THE NERVOUS SYSTEM. 203 



opposite half in that portion of the brain caudad of the 

 optic thalami. The internal portion of the medulla ob 

 longata possesses numerous transverse fibers which, with 

 the longitudinal fibers, form a kind of reticulum in the midst 

 of the gray matter, known as the formatio reticularis. 



The projection fibers (Fig. 104) are those connecting the 

 cortex with the lower brain centers and the cord. The chief 

 motor tract is the crossed pyramidal tract 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. 

 92), by slicing away the ventral portion of the brain 

 obliquely in a plane joining the cranial margin of the pons 

 and the crucial sulcus. Numerous 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. 102, 103, 104) 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. 98). The spreading out of the projection fibers just 

 beneath the cortex of the cerebrum forms the corona 

 radiata. 



