294 H. W. NORMS AND SALLY P. HUGHES 



4. The ramus dorsalis.X 366 



5. General cutaneous elements in the vagus nerve of elasmobranchs. . . 367 



6. The ramus lateralis X 369 



7. The second to the fifth branchial nerves 369 



8. The ramus intestino-accessorius X 372 



The occipital nerves 373 



The first three spinal nerves 375 



The hypobranchial nerve 376 



Sympathetic ganglia connected with cranial nerves 381 



1. The ciliary sympathetic plexus 382 



2. Branchial sympathetic ganglia 389 



Literature cited 392 



INTRODUCTION 



The common dogfishes of Europe and America have long been 

 used in biological laboratories as types introductory to courses 

 in the comparative anatomy of vertebrates, neurology, etc. An 

 exact knowledge of the structure of these forms, particularly of 

 the head, has, strangely enough despite their general use, not 

 yet been gained, except in reference to certain parts. Marion 

 ('05) has described the muscles of the head in Squalus acanthias. 

 The skull of the same form has been accurately described and 

 figured by Wells ('17). Neal's ('97, '98, '14, '18) studies on the 

 development of the nervous system, the metamerism of the 

 head, and the innervation of the hypoglossal musculature have 

 thrown much light upon adult conditions. It has long been 

 customary to regard the cranial nerves of the selachians as a 

 pattern to which may be referred the cranial nerves of all other 

 vertebrates. Without doubt, this practice has been justified, 

 but it is a matter of regret that so little has been positively known 

 of the exact composition, origin, and distribution of the main 

 nerves, to say nothing of the smaller divisions. 



There is a vast and bewildering literature upon the cranial 

 nerves of the elasmobranch fishes. Much of it is based upon 

 inexact observations, much of it has been published in support of 

 theories that have little basis in fact. Doubtless, our knowledge 

 of cranial anatomy had to be exploited in the interests of now 

 generally abandoned theories of the metamerism of the head. 

 Unquestionably, there is a fundamental segmentation of the 



