c.). There are dilatations, called ampullae, at the anterior base of the anterior, and at the posterior base of the posterior and horizontal ca.n.a.ls. Indirectly connected with the main sac is a spirally-twisted portion, resembling a snail sh.e.l.l in form, the cochlea. 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, the endolymph; between the membranous wall of the labyrinth and the enclosing bone is a s.p.a.ce containing the perilymph. 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), the ductus 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, and external auditory meatus (e.a.m.). A tightly stretched membrane, the tympanic membrane, separates this from the drum. A chain of small bones, the malleus (m.), the incus (i.), the os...o...b..culare (o.or.), a very small bone, and a stirrup-shaped stapes, swing across the tympanum, from the tympanic membrane to the internal ear. At two points the bony investment of this last is incomplete-- at the fenestra rotunda (f.r.), and at the fenestra 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 pa.s.sage, 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 essentially aquatic auditory organ, added to and patched up to fit the new needs of a life out of water.

Section 117. The impressions of smell are 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 pa.s.sage. The sense of taste has 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, the brain of a mammal consists of a linear arrangement of three hollow 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, the pineal gland (p.g.), while on its under surface is the pituitary body (pt.).

Section 119. The lower figure of (5) shows, in a diagrammatic manner, the derivation of the adult brain from this primitive state. From the fore-brain vesicle, a hollow outgrowth on either side gives rises to the (paired) cerebral hemisphere (c.h.), which is prolonged forward as the olfactory 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 the mid-brain is also thickened, and bulges up to form two pairs of thickenings, the corpora quadrigemina, (c.q.). The hind-brain sends up in front a median outgrowth, which develops lateral wings, the cerebellum (cbm.), behind which the remainder of the hind-brain is called the medulla oblongata, and pa.s.ses without any very definite demarcation into the spinal cord.

Section 120. Figure 1 is 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 the third ventricle, the original cavity of the fore-brain (1 in Figure 5), by an aperture (the foramen 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, the corpus callosum (c.c.), which is distinctive of the mammalian animals. The original fore-brain vesicle has its lateral walls thickened to form the optic 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 the thalamencephalon, the hemisphere, the prosencephalon, the olfactory lobes, the rhinencephalon.

Section 121. The parts of mid-brain (mesencephalon) will be easily recognised. Its cavity is in the adult mammal called the iter; its floor is differentiated into bundles of fibres, the crura cerebri (c.cb.), figured also in Figure 4.

Section 122. The cerebellum (metencephalon) consists of a central ma.s.s, the vermis (v.cbm.), and it also has lateral lobes (l.l.), prolonged into flocculi (f.cbm.), which lastare -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 the hind-brain, before and behind the cerebellum, consists of extremely thin plates of nervous matter. Its floor is greatly thickened to form the ma.s.s of the medulla, and in front a great transverse track of fibres is specialized, the pons Varolii (p.V.). Its cavity is called, the fourth ventricle.

Section 123. Figure 2 gives a dorsal view of the rabbit's brain; a horizontal slice has been taken at the level of the corpus callosum.

The lateral 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 in Figure 1 by the lines A and B.

This latter figure will repay careful examination. The arrow, ar., plunges into the third ventricle, behind the great middle 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 in Figure 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 of cranial nerves. From the fore-brain spring two pairs, which differ from the rest of the cranial nerves in being, first of all, hollow outgrowths of the brain-- the others are from the beginning solid. The first nerve is the olfactory lobe, which sends numerous filaments through the ethmoid bone to the olfactory organ. The second is the optic nerve, the visual sensory nerve.

Section 125. The mid-brain gives rise to only one nerve, the third, which supplies all the small muscles of the eye (see Section 114), except the superior oblique and external rectus.

Section 126. The remainder of the nerves spring from the hind-brain.

The fourth pair supply the superior obliques, and the sixth the external recti; so that III., IV., and VI. are alike purely motor nerves, small and distributed, to the orbit. The fifth nerve, the trigeminal, 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 (see Sheet 9) runs up over the recti behind the eyeball, it is the ophthalmic branch; V2, the maxillary branch, runs deeply under the eyeball and emerges in front of the malar, and V3, the mandibular branch, runs down on the inner side 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 the optic foramen for the exit of nerve II., perforating the orbito-sphenoid. Behind this there comes an irregular aperture, (f.l.a.), the foramen lacerum anterius, giving exit to III., IV., VI., and V1. V2 emerges from the foramen rotundum, and V3 from the foramen ovale, two apertures uniting behind a bony screen.* Just in front of the bulla is a foramen lacerum medium (f.l.M.), through which no nerve pa.s.ses.

* 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. pa.s.ses.

Section 128. The eighth nerve (auditory) is purely sensory, the nerve of the special sense of hearing; it runs into the periotic bone, and breaks up on the labyrinth. The seventh nerve (facial) is almost entirely motor; it pa.s.ses through the periotic anterior to VIII., and emerges by the stylo-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. The ninth (glossopharyngeal) nerve is chiefly sensory; it is the special nerve of taste, and is distributed to the tongue. The tenth nerve (vagus) arises by a number of roots, and pa.s.ses out of the skull, together with IX and XI, by the foramen 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 a superior laryngeal branch (Xa) to the larynx. The left vagus pa.s.ses ventral to the aortic arch, and sends a branch (l.x.b.) under this along the trachea to the larynx-- the recurrent laryngeal nerve. The corresponding nerve on the right (r.x.b.) loops under the subclavian artery. The main vagus, after this branching, pa.s.ses behind the heart to the oesophagus and along it to the stomach. XI., the spinal accessory, supplies certain of the neck nerves. XII., the hypoglossal, runs out of the skull by the condylar foramen (c.f.), is motor, crosses the roots of XI., X., and IX., pa.s.ses ventral to the carotid, and breaks up among the muscles of the tongue and neck.

Section 130. Of the functions of 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 the spinal cord is 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 ca.n.a.l (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, the sensory 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, the motor 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 ma.s.s of brain and spinal cord (cerebro-spinal axis), there is, on either side of the dorsal wall of the body cavity, a sympathetic nervous 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 a superior cervical ganglion, and opposite the first rib it forms an inferior cervical ganglion. Thence, backwards, there is a ganglion on each sympathetic chain opposite each spinal nerve, and the two exchange fibres through a thread, the ramus 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, the brachial plexus (plexus, literally network, but here meaning a plaited cord). The fourth cervical also sends down a phrenic 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 a sacral plexus, supplying the hind limb.

Section 134. From the sympathetic in the hinder region of the thorax a nerve, the great splanchnic nerve, arises, and runs, back to a ganglionated nervous network, just behind the coeliac artery, into which the vagus also enters; this is the coeliac ganglion, and together with a similar superior mesenteric ganglion around the corresponding artery, makes up a subsidiary visceral nervous network, the solar plexus. A similar and smaller nervous tangle, bearing an inferior mesenteric ganglion, lies near the inferior mesenteric artery.

Section 135. Finally, we may note the pineal gland and the pituitary 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 or vestigial 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 lacelet (Amphioxus).

8. _Renal and Reproductive Organs_

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 of unselfish provision, 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 a.s.sociate 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. The female organs of reproduction are shown in Sheet 10. The essential organ is the ovary (ov.), in which the ova (eggs) are formed. Figure 3 gives an enlarged and still more diagrammatic rendering of the ovary. There is a supporting ground ma.s.s, or stroma, into which numerous bloodvessels and nerves enter and break up. The ova appear 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 the ovisac as the development proceeds. (5) is a much more mature ovisac or Graafian follicle.

Section 138. The ovum (ov.), is now large, and its nucleus and nucleolus (the germinal vesicle and spot) are very distinct. The wall of the follicle consists, in the mammal, of several layers of cells, the membrana granulosa (or "granulosa" simply); the ovum lies on its outer side embedded in a ma.s.s of cells, discus proligerus, separated from actual contact with the ovum by a zona pellucida.

The ripening follicle moves to the surface of the ovary and bursts, the ovum falls into the body cavity. In Figure 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 pa.s.ses 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 the v.a.g.i.n.a (V.), which runs out into a vestibule (vb.) opening between tumid lips to the exterior. The urinary bladder (ur.b.) also opens into the vestibule, and receives the two ureters from the kidney.