spinal cord may be gathered by considering that a bifurcation of the fibres takes place in the medulla oblongata, and upon one of the resulting bundles, the crus cerebri, the cerebrum is formed, on the other the cerebellum. The crus cerebri is composed of three strands: an inferior, the fibres of which have come from the anterior pyramids, and in part from the olivary bodies. This strand ends in the corpus striatum, its fibres not, however, blending abruptly with the vesicular matter, but passing into it in bundles. It is essentially motor. A superior, which is derived from the posterior pyramids, and terminates in the thalamus. It is essentially sensory. Between these, constituting the third portion— strand it can scarcely with propriety be called-is a layer of dark vesicular material, the locus niger. It is to be understood that the motor strands of the opposite sides decussate in the medulla oblongata; the sensory strands decussate in the mesocephalon.

The other bundle, arising in the original bifurcation, assumes the desFormation of ignation of crus cerebelli. On it the cerebellum is develthe cerebellum. oped. It consists essentially of fibres from the restiform bodies, re-enforced by others which have come from the anterior pyramids under the name of arciform fibres. These together make their way to the interior ganglion of the cerebellum, the corpus dentatum, and there they end. But the crus cerebelli contains likewise two other great strands an inferior, which constitutes the commissure of the two cerebellar hemispheres, and which, running round the entire prolongations of the spinal cord, forms the pons varolii; a superior, the processus cerebelli ad testes, which unites the cerebellum and cerebrum.

Of the portions of the spinal cord on which the cerebrum is to be developed, those which are sensory end in the optic thalamus, those which are motor in the corpus striatum. The thalamus and striatum of each side may be regarded as one compound ganglion, since, like the columns of the cord, they are united by a gray and a white commissure. Of the portions on which the cerebellum is to be developed, the termination is in the central ganglion of the cerebellum, the corpus dentatum. At the place of bifurcation of the constituent strands of the crus cerebri and crus cerebelli from each other in the medulla oblongata, there is intercalated or included a ganglion, which, with its apparatus, constitutes the olivary body, the fibres of which make their way upward between the two preceding bundles, and, having bifurcated, one branch goes to the quadrigemina and the other to the optic thalamus, the latter constituting, as has been said, a part of the crus cerebri. The seat of power of the medulla oblongata is in this ganglion.

Olivary body.

Such being the anatomical construction of the crus cerebri, it may be physiologically regarded as a compound strand, the anterior portion of

THE BRAIN.

sory strands re

315 which is motor, the posterior sensory; and between these a Nerves of the dark vesicular deposit, the locus niger, which is continuous motor and senbetween the vesicular matter in the spinal cord and that of spectively. the thalamus and corpus striatum. From the lowest extremity of the cord to these great ganglia there is, therefore, an unbroken vesicular channel. In its progress onward to the corpus striatum, the anterior strand yields roots of the spinal accessory, hypoglossal, facial, abducens, the small root of the fifth, the trochlearis, and the oculo-motor nerves. If there were no other proof of the motor character of this strand, the motor property of all these nerves would be sufficient to determine it. In like manner, the posterior strand yields the pneumogastric, the glossopharyngeal, and the sensory root of the fifth, from the sensory functions of which its sensory character is established.

ter of the hem

The layer of vesicular matter which is found upon the cerebral convolutions, and which is doubtless the seat of the higher intel- Relation of the lectual qualities, has therefore no communication with the vesicular matvesicular matter of the spinal axis, by contact or continua- ispheres. tion, but only through the intervention of fibres which radiate upon it in all directions from the thalamus and striatum, or rather through some which radiate from the great sensory centre, the thalamus, to the periphery of the cerebrum, and others which converge from that periphery to the great motor centre, the striatum. If the diameter of these fibres be assumed to be 1000 of an inch, there must be many millions of them in the aggregate. The vesicular matter of the hemisphere is arranged on the superficies instead of centrally, on account of the necessities of their structure and condition of activity, for thereby a great surface is obtained, which is further increased by the artifice of convolutions, a vesicular surface which, counting in that of the cerebellum, has been estimated at 670 square inches, and blood can be copiously supplied and freely removed.

brain.

But the thalamus and striatum are only two of a chain of ganglia beneath the cerebral hemispheres. Anteriorly we find the ol- Ganglia at the factive ganglia, or bulbs of the olfactory nerves, which are base of the seated upon peduncles, though their character is manifest from the gray matter they contain. Behind these are the tubercula quadrigemina, to which the optic nerves run, and which are therefore their ganglionic centres. What answers to the auditory ganglion is lodged at a distance back, at the fourth ventricle, and the gustatory ganglion is in the medulla oblongata. These are the ganglia of special sense, and to be regarded as subordinate to the thalamus, which is their common register.

All these parts are commissured with one another, and with their fellows of the opposite half of the brain. Indeed, so likewise are all its

316

the brain.

THE BRAIN.

Commissures of parts, the different cerebral lobes, the opposite hemispheres, adjacent and distant convolutions, the cerebrum with the cerebellum. Hence arises a structure of extreme complexity. Among the commissural apparatus may be more particularly mentioned the corpus callosum, the fornix, the anterior, the posterior, the soft, and the superior longitudinal commissures.

brain.

For the sake of a clear conception of the structure of the brain, so far Aspects of the as is required for physiological purposes, the annexed representations of its superficial aspects are given. These are a preparation for the diagrammatic sketches which follow, and which enable us to understand the relation and dependence of the more prominent parts. It need scarcely be added that the uses and functions of nearly all the subordinate parts are at present wholly unknown. For the time being, they are therefore objects of interest to the anatomist rather than to the physiologist.

Fig. 154, external lateral face of the right half of the brain: 1, medulla oblongata; 2, pons varolii; 3, cerebellum; 4, pneumogastric lobule; 5, frontal convolutions; 6, parietal convolutions; 7, occipital convolutions; 8, fissure of Sylvius; 9, 9, its two branches.

9

Fig. 155, superior aspect of the brain: 1, 1, anterior lobes; 2, 2, posterior lobes; 3, 3, great median fissure; 4, 4, fissures of Rolando; 5, 5, anterior parietal convolutions; 6, 6, posterior parietal convolutions; 7, 7, rudimentary parietal convolutions; 8, 8, frontal convolutions; 9, 9, occipital convolutions.

Fig. 156, internal lateral face of the right half of the brain: 1, half of medulla oblongata; 2, half of pons varolii; 3, half of crus cerebri; 4, arbor vitæ of cerebellum; 5, aqueduct of Sylvius; 6, half of the valve of Vieussens; 7, two of the tubercula quadrigemina; 8, half of the pineal gland; 9, its inferior peduncle; 10, its anterior peduncle; 11, transverse portion of the fissure of Bichat: 12, superior face of the optic tract;

THE BRAIN.

Fig. 157.

34

11

317

13, its internal face; 14, commissura mollis; 15, infundibulum; 16, portion of pituitary gland; 17, portion of tuber cinereum; 18, pisiform tubercle; 19, locus perforatus; 20, oculo-motor nerve; 21, portion of optic nerve; 22, anterior cerebral commissure; 23, foramen of Monroe; 24, fornix; 25, septum lucidum; 26, corpus callosum; 27, splenium; 28, genu; 29, sinus of the corpus callosum; 30, gyrus fornicatus; 31, internal convoluanfractuosity; 33, convolution of pos

Fig. 157, base of the brain, photographed from a wax cast: 1, 1, anterior lobes; 2,2, middle lobes; 3, 3, posterior lobes; 4, anterior portion of great median fissure; 5, its posterior portion; 6, 6, fissures of Sylvius; 7, 7, anteroposterior portions of the great fissure of Bichat; 8, tuber cinereum; 9, 9, corpora albicantia; 10, locus perforatus medius; 11, 11, crura cerebri; 12, pons varolii; 13, medulla oblongata; 14, 14, anterior pyramids; 15, 15, olivary bodies; 16, 16, restiform bodies; 17, 17, lateral lobes of the cerebellum; 18, portion of its middle lobe; 19, 19, two small antero-posterior convolutions of the frontal lobe, separated by the groove of the olfactory nerve; 20, oblique convolution, limiting the fissure of Sylvius; 21, convolution of the great cerebral fissure; 22, olfactory nerve; 23, its bulb; 24, 24, optic nerves and their chiasm; 25, 25, oculo-motor nerves; 26, 26, pathetici; 27, 27, great and small roots of the trifacial; 28, 28, external oculo-motor nerves; 29, 29, facial nerves; 30, 30, auditory; 31, 31, glosso-pharyngeal; 32, 32, pneumogastric nerves; 33, 33, spinal accessory; 34, 34, great hypoglossal. In this engraving several of the symmetrical numbers are not repeated, for the sake of clearness.

Fig. 158 is an analytical diagram of the brain in a vertical section (from Mayo). It serves to impress on the mind the foregoing Structure of structural descriptions. 8, Spinal cord preparing for bifurca- the brain.

tion; r, restiform bodies passing to c, the cerebellum; d, corpus dentatum of the cerebellum; o, intercalation of the olivary body; f, columns continuous with the olivary bodies and central part of the medulla oblongata, and ascending to the tubercula quadrigemina and optic thalami ; p, anterior pyramids : v, pons varolii; n, b, tubercula quadrigemina; 9, geniculate body of the optic thalamus; t, processus cerebelli ad testes: a, anterior lobe of the brain; 7, posterior lobe of the brain.

Fig. 159, the motor tract (from Sir C. Bell). A, A, fibres of the hemispheres converging to form the anterior portion of the crus cerebri; B, the same tract when passing the crus cerebri; C, the right pyramidal body, a little above the point of decussation; D, the remaining part of the pons varolii, a portion having been dissected off to expose B. 1, olfactory nerve in outline; 2, union of optic nerves; 3, 3, motor oculi; 4, 4, patheticus; 5, 5, trigeminus; 6, 6, its muscular division; 7, 7, its sensory root; 8, origin of sensory root from the posterior part of the medulla oblongata; 9, abducens oculi; 10, auditory nerve; 11, facial nerve;