288 NERVOUS SYSTEM OF VERTEBRATES. 



primitive components of these nerves, namely the general visceral 

 fibers as distinguished from taste fibers. The point of special 

 interest 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 vertebrates in 

 which the cerebellum is most primitive (Petromyzon, Protopterus, 

 Urodeles) a decussation 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 considered as the homologue 

 of the somatic sensor}^ portion of the dorsal decussation of the 

 spinal cord. It is an important decussation in all lower verte- 

 brates. 



A second prominent cerebellar decussation is found in fishes. 

 This is situated in the velum medullare anterius or in the enlarged 

 equivalent of the velum, the valvula cerebelli of ganoids and bony 

 fishes. Instead of connecting the dorsal portions or lateral lobes 

 of the cerebellum, this commissure connects two nuclei which in 

 fishes lie in the lateral walls distinctly ventral to the somatic 

 sensor}' centers, the superior secondary gustatory nuclei. The 

 fibers of the secondary gustatory' tract coming from the visceral 

 sensory column end in part in the secondary gustatory nucleus 

 of the same side and in part cross to the opposite side. The 

 remainder of the decussation is formed of the neurites of the cells 

 of these nuclei. The destination of these fibers is not certainly 

 known, so that it is uncertain whether a true commissure is present. 

 It is evident, however, that the inferior cerebellar commissure 

 belongs to the visceral sensory division of the nervous system. 



The dorsal decussation of the tectum mesencephali must be 

 regarded as a somatic sensory decussation comparable with the 

 somatic sensory portion of the dorsal decussation in the spinal cord. 



The posterior commissure has been discussed above (p. 265). 



