246 COMPARATIVE ANATOMY 



of which it arises : it is therefore primitively a cerebral and not a 

 spinal nerve. It presents certain characteristic peculiarities in 

 the Amphibia, Sauropsida, and Mammalia respectively, so that 

 a direct comparison of the nerve in these groups is impossible. 1 



Owing to secondary differentiations, the accessory of Mammals 

 takes on a very different character from that of the Sauropsida : 

 only that part of it in the former arising from the spinal cord can 

 properly be described as the accessory, while its cerebral portion 

 must be included under the vagus group. In the Sauropsida, the 

 nerve is better described as the spinal portion of the vagus. In 

 Mammals, the accessory contains viscero-motor elements from 

 the dorsal roots of the 5th to 7th spinal nerves, and extending 

 along the course of the vagus gives off branches to the larynx and 

 to the trapezius and sternocleidomastoid muscles. 



Spino occipital and Hypoglossal Nerves. Under the 

 term " spino-occipital nerves " is understood a group of nerve- 

 roots in the occipital region and anterior trunk-myotomes which 

 are in close relation to the hypoglossal. As most of their com- 

 ponents are bound up in the vagus-group, they were formerly 

 erroneously described as " ventral roots of the vagus." 



In Cyclostomes they have either not been assimilated by the 

 cranium (cf. p. 85) or are not even differentiated from the cerebral, 

 nerves, so that in this case they cannot be spoken of as spino- 

 occipital. In Plagiostomes, in which, as in Amphibians, vertebral 

 elements are fused with the occipital region of the skull, a series of 

 intracranial spinal nerves can be recognised which may be described 

 as "occipital," a reduction in which, from before backwards, can 

 already be observed. In the Holocephali, owing to a still greater 

 assimilation of vertebral elements to the skull, three additional 

 spinal nerves later became intracranial, while the number of 

 occipital nerves is reduced to two. 2 The relative number of these 

 two series varies in Ganoidei, Dipnoi, and Amniota, the occipital 

 nerves having entirely disappeared in the Teleostei. 



In Fishes the first spinal nerve, which corresponds to the hypo- 

 glossal of higher forms, supplies the muscles of the trunk, the floor 



1 The evolution of the spinal accessory in the higher Vertebrates must have 

 taken place somewhat as follows. Beginning with the Amphibia, in which the 

 vagus group does not extend into the spinal cord, the accessory in the primitive 

 Amniota must have possessed the following characters : close connection with the 

 vagus, extension backwards at least as far as the first cervical segment, origin 

 from a lateral collection of cells of the ventral cornu, and course on the ventral 

 side of the dorsal cornu of the gray substance. From this primitive form the 

 nerve must have developed along two different lines in the Sauropsida and Mam- 

 malia respectively, in both of which, however, in contrast to the Amphibia, it thus 

 forms a kind of connecting link between the cerebral and the spinal nerves, this 

 region including in the Sauropsida at most three, in Mammalia seven, segments. 



- These additional nerves have been described as "occipito-spinal " to dis- 

 tinguish them from the "occipital" nerves : each series constitutes a sub-section 

 of the spino-occipital group. In Amphibians (except Ichthyophis) the occipital 

 nerves are no longer recognisable, even in the embryo. 



