MORPHOLOGY OF CRANIAL MUSCLES IN SOME YERTEEEATES. 307 



Gehuchten). As all the fibres o£ the Xltli medollary join 

 the Xtli, and all the fibres of the Xlth spinal pass to the 

 trapezius and sterno-mastoid/ it is a little questionable whether 

 the old distinction of the two parts of the accessoi'ius is worth 

 preserving. The term Xlth or accessorius mio-ht well be 

 limited to Avliat is now known as the Xlth spinal. In a 

 Mammal like the rabbit, where the whole of the second and 

 third branchial myotomes (other than their dorsal ends 

 which take part in forming- the trapezius and sterno- 

 mastoid) disappear during development, the Xth and Xlth 

 medullary motor centres contain none of the original somatic 

 efferent fibres or cell-groups, and their new centres are those 

 innervating muscles derived from cells proliferated from the 

 wall of the cephalic cojlom. They also contain motor centres 

 for certain visceral muscles which are developed in the body 

 region. 



The Xth and Xlth medullary centres overlap aiitero- 

 posteriorly the hypoglossal nucleus, probably owing to their 

 backward extension into the first three segments of the 

 spinal cord. 



The Xlth spinal is, as emphasised by Fiirbringer, a true 

 cerebral and not a spinal nerve. It innervates a special 

 group of muscles which, in the rabbit, are derived from the 

 upper ends of the three branchial myotomes. Its nucleus 

 of origin is, from a phylogenetic point of view, a backward 

 extension into the spinal cord of the (dorsal) nucleus of the 

 Xlth medullary, but it is not known what happens in 

 embryonic development. 



The hypoglossal nucleus is the motor centre of the hypo- 

 branchial spinal muscles, of the rectus system, developed 

 f rotu the first three body myotomes. Cell-groups corresponding 

 to the upper, atrophying portions of these myotomes have 

 been lost. It is not known whether the subdivision of the 

 nucleus into the parts with large and with moderate-sized 

 cells corresponds Avith individual muscles or muscle-groups. 

 The hinder part of the hypobranchial spinal muscles has a 

 ' In dog (loc. cit.) and man (Streeter). 



