THE NERVOUS SYSTEM. 213 



of the entire body (Figs. 98, 99, and 100). The crossing 

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

 medulla is known as the motor decussation or the de- 

 cussation of the pyramidal tract. 



The limits of these various fiber tracts of the central 

 nervous svstem cannot be determined by dissection. 



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They have been worked out largely by experimental 

 physiology and pathology, and by studying their embry- 

 onic development when the fibers of different tracts 

 are seen to acquire their sheaths (neurilemmae) at differ- 

 ent periods. The portion of a nerve fiber separated from 

 its cell degenerates, so that if the fibers of the crossed 

 pyramidal tract were injured 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 spinal cord (Fig. 100). Like- 

 wise some of the fibers originating in the cells of the cord, 

 medulla, cerebellum, and basal ganglia terminate about 

 the cells of the cortex. The cord, medulla, and cere- 

 bellum are connected with the higher brain centers by 

 the fibers of the cerebral peduncles (Figs. 90 and 100), 

 which are separated into two parts by an elongated mass 

 of gray matter, the substantia nigra. The dorsal part 

 is known as the tegmentum, while the ventral part is the 

 crusta . 



The fibers of the brain are of three kinds the com- 



