Johnston, Morphology of the Head. 255 



to the opposite side. The remainder of the decussation is 

 formed by axones of cells of these nuclei. The destination of 

 these axones is unknown, so that we are uncertain whether true 

 commissural elements are present. It is evident, however, that 

 the decussation belongs to the splanchnic sensory division of 

 the nervous system. 



In mammals the tract which corresponds to the secondary 

 vagus tract is the direct cerebellar tract arising from the cells of 

 Clarke's column in the cord. This tract ends in a nucleus 

 which lies in the roof of the IV ventricle in the vermis. It is, 

 however, probably homologous with the secondary vagus nucleus 

 of lower vertebrates. 



The dorsal decussation of the optic lobes requires but brief 

 mention. In accordance with the interpretation of the lobes 

 given in this and previous papers it must be regarded as a 

 somatic sensory decussation comparable wifh the somatic por- 

 tion of the decussation in the spinal cord. 



The posterior commissure is a decussation of fibers arising 

 from a nucleus in the dorsal part of the diencephalon and 

 mesencephalon (75. 68). After crossing the fibers run toward 

 the base of the medulla oblongata. The destination of the 

 fibers is unknown, and nothing is known of the fiber tracts 

 which may come to end in the nucleus of origin. The fact that 

 the decussation is somewhat intermingled with the dorsal de- 

 cussation of the optic lobes suggests that the two may be related. 

 Some of the cells of origin of the posterior commissure in 

 Petromyzon are so situated as to be indistinguishable by their 

 position from those of the optic lobes. Without knowing the 

 connections and functions of the posterior commissure, nothing 

 further can be said. 



In the roof of the diencephalon two decussations are pres- 

 ent, the well known superior or habenular commissure and a 

 decussation closely related to the base of the epiphysis known 

 only in a few forms (67, 62). The superior commissure con- 

 tains decussating fibers from the olfactory nuclei of the fore-brain 

 (tractus olfacto-habenularis) and also, according to Catois (16) 

 and Cameron (15) true commissural fibers. A certain similarity 



