494 TEXT-BOOK OF PHYSIOLOGY. 



occupying the middle three-fifths of the pes or crusta of the crus 

 cerebri, the ventral portion of the pons, and eventually the anterior 

 pyramid of the medulla oblongata. At this point the tract divides 

 into two portions, viz. : 



1. A large portion, containing from ninety-one to ninety-seven per 



cent, of the fibers, which decussates at the lower border of the 

 medulla and passes down the lateral column of the cord, con- 

 stituting the crossed pyramidal tract. 



2. A small portion, containing from three to nine per cent, of the 



fibers, which does not decussate at the medulla, but passes 

 down the inner side of the anterior column of the same side, 

 constituting the direct pyramidal tract or column of Tiirck. 

 After passing through the internal capsule, and as it descends 

 through the crus, pons, and medulla, the cortico-efferent tract gives 

 off a number of fibers which cross the median line and arborize around 

 the nerve-cells in the gray matter beneath the aqueduct of Sylvius 

 (the nuclei of origin of the third and fourth cranial nerves), and 

 around the nerve- cells in the gray matter beneath the floor of the 

 fourth ventricle (the nuclei of origin of the remainder of the motor 

 cranial nerves). The remaining fibers go to form the crossed and 

 direct pyramidal tracts and arborize around the cells in the anterior 

 horn of the gray matter of the opposite side of the cord at successive 

 levels. By this means the cortex is brought into anatomic and phys- 

 iologic relation with the general musculature of the body through the 

 various cranial and spinal motor nerves. (See Fig. 209, page 463.) 

 The fronto-cerebellar and the occipito-temporo-cerebellar tracts 

 are also efferent tracts and parts of the projection system. The 

 fronto-cerebellar, originating in the nerve-cells of the cortex of the 

 frontal lobe, passes down to and through the internal capsule, occupy- 

 ing the anterior one-third of the anterior segment. It then descends 

 along the inner side of the crus cerebri to the pons, where its fibers 

 arborize around the cells of the nucleus pontis. Through the inter- 

 mediation of these cells this tract is brought into relation with the 

 cerebellum of the same but chiefly of the opposite side. The occipito- 

 temporal tract, originating in the cells of the cortex of both the 

 occipital and temporal lobes, passes downward and inward toward 

 the lenticular nucleus, beneath which it passes to enter the outer 

 one-fifth of the crusta. It then enters the pons, and through the 

 nucleus pontis also comes into relation with the cerebellum of both 

 sides. 



THE FUNCTIONS OF THE CEREBRUM. 



The functions of the cerebrum comprehend, in man at least, 

 all that pertains to sensation, cognition, feeling, and volition. All 

 subjective experiences, which in their totality constitute mind, are 



