224 THE CEREBRUM. 



Trigeminal nucleus of the mid-brain is a very small nucleus 

 situated in the extreme lateral part of the central gray matter. 

 It is continuous with the pontine nucleus of the fifth, located 

 under the locus caeruleus, and is merely the superior end of the 

 motor nucleus of the trigeminal. It gives origin to the descend- 

 ing root of the fifth nerve, which descends to the pons and there 

 joins the main motor root. In its course downward the mesen- 

 cephalic root runs between the central gray matter and the bra- 

 chium conjunctivum cerebelli. . 



Lesions of these cerebral nerve nuclei are apt to involve the 

 tegmentum. If so, the result is paralysis of the nerves on the 

 same side and hemianaesthesia, hemiataxia, loss of taste (?) and 

 deafness on the opposite side. 



The white matter of the cerebrum is composed, in the 

 adult condition, of medullated fibers; the medullation begins in 

 the ninth month, in utero, and is continued for a considerable 

 time after birth (Flechsig). Within the cortical substance the 

 myelin sheaths continue to be laid down until late in life (Kaes, Mc- 

 Murrich). The cerebral fibers form three definite systems: 



1. Projection, or peduncular fibers. 



2. Transverse, or commissural fibers. 



3. Association fibers. 



I. PROJECTION FIBERS. 



The projection fibers are connected only with the motor and 

 sensory areas of the cerebral cortex and are, therefore, motor and 

 sensory in function (Figs. 54 and 55). Where they are present 

 they are continuous with Meynert's radiations. They are com- 

 posed, first, of the medullated axones of the pyramids and the 

 polymorphous neurones; these descend from the cerebral cortex, 

 are motor in function, or corticifugal, and constitute the upper 

 motor segment ; and, second, they comprise the medullated axones 

 of neurones whose cell-bodies are situated in gray matter below 

 the cerebral cortex; these axones ascend to the cortex and are 

 sensory in function, or corticipetal. The projection fibers run 

 from cerebral cortex through the corona radiata, the internal 



