THE NERVOUS SYSTEM. 201 



the optic thalamus to the base of the brain. Here it is one 

 of the. three main tracts forming the crus or peduncle of the 

 cerebrum, whence it extends through the pons Varolii, ap- 

 pearing along the median ventral line of the medulla as the 

 pyramid (Fig. 93). At the caudal end of the medulla it 

 crosses dorsad to the opposite side of the cord to occupy the 

 area laterad of the posterior horn of gray matter. Its 

 fibers terminate largely in arborizations around motor cells 

 of the cranial nerves in the brain, and the cells in the an- 

 terior horn of gray matter, from which originate the motor 

 fibers for the muscles of the body. Therefore it is ap- 

 parent that this tract controls largely the muscular activities 

 of the entire body (Figs. 102, 103, and 104). The cross- 

 ing of the fibers of this tract in the caudal region of the 

 medulla is known as the motor decussation or the decussa- 

 tion of the pyramidal tract. 



The limits of these various fiber tracts of the central 

 nervous system cannot be determined by dissection. They 

 have been worked out largely by experimental physiology 

 and pathology, and by studying their embryonic develop- 

 ment when the fibers of different tracts are seen to acquire 

 their sheaths (neurilemmse) at different periods. The por- 

 tion of a nerve fiber separated from its cell degenerates, so 

 that if the fibers of the crossed pyramidal tract were in- 

 jured by accident or disease in the region of the medulla, 

 all that part of the tract in the cord would degenerate, in 

 consequence of which the subject would suffer paralysis. 



The larger portions of the tracts thus far described have 

 been confined to the cord, while the remaining tracts to be 

 discussed concern chiefly the brain. In order to understand 

 these it is necessary to remember that the cortex of the 

 brain is composed of millions of nerve cells which give 

 origin to nerve fibers extending to other portions of the 

 cortex, to the basal ganglia, the cerebellum, medulla, and 



