and

Fig. 147.

complete symmetry of arrangement. The internal abdominal organs are in great measure unsymmetrical, and the corresponding nerves and nervous centres of the ganglionic system present the same want of But while the ganglionic regularity in their locality and distribution. system presides over the internal organs functions of nutrition, the cerebro-spinal system, on the other hand, is connected with the apparatus of animal life, namely, the organs of sensation and movement by which the living body is brought into relation with the exterior. As these organs, in man and the vertebrate animals, are symmetrically arranged, the cerebro-spinal nervous system presents the same character. Both the brain and the spinal cord are composed of two, right and left, lateral halves; each one of which furnishes the nerves of sensation and motion to the corresponding sides of the body.

Another striking peculiarity of this part of the nervous system is the mutual decussation of the nerve fibres belonging to its two sides. Both the brain and spinal cord have their right and left halves connected by fibres which pass across the median line from one to the other; the different bundles being often interwoven with each other, at the point of transit, in a somewhat complicated manner. This peculiarity extends to the origins of the nerves, which decussate with each other internally; so that the nerve fibres emerging from the right side of the cerebro-spinal mass have their origin from the gray substance of the left lateral half, and those emerging from the left side originate from the gray substance of the right lateral half. The only uncertainty in this respect is whether the decussation be complete or partial; that is, whether all the fibres of a given nerve root be connected with the opposite side of the central mass, or whether a part of them originate from the same and a part from the opposite side. The decussating fibres, in a large number of instances, are anatomically demonstrated. In some remaining exceptions their course is more or less a matter of doubt.

The Spinal Cord is a nearly cylindrical nervous mass, inclosed in the cavity of the spinal canal, commencing by a slightly enlarged extremity

at the brain above, and terminating below in a conical point at the leve! of the first lumbar vertebra. Its inner portions are occupied by gray substance, which forms a continuous chain of ganglionic matter, running from one extremity of the cord to the other. Its outer portions

are composed of white substance, the fibres of which run mainly in a longitudinal direction, connecting its different parts with each other, and forming a communication between it and the brain.

The spinal nerves are given off from the cord at regular intervals and in symmetrical pairs; one pair for each successive portion of the body, their branches being distributed to the integument and muscles of the corresponding regions. In fish and serpents, where locomotion is performed by means of simple, alternating, lateral movements of the spinal column, the cord is nearly or quite uniform in size, or tapers gradually from its anterior to its posterior extremity. But in the other vertebrate classes, where the body is provided with special organs of locomotion as fore and hind limbs, or wings and legs, the cord is increased in size where the nerves of these organs are given off; and the nerves supplying the limbs are larger than those which originate from other parts of the cord. In man, the lower cervical nerves, which form the brachial plexus and supply the arms, and the sacral nerves forming the sacral plexus, which supplies the legs, are larger than those given off in the upper cervical, dorsal, and lumbar regions. The cord itself, furthermore, presents two marked enlargements corresponding with the points of origin of these nerves, namely, the cervical enlargement, which is the source of the nerves for the upper extremity, and the lumbar enlargement, which gives off the nerves destined for the lower extremity.

A transverse section of the spinal cord shows that it is incompletely divided into right and left lateral halves by an anterior and posterior

as the "gray commissure," in the centre of which is a narrow longi. tudinal canal, the "central canal," but little over 0.2 millimetre in diameter, and lined internally with epithelium.

The anterior and posterior portions of gray substance, in each lateral half of the cord, are called the anterior and posterior horns. Immediately in front of the gray commissure is a transverse band of white substance, the "white commissure" of the cord.

The spinal nerves originate from the cord on each side by two distinct sets of fibres, forming the anterior and posterior roots. The anterior root (Fig. 148, d) passes out from the surface of the cord near the extremity of the anterior horn of gray matter. The posterior root (e) originates at a point corresponding with the posterior horn of gray matter. Both roots are composed of a considerable number of fibres, united with each other in parallel bundles. The posterior root is distinguished from the anterior by the presence of a small rounded mass of gray matter, or ganglion, with which it is incorporated and through which its fibres pass. The two roots unite with each other soon after leaving the cavity of the spinal canal, and mingle their fibres in a common trunk.

The white substance of each lateral half of the spinal cord is thus divided into three portions or "columns;" so called because the nerve fibres composing them run, for the most part, parallel with each other, in a longitudinal or vertical direction. The portion which is included between the anterior median fissure and the origin of the anterior nerve roots is the anterior column; that between the anterior and posterior nerve roots is the lateral column; while that between the posterior nerve roots and the posterior median fissure is the posterior column. As the posterior median fissure penetrates deeply into the substance of the cord, quite down to the gray substance, the posterior columns appear entirely separated from each other in a transverse section; but 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.

The brain, or "encephalon," is that portion of the cerebro-spinal system contained in the cranial cavity. It forms a more or less rounded mass of nervous matter, consisting, as in the spinal cord, of right and left lateral halves which remain connected with each other by their central parts. In man and the higher vertebrate animals, it presents, above and behind, two principal divisions, namely, the cerebrum and cerebellum, which are composed externally of a convoluted layer of gray substance, these two divisions together forming at least nineteentwentieths of the whole encephalon; while beneath them is a smaller portion composed externally of white substance, like the spinal cord, and forming the communication between the cord below and the brain above. This inferior portion is called the "isthmus," and comprehends the medulla oblongata, the tuber annulare, and the peduncles of the cere brum. Beside, however, the portion visible externally, there are, in

each of these divisions, various deep-seated deposits of gray substance, which are concealed by the overlying parts.

The construction of the brain, as a whole, may therefore be represented by considering it as a double series of nervous centres or deposits of gray substance, of varying size and position, connected with each other and with the spinal cord by transverse and longitudinal tracts of white substance. The number and relative importance of these nervous centres, in different kinds of animals, depend upon the perfection of the bodily organization in general, and more especially upon the develop ment of the functions and capacities connected with particular parts of the nervous system. In the inferior classes, as fish and reptiles, the brain is more simple in its anatomical characters; while it becomes successively more complicated in birds, quadrupeds, and man.

In fish and reptiles the different nervous centres of the brain are so distinctly separated, and of such moderate size, that they are frequently designated as "ganglia." In the brain of the alligator (Fig. 149) there are five pairs of these ganglia, arranged one behind the other in the cavity of the cranium. The first of these are two rounded masses (1),

Fig. 149.

lying just above and behind the nasal cavities, which distribute their nerves upon the olfactory 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 (,) are somewhat larger and of a triangular shape, when viewed from above downward. They are termed the 66 hemispherical ganglia," or the hemispheres, and correspond to the "cerebrum" in the higher classes. Immediately following them are two quadrangular masses (3) which give origin to the optic nerves, and are therefore called 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 is a single tr angular collection of nervous matter (,), 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 (,), 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 termed the "pneumogastric" or "respiratory" ganglia.

It will be seen that the posterior columns of the cord, as they diverge laterally, 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.

Fig. 150.

The successive 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 spinal cord. In the brain of the alligator this fissure appears to be interrupted at the cerebellum; but this is due to the incomplete development of the lateral portions of this organ, as coinpared with its middle. The same peculiarity is to be seen in birds and in most quadrupeds; while in man the lateral portions of the cerebellum are so highly developed as to project, on each side, above the level of its central part, and the longitudinal median fissure, accordingly, appears complete throughout.

2

BRAIN OF PIGEON-Profile view.1. Cerebrum. 2. Optic tubercle, 3. Cerebellum. 4. Optic nerve. 5. Medulla oblongata.

In birds the hemispheres, or cerebrum, are of comparatively larger size, and partially or completely conceal the optic tubercles in a view taken from above. The cerebellum is well developed in this class, and presents on its surface a number of transverse foldings or convolutions by which its gray substance is considerably increased in quantity. The cerebellum extends so far backward as to completely cover the medulla oblongata and the fourth ventricle.

In quadrupeds the cerebrum and the cerebellum attain a still greater size as compared with the remaining parts of the brain. There are also two other collections of gray substance on each side, situated in the inferior part of the hemispheres, anteriorly to the tubercula quadrigemina. These are the "corpora striata" in front, and the "optic thalami" behind. These bodies are frequently designated by the name of the "cerebral ganglia," since they are collections of gray matter which occupy the lower and central parts of the cerebrum, and intervene between the tracts of white

Fig. 151.

BRAIN OF RABBIT, viewed from above.-1. Olfactory ganglia. 2. Hemispheres of the cerebrum, turned aside. 3 Corpora striata.

4. Optic thalami 5. Tubercula quadrigemina. 6. Cerebel lum.