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the posterior knot, which converts them into muscular action.
In the Articulata, the apparatus of motion is greatly developed, and the nervous system assumes an axidal position, and consists of a line of small ganglia, some thirteen at most, of which all but the two foremost, which are in the same plane, are repetitions one of another. The exceptions are the cephalic ganglia. This arrangement is observable in the larva, but in the perfected insect a redistribution of ganglia takes place, several disappear, the two at the posterior extremity coalesce, the cephalic ganglia increase in size, and the thoracic ganglia, from which the legs and wings are supplied, are concentrated and enlarged.
In the Vertebrata the ganglia are no longer distributed like tubers with commissural filaments, but are united into one continuous column, expanding at the head into a large knot of ganglia; the whole enclosed in strong bony armour.
In the Vertebrata the cerebral ganglia consist of 1. The sensory ganglia, including those of sight and smell. 2. The cerebric hemispheres. 3. The cerebellum.
The spinal cord in the lower vertebrate species is thick, and branches at its junction with the brain. The optic and olfactory ganglia predominate, the cerebellum is small, and the cerebrum smaller yet.
In higher types, where there is great complication of muscular action, the cerebellum assumes importance.
In man, the cerebrum is far larger than any of the other portions, its hemispheres overlap all the other parts, and are marked out with convolutions.
In reptiles, birds, and mammals the fork which appears in fish at the junction of the spinal cord and brain is filled
up; the spinal cord becomes gradually less thick and important as the type rises, and the preponderating nervous tract is located in the skull.1
The vascular afferent matter of the nervous system is grey, the fibrous efferent matter is white. Throughout the whole length of the spine the grey lies within the white. Towards the loins the amount of grey is greater than at the small of the back; this is the representative of the posterior ganglion in the lower organisms. The spinal cord receives afferent fibres from every portion of the body and gives origin to efferent fibres, which unite with the former at a short distance from the spine ; thus each nerve has two roots, and has at once a centripetal and a centrifugal action. The spine being a nucleus of nervous matter, a continuous ganglion, it can resolve force without transmitting it to the brain, but this action is simply reflex.
Towards the junction of the spine and brain is the medulla oblongata, a prolongation and expansion of spinal matter. This is the ganglionic centre of the respiratory and deglutive action. This is of a strictly reflex character also, all such action being due to an impression exerted upon the periphery of the system, which being reported to the centre returns as a motor impulse.
Overhanging the fourth ventricle of the brain is a great laminated mass, the cerebellum ; on each side, this organ sends down layers of transverse fibres, which sweep across the brain and meet in the middle line of its base, forming a kind of bridge, called the Pons Varolii.
The longitudinal nerve-fibres of the medulla oblongata pass forward, and emerge in front of this bridge as two
i This is summarized from Dr. Carpenter's “Principles of General and Comparative Physiology,” and Valentin's “ Text Book of Physiology," London, 1853.
broad diverging bundles, called crura cerebri. Above these lies a mass of nervous matter, raised into four hemispherical elevations, called corpora quadrigemina.
The crura then pass into a second large mass of nervous matter, called thalami optici, from which the fibres pass on into a body of grey and white matter, called corpus striatum. Adjoining these are the olfactory ganglia. The auditory ganglia are lodged in the substance of the medulla oblongata on either side of the fourth ventricle, and the gustatory ganglion is probably another node similarly imbedded in the medulla oblongata.
The sensory ganglia form the base of the cerebral hemispheres. In the medulla oblongata the grey vesicular substance occupies the same position within the white as it does in the spinal cord, but in the cerebral hemispheres and in the cerebellum the grey matter is external.
From the thalami optici and corpora striata, fibres radiate to the surface of the cerebrum.
The cerebellum has no direct communication with the cerebrum, but possesses independent connexions with the upper part of the spinal cord; it has its white matter so disposed within the grey as to exhibit a very peculiar and beautiful tree-like appearance, termed the arbor vitæ.
Such is the structure of the human brain. We will next consider what each portion is homologous to in the lower organisms.
In the lowest forms of animal life which are provided with a nervous system, all nervous action is reflex. In man, the spine (which is a continued chain of ganglia fused into one), the visceral ganglia, and the medulla oblongata are the homologues of these retlex motor nervous structures.
To the cephalic ganglia of those insects which have
perception of smell and sound, and which have visual organs, answer the sensory ganglia in man. In myriads of animals, indeed in the whole of the invertebrate subkingdom, no cerebrum exists; and, even in the lower Vertebrata, the olfactory, optic, and auditory ganglia have no direct connexion with it, so that the totality of their life is made up of sensational consciousness, which is formularized by sensori-motor action.
In those animals which exhibit great variety of motion, the cerebellum assumes prominence. Reptiles are the most inert of vertebrate animals, and in them the cerebellum is small. The active predaceous fishes have it largely developed. The vermiform fishes, whose progression is accomplished by flexion of the body, have a cerebellum so small as to be scarcely discoverable. On turning to the class of birds, we observe that the active falcons and swift-swinged swallows have a cerebellum much larger in proportion than that of the gallinaceous birds, whose powers of flight are small, or than that of the struthious tribe, in which they are altogether absent. Among mammals, its size bears a fixed proportion to the number and variety of muscular actions requiring combined movements of which they are respectively capable.
In the adult brain of man, in whom exists a necessity for co-ordination of an immense variety of voluntary and locomotive actions, no part of the encephalon has such extensive connexions with the cerebro-spinal axis, for it is in union with each segment of the great nervous centres, upon which the sensations and movements of the body depend
The cerebrum in like manner exhibits a steady increase in size as we ascend the vertebrate scale, till it culminates in man.
In the bat, the mole, and the rat, as in birds, the
cerebral hemispheres are perfectly plain and smooth, though divided by the Sylvian fissure; in the rabbit, the beaver, and porcupine this fissure is strongly marked, but there are only a few depressions indicating the future sulci on the surface of the hemispheres. In the fox, the wolf, and the dog the simplest form of the true convolutions are first met with, the fundamental convolutions of Leuret. In the fox, as a typical example, they are six in number. In the human brain, there are three external fundamental convolutions, which are tortuous; and between the anterior and posterior portions of these three external convolutions are interposed, on the upper surface of the hemispheres, two sets of transverse convolutions, divided by a distinct sulcus. In each of the hemispheres there are four orders of convolutions. In the first there is but one, surrounding the hemispheres like a riband. Of the second order, the marginal convolutions, there are two. The internal convolutions form those of the third order. Those of the fourth, the largest, deepest, and least symmetrical of all, are especially characteristic of the human brain.
We will proceed next to consider the functions of the different portions of the brain.
The spinal cord, as has been already observed, is a distinct and independent centre of action, consisting of a series of segmental ganglia and nerves, structurally homologous, and functionally analogous to the jointed ganglionic cord of the Articulata. The excito-motory and reflex actions of which it is the seat are evidently subservient to the conservation of the organism.
The functions of the medulla oblongata have been described. We come now to the corpora striata, which are in close connexion with the cerebral hemispheres. Im