THE ELASMOBRANCH FISHES 241 



auditory capsule the nerve divides into a smaller anterior 

 and a larger posterior branch ; the former, called the pre- 

 trematic branch, runs down in front of the first gill-cleft 

 along the hinder border of the hyomandibular cartilage. 

 The hinder or post-trematic branch runs behind the first 

 gill-cleft, along the anterior face of the first branchial arch. 



Just as the ninth nerve bears a definite relation to the 

 first gill-cleft, so does the tenth or vagus nerve bear similar 

 definite relations to the remaining gill-clefts. It arises by 

 a number of roots close behind the ninth nerve and emerges 

 from the skull through a foramen situated at the side of 

 the foramen magnum, and thence runs backwards along 

 the inner wall of the anterior cardinal sinus, where it gives 

 off numerous branches and appears to be joined by the 

 first five spinal nerves, and by the slender trunk formed 

 by the so-called occipital nerves. This union, however, is 

 only apparent ; the nerve formed by the spinal and occipital 

 nerves is bound up in the same connective tissue sheath 

 as the vagus as far back as the ductus Cuvieri, but makes 

 no connections with it. The four external branches of the 

 vagus are the branchial nerves. They run backwards and 

 outwards over the pharyngo - branchial cartilages, and re- 

 spectively supply the second, third, fourth, and fifth gill-slits, 

 each nerve having a pretrematic and a post-trematic branch 

 like the ninth nerve. On its inner side the vagus, soon 

 after its emergence from the skull, gives off a large lateral 

 nerve which turns inwards and runs back to the posterior 

 end of the body in the fascia between the dorsal and ventral 

 sections of the musculature of the trunk. Its course is 

 parallel to, but a good deal deeper than the lateral line. 

 The main trunk of the vagus is continued backwards past 

 the ductus Cuvieri to the dorsal wall of the oesophagus, 

 where it breaks up into several branches and enters into 

 connection with the sympathetic system. A small branch is 

 given off to the sinus venosus, but it is difficult to trace 

 it beyond the sinus into the heart. 



We may now review the distribution of the several cranial 

 nerves and consider what may be learned from them. It has 

 already been pointed out that the spiracle is a rudimentary 

 gill-cleft. The seventh nerve has the same relation to the 

 spiracle that the ninth has to the first gill-cleft and the vagus 



II. Q 



