The disposition of the fibres in the brain is far too complex to be accurately followed. All that we can say is, that there are strands which connect one convolution with another, strands which connect one hemisphere with another, strands which connect cerebrum with cerebellum, and strands which connect the cerebrum with the lower ganglia. It is important to conceive this distinctly; for we shall hereafter see that the function of the Brain (by brain is here meant both Cerebrum and Cerebellum) is not that of innervation, but of incitation and regulation. To speak metaphorically, it is the coachman who holds in his hands the reins, and guides the team. One cardinal fact should arrest attention, namely, that not a single nerve in the body has its origin or centre of innervation in the cerebrum and cerebellum. The olfactory and optic nerves do indeed seem to issue from the cerebrum; and are commonly described as cerebral nerves. But the facts of Development, minute Anatomy, and Experiment prove this to be inexact. Although I shall continue to speak of the olfactory and optic nerves in accordance with universal usage, not wishing to burden the reader with unnecessary innovations, I must at the outset express my opinion that these nerves cannot be brought under the same general type as the other sensory nerves. Embryology and Anatomy suggest that they have no more claim to the title than the crura cerebri. Of this hereafter. Setting these aside, no one now refuses to acknowledge that Cerebrum and Cerebellum, although centres of Incitation and Association, are not the centres of direct Innervation: the organic mechanism in all its physiological processes will act independently of them (so far as such artificial distinctions are admissible at all). This does not throw a doubt on their physiological functions, nor on their participation in the normal execution of physiological processes.
8. From this rapid survey two important points may be selected for special attention. First, the continuity of the neural axis throughout; secondly, the fundamental similarity of its structure, underlying great variations in its form and connections. This, which is the anatomical expression of the Unity of the nervous system, will become more evident after we have expounded what Embryology and Microscopic Anatomy teach. We may therefore digress here awhile to consider
THE EARLY FORMS OF NERVE CENTRES.
9. In the outermost layer of the germinal membrane of the embryo a groove appears, which deepens as its sides grow upwards, and finally close over and form a canal. This canal is composed of cells all alike. Its foremost extremity soon bulges into three well-marked enlargements, which are then called the primitive cerebral vesicles. The cavities of these vesicles are continuous. Except in position and size, there are no discernible differences in these vesicles, which are known as the Fore-brain, Middle-brain, and Hind-brain.
10. The Fore-brain soon buds off from each side a small vesicle. This is the optic vesicle, the first rudiment of what subsequently becomes optic nerve and retina. At this period it is simply a vesicle with a hollow stem, the cavity being continuous with the cavity of the cerebral vesicle, and the walls continuous with the cerebral wall.
It thus appears that the retina and optic “nerve” are primitive portions of the brain—a detached segment of the general centre, identical in structure with the cerebral vesicle, and not unlike in form. A cup-like depression quickly forms the optic vesicle into an inner and an outer fold. The inner or concave fold becomes the retina, and the outer or convex fold (that nearest to the brain) becomes its choroid membrane. On the fourth day of incubation the retina of the chick is composed of spindle-shaped cells, all alike. On the seventh day there is a differentiation into layers, one of which on the eighth day is granular; on the tenth two are granular; and on the thirteenth ganglionic cells appear. Some of the cells have elongated into radial fibres (known as Müller’s fibres); and with the appearance of rods and cones the normal retinal elements are complete.
11. The researches of Foster and Balfour[84] confirm the statement that all the different parts of the retina (whether nervous or connective) are derived from one and the same layer of embryonic cells, which originally formed a portion of the first cerebral vesicle.
12. Meanwhile the hollow stem of this optic vesicle begins to develop fibres amidst the nuclei of its walls. The “optic nerve” arises: it is still hollow; and in birds remains so through life. The fibres as they are developed grow forwards towards the retina, and spread over its internal surface. They also grow forwards towards the brain, and spread over its substance; but it is not, as might be supposed, and is generally believed, with the cerebral hemispheres (or that portion of the Fore-brain from which these are derived), but with the Middle-brain (which becomes the corpora quadrigemina), that the optic fibres are in connection.[85]
13. This will be understood when the further development is traced. The Fore-brain, after budding off the optic vesicles, buds off two larger vesicles—the future cerebral hemispheres. This is noticeable on the second day of incubation, and by the third day each vesicle is as large as the whole of the original Fore-brain. Their development is essentially like that of the optic vesicles; both as to the cellular and the fibrous elements.
The convolutions, corpus callosum, nucleus lentiformis, and corpora striata are then indicated. Meanwhile, that which originally was the Fore-brain has lapsed into the secondary rank as Intermediate-brain (Zwischenhirn), and becomes the parts surrounding the third ventricle, namely, the thalami, corpora candicantia, infundibulum, and what is called the “posterior perforated substance.”