2 54 Journal of Comparative Neurology and Psychology. 



dorsal decussation of the medulla oblongata is not obliterated 

 on account of the non-nervous roof, but its elements are crowded 

 forward or backward. Behind tiie choroid plexus the commis- 

 sura infima contains the splanchnic sensory elements proper to 

 the segments of the VII, IX and X nerves. It is probable that 

 the course of the root fibers of these nerves within the brain has 

 been influenced by the crowding backward of their decussation 

 and median nucleus by the choroid plexus. It is further prob- 

 able that those fibers which take this caudal course are the more 

 primitive components of these nerves, namely the general vis- 

 ceral fibers as distinguished from end bud fibers. It is known 

 that the end bud fibers in teleosts (48) end chiefly in the lobus 

 facialis. The point of especial interest in the present connec- 

 tion is that the concentration of the visceral decussation for the 

 VII, IX and X nerves behind the choroid plexus precludes the 

 expectation that the visceral elements of the first order will be 

 found in the dorsal decussations farther forward. There are no 

 visceral nerves anterior to N. VII. 



The somatic sensory elements have behaved differently 

 with reference to the IV ventricle. Instead of concentrating 

 behind it they have concentrated in front of it. In those verte- 

 brates in which the cerebellum is most primitive (Petromyzon, 

 Protopterus, Urodeles) a commissure constitutes a prominent 

 part" of it. This decussation consists of axones of granule cells 

 situated in the cerebellum destined to the somatic sensory 

 nuclei of the medulla oblongata. This is therefore to be con- 

 sidered as the homologue of the somatic sensory portion of the 

 dorsal decussation of the spinal cord. It is an important de- 

 cussation in all lower vertebrates. 



A second prominent cerebellar decussation is found in fishes 

 (67). This is situated farther ventrally and cephalad and in- 

 stead of connecting the dorsal portions or lateral lobes of the 

 cerebellum, connects two nuclei which in fishes lie in the lateral 

 walls distinctly ventral to the somatic sensory centers, the sec- 

 ondary vagus nuclei. The fibers of the secondary vagus 

 tract coming from the vagus lobe end in part in the 

 secondary vagus nucleus of the same side and in part cross 



