DISSECTION OF THE SHARK 33 



nerve, as illustrated in Figs. 1, 2, 5A, and 6. The branchial 

 trunk typically divides into: (1) a postbranchial (or post- 

 trematic) nerve, containing visceral sensory and visceral motor 

 components, (2) a prebranchial (or pretrematic) nerve (visceral 

 sensory) , (3) a pharyngeal nerve (visceral sensory) . The post- 

 branchial division of the IX nerve of fishes is homologous with 

 the human lingual branch, for distribution to the tongue. 



The roots of the IX nerve connect with the medulla oblongata 

 a short distance behind the VIII nerve, and the IX nerve itself 

 can be seen, after the dissection of the internal ear, running 

 across the floor of the auditory chamber. Dissect the IX 

 nerve outward and note its division into prebranchial and post- 

 branchial rami. 



The roots of the X nerve arise behind those of the IX (except 

 the lateral line root which arises farther forward from the 

 acoustico-lateral area) . Dissect the vagus trunk outward and 

 note its division into lateralis and branchial trunks. The latter, 

 after giving off the branchial rami to the second to fifth gills, is 

 continued backward to form the ramus intestinalis and ramus 

 cardiacus. 



21. The student should at this time acquire a general famil- 

 iarity with the arrangement of the human spinal and cranial 

 nerves. The cranial nerves of the dogfish are broadly similar 

 to the corresponding nerves in the human body. Some of the 

 more important differences are as follows: the absence in the 

 dogfish of the XI and XII pairs and of the cochlear branch of 

 the VIII pair; the presence in the dogfish of functional gills with 

 corresponding modifications of the IX and X pairs of nerves; 

 the presence in the dogfish of an extensive special system of 

 subcutaneous sense organs structurally (and probably physio- 

 logically) related to those of the internal ear. These are the 

 sense organs of the lateral line canals and ampullar of Lorenzini, 

 for which there is a special system of nerves, the lateralis com- 

 ponents of the VII, IX, and X nerves (see Section 18) . 



Draw the dissection of the brain and cranial nerves, indi- 

 cating the functional components in each peripheral nerve- 

 trunk by colors in accordance with the scheme given in Section 

 ,62 (p. 60), coloring the lateral line nerves the same as the VIII 

 nerve (brown or green) . 

 3 



