THE PYRAMIDAL TRACT.

35

diate neighborhood, and, after forming part of the corona radiata (Fig. 9), enter the internal capsule, where they are found in the knee and in the anterior third of the posterior limb. In the crusta they occupy a position between the bundles mentioned under a and. (See Plate 68.) This portion of the ventral peduncular fibers emerges at the posterior border of the pons in a compact bundle, known as the pyramid, and continues its way down the spinal cord as the pyramidal tract. Most of the fibers,

as has already been stated, undergo decussation and occupy the lateral column, while the smaller uncrossed portion remains in the anterior column. This tract contains the longest fibers of the corona radiata, and can be followed in the lateral column of either side as far down as the conus medullaris. From the cerebral peduncle onward fibers constantly leave the main tract and pass to the motor nuclei of the opposite side. This tract forms the central motor pathway.

The position of the various tracts is well shown in the accompanying schema of the capsule (Fig. 10), modified from Edinger.

The fibers of the corona radiata, which, as we have seen, are derived from the cortex, are later joined by other fibers from the striate body and optic thalamus, which continue downward as short tracts. The latter include:

1. The fibers running from the caudate nuclei and putamen to the optic thalamus (inner segment of the lenticular nucleus) and further downward to the substantia nigra in the crus (basal forebrain bundle).

2. Fibers beginning in the optic thalamus and ending in the tegmentum and corpus albicans. (See Fig. 11.) These contain the more important short tracts of the interbrain.

The following fibers take their origin in the tegmentum: The posterior longitudinal bundle, situated immediately beneath the gray matter of the ventricle, which connects several cranial nerves and extends as far down as the cervical enlargement (see Fig. 11); a massive bundle of fibers which emerge from the red nucleus and, after decussating with a corresponding bundle from the opposite side, form the superior cerebellar peduncle, passing as such to the hemispheres of the cerebellum and ending in the corpus dentatum and cerebellar cortex.

In the cerebellum the following fibers are found:

The middle cerebellar peduncle, which passes to the pontine ganglia of the other side, where its fibers end, and the inferior cerebellar peduncle, going to the medulla oblongata (restiform body). These bundles contain fibers connecting the cerebellum with the olivary bodies and the posterior and lateral columns (direct cerebellar tract) of the spinal cord; some of these form long, and others short, tracts.

The white matter of the cerebellum also contains commissural and association fibers similar to those found in the cerebrum.

SHORT TRACTS IN BRAIN-STEM AND INTERBRAIN. 37

For the manner in which the white matter of the spinal cord is constructed from the projection fibers of the cere

Fig. 11. The short tracts in the brain-stem and interbrain. Sagit

tal section through the middle of the brain-stem: inf, Infundibulum; c. mam, corpus mammillare; fasc. Vic, bundles of Vicq d'Azyr; f. teg, tegmental bundle; ped. m, peduncle of corpus mammillare; fase. rt, fasciculus retroflexus; fase. l. inf, posterior longitudinal bundle; II, III, VI, XII, corresponding cranial nerves; c, pineal gland.

bral and cerebellar cortex, the pyramids, restiform bodies, etc., consult a former paragraph (p. 15).

In a section of the spinal cord we see the two halves separated in front by the anterior longitudinal fissure. Immediately behind this, in the median line, is the white anterior commissure, connecting one anterior column or, rather, anterior horn, with that of the other side. Ventrally from this is the central canal, which is for the most part obliterated, and behind the latter, the narrow posterior commissure.

The gray matter forms the anterior and posterior horns, and is surrounded by the anterior, lateral, and posterior columns of the cord.

1. The anterior column contains the uncrossed pyramidal tract and the anterior ground bundle (continuations of the constituent parts of the substantia reticularis in the tegmentum).

2. The lateral column contains the crossed pyramidal tract, the direct cerebellar tract (from the restiform body), the tract of Gowers (from the vermiform process of the cerebellum), the anterolateral ground bundle, and the mixed lateral zone (partly derived from the tegmentum). 3. The posterior column contains the columns of Goll and Burdach and the root-zones.

In addition to the fibers derived from the brain the spinal cord contains numerous fibers originating within its own substance (myelogenic, intramedullary fibers), and also fibers which come from the periphery (extraspinal fibers).

A more detailed description of the course and significance of the pathways that have been mentioned and of others will be found in Part III. The foregoing, it is hoped, will serve to explain the serial sections contained in Plates 28 to 52, which are intended as an introduction to the study of the topographic anatomy of the brain. The more complex relations between the gray matter of the cortex, basal ganglia, optic thalamus, and nuclei of the cranial nerves can only be studied from the illustrations, and the reader is therefore referred to the plates and to the explanatory notes which accompany them.

PART III.

ANATOMY AND PHYSIOLOGY OF THE MORE IMPORTANT NERVOUS PATHWAYS.

(Plates 54 to 57.)

WE understand by nervous pathway the anatomic basis for the transmission of a definite physiologic impulse from the ganglion cell to the end-organ which receives the impulse, including all the connections.

Every physiologic function requires for its completion a nervous pathway which is composed of a number of communicating neuron complexes, each individual neuron complex completely retaining its physiologic and anatomic independence. Thus there are pathways which consist of two, three, or even more successive neuron complexes. The more important of these belong to the projection system of the cerebral and cerebellar cortex. They are the following:

Nerve Tracts Associated with Known Functions.

1. The motor, corticomuscular, or centrifugal tract, consisting of two neuron complexes.

2. The sensory, centripetal tract, consisting of at least three, and probably more, neuron complexes.

These two tracts communicate with each other in two different divisions of their course :

(a) They have a lower connection, which forms the reflex tract and is not under the influence of the will. It is