SURFACES. 151 



which innervate the muscles of the arm. Still behind these, in 

 the internal capsule, and external to them, in the pyramidal 

 tract of the basis pedunculi, are fibers for the innervation of the 

 trunk and leg muscles. 



(3) The inner fifth of the basis pedunculi is composed of the 

 fronto-pontal tract (tractus cerebro-cortico-pontalis frontalis) 

 (Figs. 47 and 48). The origin of the latter is probably in the 

 prefrontal cortex. It is motor. This motor tract is contained 

 in the frontal part of the upper lamina of the internal capsule. 

 According to Hoche, it is interrupted and relayed either in the 

 thalamus or striated body (Barker). Its termination is in the 

 nucleus of the pons and in the motor nuclei of cerebral nerves 

 (Flechsig). It constitutes a stage of an indirect motor path, like 

 the fibers of the outer fifth of the basis pedunculi, and the indirect 

 path is continued to the opposite half of the cerebellum by neurones 

 whose cell-bodies are in the nucleus pontis (Flechsig). 



The Substantia Nigra (Figs. 46, 47 and 48). 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 sub- 

 stance (substantia per for at a posterior}, arid 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. Trans- 

 versely, the substantia nigra is convex forward, but it is slightly 

 concave longitudinally. The third nerve pieces it and comes 

 out through the oculomotor groove. It contains small pigmented 

 multipolar cell-bodies, some of which constitute a relay for cer- 

 tain fibers of the medial fillet (Barker). There is a median 

 aggregation of these cells located just in front of the pons, the 

 inter peduncular ganglion (ganglion inter peduncular e). According 

 to Forel, this ganglion is connected by a bundle of fibers, the fas- 

 ciculus retroflexus, with the nucleus habenulae 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. 



