Problems of Life and Mind. Second series, George Henry Lewes [e book reading free TXT] 📗
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Like the Cerebrum, it is a double organ formed by two symmetrical halves, as the cerebrum is of two hemispheres. Each half innervates the corresponding half of the body. The cord is unlike the cerebrum in external form, though very like it in internal structure. The gray structure is mainly external in the cerebrum, and is internal in the cord.
From the anterior side of the cord (that which in animals is the under side) the motor nerves issue; from the posterior (in animals the upper) side, issue the sensory nerves. On each of the sensory nerves there is a ganglion. The roots of each nerve, formed of several rootlets issuing from the anterior and posterior columns, subsequently unite together, and proceed in a single sheath to muscles and skin, separating again, however, before they reach muscles and skin. Fig. 2 represents this arrangement.
Fig. 2.—A portion of the spinal cord with its nerves (after Bernard). The left-hand figure shows the anterior side; the right-hand the posterior. A the anterior, and P, the posterior root, they meet at g, the ganglion; c and d are filaments connecting two posterior roots.
5. There are thirty-one pairs (sometimes thirty-two) of such nerves—namely, eight cervical, twelve thoracic, five lumbar, five sacral, and one (or two) coccygeal. Figs. 3 to 6 represent transverse sections, which display the entrance of the roots of the nerves into the anterior and posterior horns.
6. Similar masses of gray substance in the Medulla Oblongata (which is the name given to the cord when it passes into the skull)83 are supposed to be the origins of some other nerves (the cranial).
Fig. 3.—Transverse section of one half of the spinal cord in the lumbar region (after Kölliker). a, anterior root entering the anterior gray horns, m and l, where cells are clustered; c, central canal; d and e, the anterior and posterior commissures uniting the two halves of the cord; b, posterior root entering the posterior gray horn.
Fig. 4.—Transverse section of both halves of the cord, cervical region. a, Fissure separating the anterior columns; b, fissure of the posterior.
Although the Medulla Spinalis is unquestionably continued as the Medulla Oblongata, the arrangement of its tissues here becomes gradually changed, and so complicated that it baffles the scalpel. Anatomists are, however, agreed on the one point of fundamental importance to us here—namely, that there is only a rearrangement, not a new tissue. Accepting the artificial division into two organs, we may say that their functions are different, inasmuch as they are different in their anatomical connections—they innervate different parts; but as nerve-centres they have one and the same property.
On its posterior surface the Medulla Oblongata opens as the fourth ventricle. It is then no longer a closed canal, but an expansion of the spinal canal, which is covered by the Cerebellum. On its anterior surface projects the pons varolii. Figs. 7 and 8 represent these.
Fig. 7.—Back, or upper view of the Medulla Oblongata as it continues the Med. Spinalis. 1, Section of the thalami; 2, corpora quadrigemina (the two lower bodies are imperfectly represented in the engraving); 3, section of the crura cerebelli; 4, the fourth ventricle; 5, the restiform bodies; 6, the calamus scriptorius.
Fig. 8.—Front, or under view of the Med. Oblong. 1, Optic nerves cut off at the chiasma; 2, crura cerebri; 3, pons varolii; 4, olivary bodies; 5, anterior pyramids; 6, spinal columns.
While thus on the one hand continuing the Medulla Spinalis, the Medulla Oblongata is seen on the other hand to be continuous with the Brain—its white columns passing upwards in the crura cerebri, its cavity repeated in the other ventricles. Above it lie the ganglionic masses, the corpora quadrigemina, optic thalami, and corpora striata. Crowning these are the big and little brains, Cerebrum and Cerebellum. Figs. 9 and 10 represent this relation of Medulla Spinalis, Medulla Oblongata, and Brain. Fig. 11 is a purely artificial diagram which will give the reader some idea of the disposition of the white and gray substances.
Fig. 9.—Human Brain in Profile. 1, Cerebrum; 2, cerebellum; 3, pons varolii and medulla oblongata.
Fig. 10.—One half of the Brain in Profile, from the inside. 1, Convolutions of the cerebrum; 2, corpus callosum or great commissure uniting the two hemispheres; 3, arbor vitæ or branching arrangement of gray and white matter in the cerebellum; 4, pons varolii and medulla.
Fig. 11.—Diagram of a vertical section of the Brain (after Dalton). 1, Olfactory ganglion; 2, cerebral hemisphere; 3, corpus striatum; 4, thalamus; 5, corpora quadrigemina; 6, cerebellum; 7, ganglion of the pons varolii; 8, olivary body.
7. In man the Cerebrum is to the Cerebellum as 9 to 1. In the lower vertebrates the preponderance is still greater. The cerebrum is in our artificial systems commonly divided into three lobes. The frontal lobe is that portion which lies in front of the deep fissure named after Rolando; between that fissure and the “internal perpendicular fissure” lies the parietal lobe; behind this we have the occipital lobe; and, below the fissure of Sylvius, the tempero-sphenoidal lobe. Each lobe is again subdivided according to its convolutions.
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 Balfour84 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.”
14. The Middle-brain, or Second Vesicle, develops the corpora quadrigemina from the roof of its cavity, and the crura cerebri from its floor.
The Hind-brain, or Third Vesicle, divides into two, like the First Vesicle; it buds off the hemispheres of the cerebellum; its cavity forms the fourth ventricle; its walls the medulla oblongata.
15. It thus appears that the primitive membrane forms into a canal, which enlarges at one part into three vesicles, and from
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