420 DAVIDSON BLACK 



ependymal gray. In this position it is placed laterad to but on 

 the same level as the hypoglossal nucleus. Below the calamus 

 the motor X nucleus in Rana is even more ventrally placed, so 

 that in its caudal end it becomes associated with the most dorsal 

 elements of the ventro-lateral nucleus of the anterior born. In 

 this relation the motor perikaryons giving origin to the acces- 

 sory nerve come to lie almost side by side with those from which 

 fibers pass out to innervate the somatic shoulder muscles. It 

 is evident that the increased differentiation of the levator mus- 

 culature of the shoulder in Rana as compared with selachians 

 has been accompanied by a corresponding specialization in the 

 arrangement of the elements of the motor nuclei from which 

 these muscles receive their innervation. This closer association 

 of vagal and spinal motor nuclei in Rana represents what may 

 be considered as the second stage in the phylogenetic develop- 

 ment of the accessory nucleus, the first stage of which is seen 

 in the condition obtaining among sharks (v. 10, p. 495). 



Eye-muscle nerves 



' Nerve VI. But little remains to be said concerning the 

 abducens nucleus and its emergent roots. The diffuse nature 

 of the nucleus and its relation to the fasciculus longitudinalis 

 medialis and octavo-motor fibers have been noted above, and 

 Kappers has remarked already upon the primitive relation 

 evident in the emergence of the VI rootlets caudad of the motor 

 VII root exit. The chief bulk of the anuran superior olive is 

 also located between the exit levels of the motor VII and IX 

 roots but whether this nucleus establishes such direct reflex 

 connection with the abducens cells as obtains in higher forms is 

 not known. 



Nerves III and IV. In most amphibians in which thje troch- 

 lear nucleus has been identified, it lies almost directly behind the 

 oculomotor nucleus and on a level rostrad of the trochlear root 

 exit. In the latter relation these amphibians differ from all 

 petromyzonts and from some selachians and teleosts; on the 

 other hand, the position of the trochlear nucleus in amphibians 



