Johnston, Morphology of the Head. 205 



The somite which gives rise to the muscles innervated by 

 N. Ill is universally agreed to be somite i (praemandibular) of 



V. WlJHE. 



The third nerve arises from the ventral wall of the mid- 

 brain, in some forms far forward toward the point at which the 

 floor of the brain bends down into the inferior lobes. The 

 nucleus of N. Ill is well forward in the base of the mid-brain 

 and that of N. IV follows not far caudal to it. These relations 

 in the adult would seem to show that the third nerve belongs 

 to the first of the two mesencephalic neuromeres, to which, 

 indeed, some authors have referred it. On this assumption 

 the IV nerve would naturally be assigned to the second 

 mesencephalic neuromere. The trochlearis, however, supplies 

 the mandibular somite (somite 2) and the sensory nerve of this 

 segment is the trigeminus. The trigminus can not on any 

 ground be placed farther forward than the cerebellar neuromere. 

 If, then, Nn. IV and V be assigned to the cerebellar neuromere, 

 N. Ill would belong to the second mesencephalic or fifth 

 neuromere of Locy. To account for the position of the nucleus 

 and root somewhat farther forward in the adult we have to con- 

 sider that the nucleus lies at the anterior border of the large 

 ventral commissure of the cord and brain, which here in the 

 base of the mid-brain reaches its greatest size and importance 

 and is known as the ansulate commissure. This great commis- 

 sural system is composed fundamentally of the decussating 

 fibers from "tract cells." It has added to it in the base of the 

 cerebellum and mesencephalon many internal arcuate fibres from 

 the cutaneous centers, descending tracts from the tectum opticum 

 to the oblongata and the inferior lobes, the decussation and end 

 nucleus of the bundles of Meynert, and other fibers. Most of 

 these fiber tracts have developed at a period later than the 

 formation of the infundibular depression which sets a cephalic 

 limit to the commissure. The development of these voluminous 

 tracts has crowded the nuclei and roots of Nn. Ill and IV for- 

 ward. Not only the mere crowding due to the size of the 

 bundles but also the fact that the fiber tracts are related to brain 

 centers situated farther cephalad, has contributed to this result. 



