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 (neurilemme) 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 
