Text Book of Biology, Part 1: Vertebrata

by H. G. Wells

Section 122. The cerebellum (metencephalon) consists of a central mass, the vermis (v.cbm.), and it also has lateral lobes (l.l.), prolonged into flocculi (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 the hind-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, 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 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. 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 passes 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 passes 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 passes 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, passes 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., passes 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.