210 LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



ependyma of the ventricle, forming a dark group of cells on 

 either side, and at the anterior extremity of the fourth ventricle. 

 This area is called the locus cceruleus. 



The main part of the trigeminus, the sensory portion, gets 

 but very few fibres from the pons. They come rather from all 

 levels above the cervical spinal cord up to its point of emergence 

 from the brain. It has often been mentioned before, and is the 

 thick, cresceiitic bundle of medullary fibres which surrounds the 

 head of the posterior horn in almost all the illustrations shown 

 in the last two lectures. Near the motor nucleus this main 

 trunk dips forward into the pons, and emerges from it as the 

 portio major. (See Fig. 113.) 



Fibres also come from the dorsal region to enter the portio 

 major. They are fasciculi from the direct sensory cerebellar 

 tract. There are not many of them. In lower animals, how- 

 ever, particularly in fishes, they constitute the greater part of 

 the nerve-root, and their point of origin in the cerebellum is so 

 largely developed as really to form one of the lobes of that body. 



Lastly, there is an enlargement of the substantia gelatinosa 

 at its cerebral termination, which is regarded as a sensory 

 nucleus of the trigeminus. It receives masses of fibres which 

 run to it from the raplie across the floor of the rhomboidal fossa. 

 Similar fibres also pass to the locus cceruleus. They represent 

 the central tract which we have discovered as belonging to all 

 the cranial nerves.- Comparative embryology teaches us that 

 this considerable bundle of fibres comes from the opposite fillet, 

 and that after leaving it they run a short distance horizontally 

 through the substantia reticularis. 



The trigeminus roots cannot be shown in a single section. 

 They come from dorsal and ventral areas to their junction on 

 the floor of the rhomboidal fossa, where they turn and pass 

 downward to their exit. 



From the level of exit of the fifth nerve up to the level 

 where the trochlear nerve emerges, the tegmentum has a much 

 simpler structure than we have previously made out. 



