34 LABORATOKY OUTLINE OP NEUROLOGY 



The components of these peripheral nerves can be com- 

 pletely separated only by microscopic methods, though a skil- 

 ful dissector can separate the lateralis components of many of 

 the nerves (see Ewart, '93 and Norris and Hughes, '19). The 

 nerve components of vertebrates are fully described by 

 Johnston ('06 and '09). 



20. The gills are visceral structures. Their sensory nerves 

 terminate centrally in the visceral sensory column of the 

 medulla oblongata, and their motor nerves arise from the 

 visceral motor column (see Sections 15 and 23). Each gill cleft 

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

 The branchial trunk typically divides into: (1) a postbranchial 

 (or posttrematic) nerve, containing visceral sensory and 

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

 nerve (visceral sensory), (3) a pharyngeal nerve (visceral 

 sensory). The postbranchial 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 oblon- 

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

 itself can be seen, after the dissection of the internal ear, run- 

 ning 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 famili- 

 arity 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 rudiments of these nerves are present- 

 see Section 17); the presence in the dogfish of functional gills 



