mentum cruris) are connected chiefly with the optic thalamus or the corpus quadrigeminum.

Physiologically, the basis cruris or crusta (Fig. 14) may be regarded as a centrifugal or motor tract, and the tegmentum cruris (Fig. 14) as a centripetal or sensory tract. This statement is not absolutely correct, but it is practically advisable to so regard it.

FIG. 14.-Diagram of the course of sensory and motor tracts in the mesocephalon and

hemispheres. (Seguin.)

S, sensory tract in posterior region of mesocephalon, extending to O and T, occipital and temporal lobes of hemispheres; M, motor tract in basis cruris, extending to P and F, parietal and (part of) frontal lobes of hemispheres; C. Q., corpus quadrigeminum; O. T., optic thalamus; N. L., nucleus lenticularis; N. C., nucleus caudatus ; 1, the fibers forming the "tegmentum cruris " (Meynert); 2, the fibers forming the "basis cruris " (Meynert).

In studying the brains of mammals, these two bundles of fibers and the ganglia connected with them give evidence of an independence of one another which governs the development of each. Where the frontal and parietal lobes are large, we find the "basis cruris" and the two nuclei of the " corpus striatum" (Figs. 8 and 14) highly developed; on the other hand, when these lobes are at their minimum we find the "tegmentum cruris" and its ganglia developed in excess.

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There is also physiological evidence to sustain the opinion that the basal ganglia and the two bundles of the crus are capable in themselves of executing, in response to excitation from without, all varieties of movements in an animal deprived of its cerebral lobes (above the level of the basal ganglia) with a nicety and exactness which are astonishing.

The "crus cerebri" suffers a diminution in the fibers of its motor bundle (basis cruris) after its entrance into the substance of the pons Varolii. This is very apparent when the large size of the tract, before its entrance into the pons, is contrasted with the small anterior pyramid of the medulla oblongata, which is its direct continuation after its exit (Fig. 6). The explanation of this fact is as follows: All of the

FIG. 15. A diagram of the brain in transverse vertical section. (Dalton.) 1, crus cerebri; 2, internal capsule; 3, optic thalamus; 4, caudate nucleus of corpus striatum; C. C., corpus callosum; L. N., lenticular nucleus of corpus striatum; S, fissure of Sylvius; Fo, gyrus fornicatus; F', first frontal convolution; F", second frontal convolution; F", third frontal convolution; T', first temporal convolution; T', second temporal convolution; T'", third temporal convolution; H, gyrus hippocampi.

peduncular fibers of the cerebrum, which become intermingled with the gray matter of the corpus striatum (the caudate and lenticular nuclei), and which escape from that ganglion as fibers of the basis cruris (Figs. 6 and 14), are not destined to form parts of the projection system.

The ganglion-cells of the pons Varolii exercise, in the case of some special cerebral fibers,' the switch-like action previously referred to, and deflect the impulses, which they carry, to the opposite hemisphere of the cerebellum; hence, in the pons, quite a large bundle of distinct fibers appear to leave the direct tract of the basis cruris (Fig. 14) and pass to the cerebellum (through the processus e cerebello ad pontem). We have come to learn that a communication between the cerebral cortex and that ganglion is thus established, but its physiological function is not yet ascertained with scientific exactness. This fact, in addition to others which will be brought forward later, leads to the conclusion that the cerebellum is, in some imperfectly understood way, brought into direct relation with the motor tract of the projection system of the cerebrum, and is endowed with some power either of control of or subtle influence over motor impulses."

If we examine cross-sections of the "pons Varolii” and "crura," we shall perceive that the pons performs for the cerebellum an office analogous to that which the corpus callosum performs for the cerebral hemispheres-the transmission of commissural fibers which possibly connect homologous portions of the two lobes, although they seem to become united with the cells of the gray substance of the pons. We may note, furthermore, that these commissural fibers of the pons subdivide the fibers of the basis cruris and tegmentum cruris into smaller bundles or fasciculi. In addition, nodal masses of gray matter may be detected in both the crus and pons.

It is reasonable, therefore, to conclude that the cells of these nodal masses of gray substance establish some form of

1 These fibers are chiefly grouped during their passage through the lower part of the cerebral hemisphere within the anterior half of the internal capsule. The fibers which arise from the cerebral cortex and apparently terminate in the gray matter of the pons, scem to spring in part from the frontal lobe and in part from the parietal and temporosphenoidal lobes. The frontal fibers pass through the anterior half of the internal capsule, and, after their escape from the cerebrum, occupy the inner one third of the basis cruris. The fibers from the parietal and temporo-sphenoidal lobes pass through the posterior half of the internal capsule, and occupy (after their escape from the cerebrum) the outer one third of the basis cruris.

2 This subject will be discussed in connection with the architecture of the cerebellum.

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41

communication between the fibers of the cerebral projection tracts and the commissural fibers of the cerebellum, independent of the fibers of the basis cruris which appear to deflect themselves from the path of the projection system into its substance.

The cerebellum, furthermore, has undoubted association with special fibers of the cerebrum (which are prolonged, subsequently, into the basis and tegmentum cruris) by means of two of its prolongations, viz., the processus e cerebello ad testes and the valve of Vieussens. The multiplicity of connections which this ganglion has with fibers of the projection system leaves its probable functions a matter of speculation. The theories advanced will merit consideration later in the course.

Finally, the GANGLIA of the brain have intimate relation with certain nerve-tracts which are independent of the projection system proper-viz., the fibers of special cranial nerves, which are more or less independent of the tubular gray

matter.

The olfactory, optic, and auditory apparatuses must be considered, therefore, as modified types of projection systems, which bear, however, striking analogies to the projection system extending to nerves of spinal origin, although possessing peculiarities of structure essentially their own (Fig. 12). In these modifications of the general arrangement, the middle projection fibers appear, at a first glance, to be wanting, as there is with some cranial nerves, as far as we at present know, no organ which corresponds exactly with the central gray tube of Meynert's projection system. Many observers, however, incline to the view that the peripheral ganglion-cells are analogous to the tubular gray matter. These consider, for example, the fibers of the optic tract as a middle system of projection, and the radiating fibers in the retina as the external system of projection.

The projection tracts of the crus are prolonged into the medulla oblongata and spinal cord (Fig. 8), where they become more or less intimately associated with the tubular gray

matter.

The third member of the projection system exhibits an augmentation in the actual number of fibers over those found in the crus; as there can be no doubt that the total number of fibers in the spinal nerves exceed greatly those comprised in the basis and tegmentum cruris. Here, again, we have undisputable evidence that the gray matter of the spinal cord, by means of its cell elements, serves as a means of conduction of nerve impulses, and also as a point of origin for additional nerves, whenever demanded.

The motor tracts of the basis cruris become joined to cells in the gray matter of the spinal cord, which are connected with the anterior or motor roots of the spinal nerves (see Fig. 8). The fibers of the tegmentum cruris unite with similar cells which lie more posteriorly, and are associated with the posterior or sensory roots of the cranial and spinal nerves (see Fig. 8). The individual course of the various bundles (that help to form the motor and sensory tracts of the crus cerebri) through the medulla and spinal cord will be described in subsequent pages.

It may be well, however, to state in general terms that each separate nerve-fiber which properly belongs to the projection tracts of the crus finds its course interrupted by the interpolation of a ganglion-cell before it reaches the particular spinal nerve, with the action of which it is to become intimately associated. The nerve-cells of the spinal cord help to explain the various phenomena which are comprised under the head of spinal automatism; since, in the beheaded animal, no other source of reflex motor action can be discovered, although its existence has been demonstrated beyond a doubt, both in animals (Pflüger) and even in man (Robin). By the interpolation of nerve-cells in the course of nerve-fibers, sensory impressions may be carried to any one of the three main divisions of gray matter, and there excite a response in the form of a motor impulse, viz., the tubular gray substance and its expansions, the basal ganglia, or the cortex of the cerebrum. These points will be discussed in subsequent

pages.