nasal sac 



olfactory bulb 

 olfactory tract 



superficial ophthalmic VII 



V, and buccal branch VII 



palatine branch VII 



lobus lineae lateralis (restiform body) 



hyomandibular branch VII 



membranous roof 

 medulla oblongata 



occipitospinal nerves 



optic lobe 

 pineal body 

 velum tronsversum 

 telencephalon^ paraphysis 

 olfactory bulb 



basilar artery 



posterior commissure 



^cerebellum 



lateral-line lobe 



membranous roof 



anterior commissure 

 preoptic recess 



hypothalamus 



fourth ventricle spinal cord 



saccus vosculosus 

 pituitary 



Figure 13-12. Broin and cranial nerve roots of Squa/us A, dorsal; B, ventral; C, medial view 

 right half. 



of 



The cranial nerves of the lamprey are in general compar- 

 able to those of the gnathostomes (Figure 13-16). The ol- 

 factory consists of many fibers. The optic nerves cross with- 

 out complication; the oculomotor is comparable to that of 

 the higher forms, but the trochlear does not decussate; the 

 nerve of either side goes to the superior oblique muscle of 

 the same side. There is doubt as to vv-hether there is a si.xth 

 nerve since the nerve in question comes out along with the 

 fifth and is said to contain at least some oculomotor fibers. 

 The profundus division of the fifth arises from the brain 

 wall independent of the combined maxillary and mandibu- 

 lar branch. Their ganglia are in part separate. 



Unlike the other fishes, the lamprey lacks a superficial 

 ophthalmic division of the fifth and seventh nerves, but the 



lateral-line sensory system is only poorly developed. The 

 fifth and seventh nerves are closely associated, making it 

 difficult to identify distinct lateral-line branches similar to 

 those of other fishes. The seventh and eighth nerve roots are 

 close together and both perforate the otic capsule, the eighth 

 entering the capsule, the seventh passing down through the 

 anterior capsule wall. The glossopharyngeal and vagus are 

 similar to those of other fishes with lateral-line and bran- 

 chial arch divisions. Each branchial nerve has typical pre- 

 and post-trematic divisions. The vagus also continues into 

 the body as the visceral component of the parasympathetic 

 system. 



The hypoglossal nerve is a derivative of the more anterior 

 spinal nerves and as such is like that in other fishes. 



396 



THE NERVOUS SYSTEM 



