Johnston, Functional Divisions of Nervous System. 105 



sation of the tectum can be compared with the (somatic sensory 

 portion of the) dorsal decussation of the cord. The posterior, 

 superior, and anterior commissures are all peculiar to themselves, 

 not comparable with any structures in the hind brain or cord. 



The general result of this study is to divide the nervous 

 system into four main functional divisions: somatic and splanch- 

 nic sensory and motor divisions. The recognition of these 

 divisions together with the connective elements in the central 

 nervous system gives an adequate basis for the description of 

 the spinal cord and the hind brain. The somatic motor division 

 extends forward almost to the extreme rostral end of the brain 

 axis, and there is reason for thinking that a large part of the 

 mid, 'tween, and fore brains consists of cells belonging to the 

 same category as the connective elements. The splanchnic sen- 

 sory and motor divisions do not extend rostrally beyond the 

 medulla. The somatic sensory division, on the other hand, 

 includes the cerebellum and probably also the tectum opticum. 

 There are certain parts of the central grey matter in the rostral 

 region of the brain (for example, the nucleus of the posterior 

 commissure and the corpus striatum) which hold such relations 

 as to defy attempts to interpret them as modified representa- 

 tives of any structure in the spinal cord or hind brain. The 

 same is to be said of the dorsal part of the brain at its extreme 

 rostral end. A part of this region seems once to have received 

 somatic sensory nerves, but it has now so far lost its primitive 

 character that attempts at comparison with more caudal regions 

 fail. This is due in part to the great development of the olfac- 

 tory apparatus, but it is probable that a portion of this region 

 is to be regarded as a cephalic enlargement of the central axis 

 older than the typical structure of the vertebrate nervous sys- 

 tem. Behind this region the nervous system consists of four 

 longitudinal divisions, each of which has its peculiar function 

 and presents a segmental arrangement corresponding to the 

 metamerism of the other system of organs. 



Morgantown, W. Va., 

 November //, igoi. 



