The bases pedunculi (Figs. 57, 58, 59 and 6o) are two rounded bands of medullated fibers, limited by the inter peduncular fossa and lateral sulcus of the mid-brain. Each basis pedunculi is 13 mm. broad and is distinctly striated longitudinally. It issues from the under surface of the fore brain at the junction of the hemisphere with the thalamus and, trending toward the median line, descends to the pons. At its supperior end it is continuous with the efferent tracts of the internal capsule. Four efferent tracts make up the basis pedunculi, viz., the intermediate, the temporo-pontal, the pyramidal, and the fronto-pontal.
The deep portion of each basis pedunculi (Fig. 59) is occupied by the intermediate path which rises in the corpus striatum, is relayed in substantia nigra and terminates in the nucleus pontis (Flechsig). The intermediate path is made up of two tracts, the strio-nigral and the nigro-pontal tract. The super ficial part of the basis is made up of three tracts.
1. The lateral fifth of each basis contains the temporo pontal tract (tractus temporo-pontalis). It is composed of neurones which reach from the temporal lobe to the pons. The temporo-pontal tract rises in the superior, middle and inferior temporal gyri (Dejerine) and, perhaps, in parts of the occipital lobe (Zacher) and the parietal lobe (Sioli); it terminates chiefly in the nucleus of the pons, a few fibers going to the motor nuclei of cranial nerves (Spitzka). The temporo-pontal tract is an efferent one, but is not voluntary motor. Its fibers are medullated later than the pyramidal tract (Flechsig).
2. The middle three-fifths of the basis pedunculi (Figs. 59 and 6o) is occupied by the pyramidal tract (fasciculus cerebra spinalis). Its fibers rise in the anterior central gyrus of the cere bral cortex; they run through the genu and anterior two-thirds of the occipital part of the internal capsule, form the middle three-fifths of the basis, a part of the basilar longitudinal fibers of the pons, and the pyramid of the medulla. Below the medulla they are continued in the anterior and lateral pyram idal tracts of the spinal cord. Those fibers of the pyramidal tract which innervate the muscles of speech and of the face run through the genu of the internal capsule and constitute the medial portion of the tract in the mid-brain and the accessory lemniscus (of Bechterew). Immediately behind the face fibers, in the capsula interna, and lateral to them, in the basis pedunculi, are fibers which innervate the muscles of the arm. Still behind
these, in the internal capsule, and lateral to them, in the pyramidal tract of the basis pedunculi, are fibers for the inner vation of the trunk and leg muscles.
3. The inner fifth of the basis pedunculi is composed of the fronto-pontal tract (tractus fronto-pontalis) (Figs. 59, 63). The origin of this tract is in the middle and posterior parts of the three frontal gyri. It is medullated later than the pyramidal tract, like the temporo-pontal tract. It is an efferent tract. It terminates very largely in the nucleus pontis; Flechsig claims that a few of its fibers end in motor nuclei of cranial nerves. The fronto-pontal and temporo-pontal tracts are probably con cerned with some form of coordination or inhibition rather than with voluntary motion. The intermediate path belongs to the strio-fugal system of fibers and, according to S. Kin nier Wilson, it exercises a steadying influence over the lower motor neurones.
The Substantia Nigra (Figs. 58, 59 and 6o) .—The central part of the crura cerebri is a sheet of pigmented gray matter. The substantia nigra is visible at the base of the brain between the bases pedunculi, where it is called the posterior perforated substance (substantia perforata posterior), and its margin comes to the surface in each lateral sulcus of the mid-brain. It extends from the pons upward to the corpora mammillaria and nucleus hypothalamicus (Luysi). Dorsal to it are the tegmenta. Transversely, the substantia nigra is convex forward, but it is slightly concave longitudinally. The third nerve pierces it and comes out through the oculomotor groove. It contains small pigmented multipolar cell-bodies, some of which constitute a relay for certain fibers of the medial fillet (Barker). There is a median aggregation of these cells located just in front of the pons, the interpeduncular nucleus (ganglion interpedunculare). According to Forel, this nucleus is connected by a bundle of fibers, the fasciculus habenulo-peduncularis (fasciculus re troflexus), with The nucleus habenulx of the thalamus. The superior portion of the substantia nigra lies ventral to the nucleus hypothalamicus (Luysi) on either side. The nucleus hypothalamicus lies ventro-lateral to the red nucleus, and is separated from it by the zona incerta.