the white portions of the two lateral halves. It is called the white commissure of the cord.

The spinal nerves originate from the cord on each side by two distinct roots; one anterior, and one posterior. The anterior root (Fig. 127, d) arises from the surface of the cord near the extremity of the anterior peak of gray matter. The posterior root (e) originates at the point corresponding with the posterior peak of gray matter. Both roots are composed of a considerable number of ultimate nervous filaments, united with each other in parallel bundles. The posterior root is distinguished by the presence of a small ganglion (c) which appears to be incorporated with it, and through which its fibres pass. There is no such ganglion on the anterior root. The two roots unite with each other shortly after leaving the cavity of the spinal canal, and mingle their filaments in a single trunk.

It will be seen, on referring to the diagram (Fig. 127), that each lateral half of the spinal cord is divided into two portions, an anterior and a posterior portion. The posterior peak of gray matter comes quite up to the surface of the cord, and it is just at this point (e) that the posterior roots of the nerves have their origin. The whole of the white substance included between this point and the posterior median fissure is called the posterior column of the cord. That which is included between the same point and the anterior median fissure is the anterior column of the cord. The white substance of the cord may then be regarded as consisting for the most part of four longitudinal bundles of nervous filaments, viz., the right and left anterior, and the right and left posterior columns. The posterior median fissure penetrates deeply into the substance of the cord, quite down to the gray matter, so that the posterior columns appear entirely separated from each other in a transverse section; while the anterior median fissure is more shallow and stops short of the gray matter, so that the anterior columns are connected with each other by the white commissure above mentioned.

By the encephalon we mean the whole of that portion of the cerebro-spinal system which is contained in the cranial cavity. It is divided into three principal parts, viz., the cerebrum, cerebellum, and medulla oblongata. The anatomy of these parts, though somewhat complicated, can be readily understood if it be recollected that they are simply a double series of nervous ganglia, connected with each other and with the spinal cord by transverse and longitudinal

commissures. The number and relative size of these ganglia, in different kinds of animals, depend upon the perfection of the bodily organization in general, and more especially on that of the intelligence and the special senses. They are most readily described by commencing with the simpler forms and terminating with the more complex.

Fig. 128.

The brain of the Alligator (Fig. 128) consists of five pair of ganglia, ranged one behind the other in the interior of the cranium. The first of these are two rounded masses (1), lying just above and behind the nasal cavities, which distribute their nerves upon the Schneiderian mucous membrane. These are the olfactory ganglia. They are connected with the rest of the brain by two long and slender commissures, the "olfactory commissures." The next pair (2) are somewhat larger and of a triangular shape, when viewed from above downward. They are termed the "cerebral ganglia," or the hemispheres. Immediately following them are two quadrangular masses (3) which give origin to the optic nerves, and are called therefore the optic ganglia. They are termed also the "optic tubercles;" and in some of the higher animals, where they present an imperfect division into four nearly equal parts, they are known as the "tubercula quadrigemina." Behind them, we have a single triangular collection of nervous matter (4), which is called the cerebellum. Finally, the upper portion of the cord, just behind and beneath the cerebellum, is seen to be enlarged and spread out laterally, so as to form a broad oblong mass (5), the medulla oblongata. It is from this latter portion of the brain that the pneumogastric or respiratory nerves originate, and its ganglia are therefore sometimes terined the "pneumogastric" or "respiratory" ganglia.

It will be seen that the posterior columns of the cord, as they diverge laterally, in order to form the medulla oblongata, leave between them an open space, which is continuous with the posterior median fissure of the cord. This space is known as the "fourth ventricle." It is partially covered in by the backward projection

of the cerebellum, but in the alligator is still somewhat open posteriorly, presenting a kind of chasm or gap between the two lateral halves of the medulla oblongata.

The chain of ganglia which compose the brain, being arranged in pairs as above described, are separated from each other on the two sides by a longitudinal median fissure, which is continuous with the posterior median fissure of the cord. In the brain of the alligator this fissure appears to be interrupted at the cerebellum; but in the higher classes, where the lateral portions of the cerebellum are more highly developed, as in the human subject (Fig. 126), they are also separated from each other posteriorly on the median line, and the longitudinal median fissure is complete throughout.

In birds, the hemispheres are of much larger size than in reptiles, and partially conceal the optic ganglia. The cerebellum, also, is very well developed in this class, and presents on its surface a number of transverse foldings or convolutions, by which the quantity of gray matter which it contains is considerably increased. The cerebellum here extends so far backward as almost completely to conceal the medulla oblongata and the fourth ventricle.

Fig. 129.

In the quadrupeds, the hemispheres and cerebellum attain a still greater size in proportion to the remaining parts of the brain. There are also two other pairs of ganglia, situated beneath the hemispheres, and between them and the tubercula quadrigemina. These are the corpora striata in front and the optic thalami behind. In Fig. 129 is shown the brain of the rabbit, with the hemispheres laid open and turned aside, so as to show the internal parts in their natural situation. The olfactory ganglia are seen in front (1) connected with the remaining parts by the olfactory commissures. The separation of the hemispheres (2, 2) shows the corpora striata (3) and the optic thalami (4). Then

come the tubercula quadrigemina 1. Olfactory ganglia. 2. Hemispheres, turned

(5), which are here composed, as

aside. 3. Corpora striata. 4. Optic thalami. 5. Tubercula quadrigemina 6. Cerebellum.

above mentioned, of four rounded masses, nearly similar in size. The cerebellum (6) is considerably enlarged by the development of its lateral portions, and shows an abundance of transverse convolutions. It conceals from view the fourth ventricle and most of the medulla oblongata.

In other species of quadrupeds the hemispheres increase in size so as to project entirely over the olfactory ganglia in front, and to cover in the tubercula quadrigemina and the cerebellum behind. The surface of the hemispheres also becomes covered with numerous convolutions, which are curvilinear and somewhat irregular in form and direction, instead of being transverse, like those of the cerebellum. In man, the development of the hemispheres reaches its highest point; so that they preponderate altogether in size over the rest of the ganglia constituting the brain. In the human brain, accordingly, when viewed from above downward, there is nothing to be seen but the convex surfaces of the hemispheres; and even in a posterior view, as seen in Fig. 126, they conceal everything but a portion of the cerebellum. All the remaining parts, however, exist even here, and have the same connections and relative situation as in other instances. They may be best studied in the following order.

As the spinal cord, in the human subject, passes upward into the cranial cavity, it enlarges into the medulla oblongata as already described. The medulla oblongata presents on each side three projections, two anterior and one posterior. The middle projections

on its anterior surface (Fig. 130, 1, 1), which are called the anterior pyramids, are the continuation of the anterior columns of the cord. They pass onward, underneath the transverse fibres of the pons Varolii, run upward to the corpora striata, pass through these bodies, and radiate upward and outward from their external surface, to terminate in the gray matter of the hemispheres. The projections immediately on the outside of the anterior pyramids, in the medulla oblongata, are the olivary bodies (2, 2). They contain in their interior a thin layer of gray matter folded upon itself, the functions and connections of which are but little understood, and are not, apparently, of very great importance.

The anterior columns of the cord present, at the lower part of the medulla oblongata, a remarkable interchange or crossing of their fibres (4). The fibres of the left anterior column pass across the median line at this spot, and becoming continuous with the right anterior pyramid, are finally distributed to the right side of the cerebrum; while the fibres of the right anterior column, passing over to the left anterior pyramid, are distributed to the left side of the cerebrum. This interchange or crossing of the nervous fibres is known as the decussation of the anterior columns of the cord.

The posterior columns of the cord, as they diverge on each side the fourth ventricle, form the posterior and lateral projections of the medulla oblongata (3, 3). They are sometimes called the "restiform bodies," and are extremely important parts of the brain. They consist in great measure of the longitudinal filaments of the posterior columns, which pass upward and outward, and are distributed partly to the gray matter of the cerebellum. The remainder then pass forward, underneath the tubercula quadrigemina, into and through the optic thalami; and radiating thence upward and outward, are distributed, like the continuation of the anterior columns, to the gray matter of the cerebrum. The restiform bodies, however, in passing upward to the cerebellum, are supplied with some fibres from the anterior columns of the cord, which, leaving the lower portion of the anterior pyramids, join the restiform bodies, and are distributed with them to the cerebellum. From this description it will be seen that both the cerebrum and the cerebellum are supplied with filaments from both the anterior and posterior columns of the cord.

In the substance of each restiform body, moreover, there is imbedded a ganglion which gives origin to the pneumogastric nerve, and presides over the functions of respiration. This ganglion is surrounded and covered by the longitudinal fibres passing upward from the cord to the cerebellum, but may be discovered by cutting into the substance of the restiform body, in which it is buried. It is the first important ganglion met with, in dissecting the brain from below upward.

While the anterior columns are passing beneath the pons Varolii, they form, together with the continuation of the posterior columns and the transverse fibres of the pons itself, a rounded prominence or tuberosity, which is known by the name of the tuber annulare. In the deeper portions of this protuberance there is situated, among the longitudinal fibres, another collection of gray matter, which,