GRAY MATTER OF THE BRAIN.

23

If we study transparent sections of the brains of small mammals, where the different portions are more clearly defined than in animals of a higher type, and where a low

FIG. 6.-Plan in outline of the brain in profile. (Quain.)

The cerebrum is represented in this diagram as separated from the cerebellum more than it naturally should be, in order to show certain important parts. A, the cerebrum; B, the cerebellum; C, the pons Varolii; D, the medulla oblongata; E, the crus cerebri; F, the olivary body; G, the tubercula quadrigemina; S, the fissure of Sylvius; R, the fissure of Rolando; a, peduncles of the cerebrum; b, superior peduncles of the cerebellum; e, middle peduncle of the cerebellum; d, inferior peduncles of the cerebellum; b, E, a, form the isthmus encephali. The convolutions of the cerebrum are not correctly drawn in this cut.

magnifying power reveals the general course of the nervefibers as well as the arrangement of the masses of gray matter, we are enabled to grasp some general features of construction which are applicable to the brain of man.

THE GRAY MATTER OF THE BRAIN.

We find, in the first place, that the nerve-cells occur in large and distinctly isolated masses, which may be thus enumerated:

1. The gray matter of the exterior of the cerebrum (the cortex cerebri) that invests the convolutions. This collection of cells, as an undulating layer folded constantly upon itself, forms a layer, marked by eminences and depressions whose sole object is to obtain an increase of the brain's surface over that of the interior of the cranium.

2. Buried within the substance of each hemisphere of the cerebrum may be found two nodal masses of cells, which rest nearly upon the level of the floor of the cerebrum, and which are named, from their contiguity to this plane, the "basal ganglia."

RTEX OF FRONTAL LL

CORTEX OF PARIETAL LOBE

CORTEX OF OCCIPITAL LOBE

FIG. 7.-A diagram designed by the author to elucidate the chief component parts of

the human brain.

The lettering upon the figure will be explained in the text. C. Q., the corpora quadragemina, or "optic lobes." The lines within the white substance of the cerebrum or in the "crus" are not intended to convey any impression to the reader of the actual arrangement of the fibers.

Each anterior mass is called the "corpus striatum," from the striated appearance of a section made through its substance.'

Each posterior mass is called the "optic thalamus," from a supposed association with vision attributed to it by early investigators.

1 This ganglion has two parts (as shown in Fig. 8)-the caudate nucleus, and lenticular nucleus. These parts are separated by a bundle of nerve-fibers, the so-called "internal capsule" of the cerebrum.

FIG. 8.-A diagram designed by the author to show the general arrangement of the fibers of the cerebro-spinal system. (Modified from Landois.)

The shaded portions represent the collections of gray matter. On the left side of the diagram, the sensory fibers of the crus are traced upward from the spinal cord to different portions of the cerebrum; on the right side, the motor fibers are similarly represented. Numerals are used in designating the sensory and commissural fibers; the motor fibers are lettered in small type. The cortical layer is shown at the periphery of the cerebral section, with commissural fibers (1) connecting homologous regions of the hemispheres, and associating fibers (a. s.) connecting different convolutions of each hemisphere. C. N., caudate nucleus of the CORPUS STRIATUM; L. N., lenticular nucleus of the same; O. T., OPTIC THALAMUS of each hemisphere, united to its fellow in the median line; c. q., CORPORA QUADRIGEMINA; e. l., CLAUSTRUM, lying to the right of the letters; c. c., CORPUS CALLOSUM, with its commissural fibers; S, FIS

SURE OF SYLVIUS; V, LATERAL VENTRICLE, the fifth ventricle being shown between the two layers of the septum lucidum; C, the motor tract of the CRUS CEREBRI (basis cruris-crusta); T, the sensory tract of the CRUS CEREBRI (tegmentum cruris); Cf, the cerebellar fasciculus, which is turned to the right for perspicuity, but which in reality decussates; e, the point of decussation of the motor fibers of the spinal cord;, the course of the motor fibers of the spinal cord below the medulla, showing their connection with the cells of the anterior horns of the gray matter, and their continuation into the anterior roots of the spinal nerves (g); a, fibers which radiate through the caudate nucleus; b, fibers of the "internal capsule"; c, fibers which radiate through the lenticular nucleus; d, fibers of the "external capsule "; 2, 3, 4, 5, 6, 7, 8, 9, sensory fibers radiating from the tegmentum cruris to the cortex by means of various nodal masses of gray matter; 11, course of the sensory fibers of the spinal cord (shown by dotted lines), intimately connected with the posterior root of the spinal nerve (12), and decussating at or near the point of entrance into the spinal cord. This diagram may be studied in connection with Figs. 7, 9, 10, 11, and 13, with possible benefit to the general reader.

3. The cerebellum presents collections of gray matter which occur partly in layers (cerebellar cortex) and partly as scattered masses within its substance.

4. The so-called "tubular gray matter" (which may be traced as a lining to the inner portions of the cerebro-spinal axis' from the tuber cinereum to the conus medullaris of the spinal cord) must be recognized as the "permanent expression of the primitive and generic type of brain."

5. Distinct groups of cells are found within the substance of the crus cerebri, the pons Varolii, and the medulla. These will be discussed later.

The diagram (Fig. 8) will make some of these subdivisions of the gray matter of the cerebrum, as well as the fibers which connect them, more apparent than a long verbal description.

It will be seen that the gray matter of the cortex is arranged like a cap to the brain, and embraces the "basal ganglia," the "claustrum," and "corpora quadrigemina intermediate portions being left which in the brain itself appear as a white, cheesy mass. These are filled in with lines in the diagram. They indicate the different sets or "systems" of nerve-fibers, as the microscope in the hands of late observers has shown them to exist, and of which this socalled "white substance" of the brain is chiefly composed. [A careful study of the text accompanying Figs. 8, 9, and 10

1 The "cerebro-spinal axis" is a term used to include the brain and the spinal cord collectively.

WHITE MATTER OF THE CEREBRUM.

27

will enable the reader to grasp the general direction and terminations of the sets of fibers described.]

FIG. 9.-Diagram of the commissural fibers of the anterior region of the brain.

(After Luys.)

These form a series of curves one within another, the extremities of each of which plunge into the homologous regions of each cerebral lobe, 1, 1', 2, 2', 3 and 3'. They pass through the middle line, and at 4 and 4' give rise to the various appearances which the " corpus callosum " presents. 5, Commissural fibers of the inferior regions. These are curved in an inverse direction as regards the former, the convexity of each set being presented toward that of the other.

THE WHITE MATTER OF THE CEREBRUM.

The FIRST SET OF FIBERS (1 in Fig. 8) will be seen to spring from the cortex, and, after taking a direction which tends to bring them to the level of the superior point of union of the two cerebral hemispheres, to cross over to the side opposite to that from which they arise. After crossing, they can be traced to homologous regions of the cortex of the opposite hemisphere. These are commonly called "commissural fibers." They are supposed to be the connecting wires between corresponding portions of the cortex of the cerebral lobes, by the aid of which the right and left hemispheres can act in unison with each other when circumstances chance to demand it.

These fibers have a direction which corresponds as a rule to the form of the letter U, and they constitute the transverse fibers of the "corpus callosum "-the connecting band of white matter between the hemispheres, seen at the bottom