MORPHOLOGY OP CRAXIAL :\[USCLES IN SOME VERTEBRATES. 305 



Xltli medullary, which innervates tensor and levator palati, 

 palato-glossus, stylopharyngeuSjphar^-ngeal constrictor, laryn- 

 geal muscles, crico-tbyroid. 



The primary cranial nerves are the llird, Vth, A Ilth. 

 IXth, and Xth ; the Xth innervating in the rabbit two 

 myotomes (second and third branchial), the others one each. 

 The primary dorsal position of their motor nuclei (other 

 than that of the Ilird), the dorso-hiteral emergence of their 

 motor with their sensory fibres, and the relationship — 

 external — of the issuing nei-ves to the cori'esponding 

 myotomes, are related phenomena. If Balfour's theory, that 

 the head and trunk became " differentiated from each 

 other at a stage when mixed dorsal and sensory posterior 

 roots were the only roots present," be associated with 

 Fiirbringer's theory that the myotomes primitively lay 

 exclusively lateral to the notochord, it would follow that in 

 the body region anterior nerve roots were secondarily 

 developed in correlation with the upgrowth of the myotomes 

 to the mid-dorsal line, and the posterior roots became exclu- 

 sively, or almost exclusively, sensory. In the head, where 

 this upgrowth does not take place, or to a very limited 

 extent, a more primitive condition persists both in the 

 position of the motor nuclei and the emergence of their 

 efferent fibres. 



A further, probable, distinction between the somatic muscles 

 of the body and those of the head is that ganglionated muscle- 

 sensory nerve-fibres pass to the former but not to the latter.^ 



The position of the nucleus of the Ilird nerve and the path 

 of its nerve-fibres may be associated wnth the loss of cutaneous 

 sensory fibres. Evidence of such loss and of a primitive dorso- 

 lateral emergence of its nerve-fibres is found in the observa- 

 tion of Neumeyer that in the twenty-nine and forty-three hours 

 old chick " derNerf vom dorsalen Theile des Mittelhirns, also 

 in der Gegend der Ganglionleiste seinen Ursprung nehme, 

 sich also sekundar mit seineni definitiven Abgangsort 

 vereinige." 



' T hope to give tlie evidence for this iu a future paper. 



