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. 
