100 Journal of Comparative Neurology. 



hypothesis receives some further support from the fact that the 

 fibers from the tectum, at least for the most part, connect not 

 with the motor centers directly but with intermediate neurones. 

 The fact last mentioned, taken together with the connec- 

 tions of the lobi inferiores, suggests an interpretation of the 

 latter. These lobes are the hypertrophied ventral region of the 

 'tween brain whose tracts go. partly direct, partly crossed, to 

 the medulla and the fore brain. The lobes receive important 

 tracts from the tectum and from the secondary olfactory centers. 

 Thus the inferior lobes do not stand in immediate connection 

 with any sensory center, but do send fibers to make (prob- 

 ably) immediate connections with motor centers. With respect 

 to the tectal tracts these neurones stand in the same relation as 

 intermediaries between the tectum and the motor centers, as 

 do the commissural and tract cells in the medulla which receive 

 the endings of the tractus tecto-bulbaris. This description 

 places the cells of the inferior lobes in the category of commis- 

 sural and tract cells. The inferior lobes may be regarded as a 

 special collection, at the rostral end of the brain axis, of these 

 cells which in general form the commissural and connective 

 elements in the central nervous system. The fibers going from 

 the inferior lobes to the fore brain arise chiefly from the corpus 

 mammillare. The tracts are essentially dorso-ventral and not 

 longitudinal. They are to be regarded as a late formation due 

 to the influence of the olfactory and optic apparatus. Fibers 

 from other cells of the same nucleus pass backward to the 

 medulla. 



The presence of the somatic motor column in the thala- 

 mus and the fact that this column extends practically the whole 

 length of the central nervous system, have been noted. Sur- 

 rounding the nucleus of the somatic motor fasciculus laterally 

 and dorsally the central grey of the 'tween and mid brain is 

 composed of rich masses of cells which in the lower craniates 

 constitute for the most part a nucleus diffusus, but which form 

 the material for later differentiated special nuclei. One of these 

 is the nucleus of the posterior commissure, already distinguish- 

 able in Petromyzon. This does not at present offer any data 



