Johnston, Functional Divisions of Novous System. 97 



The most conservative elements in the cord and brain are 

 purely central and do not belong to either of the main divisions 

 of the central system. These are the connective elements, the 

 celb which set up connections between successive or distant 

 segments of the brain and cord. These tract and commissural 

 cells are present in almost all parts of the cord and in the 

 medulla they and their tracts continue to form the bulk of the 

 ventrolateral region. They are practically absent from both of 

 the sensory centers, but are intermingled with the motor cells. 



The decussation of fibers ventral to the ventricle is con- 

 tinued from the cord into the medulla and is greatly increased 

 by the large number of internal arcuate fibers, secondary fibers 

 from the somatic sensory centers to the tectum opticum. The 

 dorsal commissure of the cord is interrupted in the medulla 

 by the choroid plexus and its elements are gathered in two 

 commissures, the commissura infima Halleri and the cerebel- 

 lar commissure. The former is at the caudal end of the 

 medulla and represents the splanchnic portion of the dorsal 

 commissure of the cord. The latter connects the somatic cen- 

 ters, although it is not clear that it contains the same somatic 

 elements as does the dorsal commissure. 



From this survey it appears that the hind brain, including 

 the cerebellum, is readily reducible to four longitudinal zones or 

 columns which correspond directly to the chief zones of the 

 cord, and that where modifications of these columns are found 

 in the hind brain they are due to modifications of the corre- 

 sponding peripheral end-organs in the head region. An excep- 

 tion to this statement must be made for the lower olive, a body 

 which appears in the lowest craniates (Petromyzon, Acipenser) 

 as a collection of cells about the ventral roots of the occipito- 

 spinal nerves (hypoglossus), whose neurites break up among 

 the ventro-lateral tracts of the opposite side. The only sug- 

 gestion which the writer is able to offer toward the interpreta- 

 tion of the olive is that its cells must be derived from the com- 

 missural cells of the region, and that the formation of a special 

 nucleus may be in some way connected with the degeneration 

 and disapp^earance of dorsal roots in the occipital region. The 



