224 NERVOUS SYSTEM OF VERTEBRATES. 



in the region of the cranial nerve roots. Their neurites run back 

 into the spinal cord. In other fishes a pair of such cells lies 

 adjacent to the motor root of the VII nerve, whose neurites cross 

 and run back into the spinal cord. These elements remind one 

 of the large cells and thick fibers characteristically found in the 

 nervous system of invertebrates and the fact that they are found 

 only in lower vertebrates and most numerous in the cyclostomes, 

 suggests that they are very archaic elements which are not to be 

 counted among the typical elements of the vertebrate nervous 

 system. 



In the region of the myelencephalon the substantia reticularis 

 has on the whole the same relations as in the spinal cord. In 

 lower vertebrates the formation of special nuclei from this material 

 has not proceeded far, but it is probable that further study will 

 show a tendency even in fishes for the segregation of the neurones 

 related to somatic centers from those related to visceral centers. 

 Especially is this to be expected in any forms in which one system, 

 such as the acustico-lateral in selachians or the gustatory in bony 

 fishes, is greatly developed. Indeed it seems clear that the inferior 

 and superior secondary gustatory nuclei, which have already been 

 described, are specially developed parts of the substantia reticularis 

 which were primitively closely related to the visceral sensory 

 column. 



At the caudal end of the myelencephalon in fishes a collection 

 of substantia reticularis cells about the roots of the first ventral 

 spinal nerves (hypoglossus) forms the inferior olive. This nucleus 

 becomes of great size and complexity in mammals but its functional 

 relations are not yet well understood in any vertebrate. 



In the ventral part of the cerebellar segment occurs a collec- 

 tion of several nuclei which in mammals causes the large protu- 

 berance known as the pons. These nuclei are highly developed 

 only in the mammals, where they are related to the pyramidal 

 tracts and the cerebellum. A part of them receive numerous 

 collaterals and terminal branches from the pyramidal tracts and 

 are believed to send fibers to the motor centers of the spinal cord 

 and to the cerebellum. These pontial nuclei are thus interpolated 

 in the direct pathway between the cerebral hemisphere and the 



