Johnston, Functional Divisions of Nervous System. 93 



motor column with varying degrees of clearness in different 

 fishes. In Petromyzon the two columns are distinct in position 

 and have different histological characters. The same seems to 

 be true in Mustelus (Houser). In Acipenser, on the other 

 hand, only that part of the splanchnic motor column which is 

 related to the V and VII nerves lies in a separate position lat- 

 eral to the somatic column. There are, however, indications of 

 a complete separation throughout the medulla, chief of which 

 is the fact that the fibers of the motor VII, IX, and X nerves, 

 which seem to come from the somatic motor fasciculus, in real- 

 ity only run along the lateral side of that fasciculus and can be 

 distinguished from it. The splanchnic motor nucleus ends quite 

 abruptly with the nucleus of the motor V, although it primi- 

 tively gave rise to one or more pairs of nerves rostral to V, of 

 which the motor component of the ophthalmicus profundus V 

 is still found in some cases in the ontogeny. 



The somatic sensory division is in most Anamnia the larg- 

 est of the four divisions in the head region and presents import- 

 ant modifications in its centers owing to the differentiation of its 

 peripheral organs. In the trunk region the somatic sensory 

 components of the dorsal roots belong to one system only, the 

 general cutaneous. In the head region the presence of the 

 various organs of the lateral line system and the ear has called 

 forth the acustico-lateral system of nerves and, correlatively, 

 great specialization in the somatic sensory centers in the hind 

 brain. In consequence of these changes there are to be recognized 

 two fairly distinct sub-divisions of the somatic sensory division, 

 the general cutaneous and the acustico-lateral. The first of 

 these seems on superficial examination to correspond fully to 

 the somatic sensory division in the trunk, since it is continuous 

 centrally with the dorsal horns and tracts of the cord and peri- 

 pherally supplies general cutaneous fibers to the whole head. 

 More careful examination of the central relations (12, 13) has 

 shown, however, that the acustico-lateral centers have almost 

 equally close relations with the somatic centers in the cord, that 

 these relations are most intimate in the lowest forms studied, 

 and that the progress of histological differentiation of the more 



