MOTOR NUCLEI IN PHYLOGENY 419 



first and second spinal nerves. The latter ''shortly before they 



recurve to their point of union give off a few 



small branches, some of which anastomose with the branch of the 

 ramus intestino-accessorius X supplying the trapezius muscle, and 

 others innervate the basi-scapularis" (46, p. 331).' The m. 

 basi-scapularis is apparently the homologue of the anuran m. 

 levator scapulae inferior. 



Thus in the double innervation of the m. trapezius, a condi- 

 tion obtains in Siren which is essentially similar to that which 

 becomes characteristically developed only in higher vertebrates. 

 Whether such a condition obtains elsewhere among urodeles is 

 at present uncertain. In Norris' earlier account of these nerves 

 in Amphiuma means he states "we may confidently deny the 

 occurrence of any anastomosing between hypoglossal and vagal 

 nerves" (45, p. 553). However, in Plate IV the branch of the 

 R intestino-accessorius to the trapezius is labeled 'ace,' and the 

 abbreviation 'ace' is explained as "branch of X nerve supphdng 

 the anterior part of the trapezius muscle." If this implies a 

 different innervation for the posterior part of the trapezius of this 

 animal, it is possible that spinal nerves may be involved without 

 anastomosing with branches of the vagus. 



In view of the above observations, the relations of the caudal 

 part of the motor X nucleus in Rana are significant. It has 

 already been pointed out that in the caudal part of this nucleus 

 the cells become scattered, so that it is difficult to set an exact 

 caudal limit to this column. Further, the condition obtaining 

 here presents an interesting contrast to that obtaining in the 

 corresponding region in sharks. In the latter animals, rostrad 

 of the calamus (10, fig. 10) the motor X nucleus lies just beneath 

 the ventricular floor, some distance laterad of the spino-occipital 

 nucleus and on a more dorsal plane. Caudad of the calamus 

 in the upper cervical cord the motor X nucleus in sharks lies 

 in the central gray completely dorsal to the spino-occipital 

 motor column. In Rana (figs. 1 and 2) the motor X nucleus 

 above the calamus is separated by a relatively great distance 

 from the ventricular floor on account of the thick layer of peri- 



'' The italics are my own. 



