THE NERVOUS SYSTEM. 215 



already described in the cord. It may be traced from 

 the pyramids to its origin in the cortex in the region of 

 the crucial sulcus (Fig. 89), by slicing away the ventral 

 portion of the brain obliquely in a plane joining the 

 cephalic margin of the pons and the crucial sulcus. Num- 

 erous fibers are given off by this tract to the motor roots 

 of the cranial as well as the spinal nerves. 



The secondary motor tract, cortico pontine tract, carries 

 motor impulses from the frontal cortex to the medulla, 

 whence other fibers convey them to the opposite half of 

 the cerebellum. The axis-cylinders of the cells here 

 transmit the impulses through the inferior peduncle to 

 the cells in the anterior horn of gray matter of the cord. 



The great sensory tract of the brain is the fillet. Its 

 fibers originate largely in the cells of the nuclei gracilis 

 and cuneatus of the medulla (Figs. 98, 99, 100) and cross 

 over to the opposite side of the medulla, forming the 

 sensory or superior pyramidal decussation. This tract 

 receives also fibers from the spinal cord, the cerebellum, 

 and the medulla oblongata. 



These projection fibers, after leaving the peduncular 

 region, turn dorsad to pass with others through the 

 corpus striatum and laterad of the optic thalamus. In 

 this part of their course they form what is known as the 

 internal capsule (Fig. 95). The spreading out of the 

 projection fibers just beneath the cortex of the cerebrum 

 forms the corona radiata. 



The association fibers are those which connect different 

 portions of the same cerebral hemisphere. Two kinds 

 are recognized. The short fibers connect adjacent 

 convolutions, while the long ones place in communication 

 two remote portions of a hemisphere. 



