486 



COMPARATIVE ANATOMY 



occipital nerves, may be seen the beginnings of the cervical plexus which 

 persists throughout the vertebrate series. Since paired appendages are 

 wanting in cyclostomes, no thoracic or lumbar plexuses are formed. 



Cerebellum 



Optic lobe 



Thalamus 



Cerebral hemisphere 



Olfactory bulb 



Olfactory tract 



Vagus nerve N. X J / 



Glossopharyngeal nerve N. IX ' , 



Acoustic nerve N. VIII / 



Abducens nerve N. VI 



Optic nerve N. II 



Inferior lobe 



Oculomotor nerve N. Ill 

 i Saccus vasculosus 

 Trochlear nerve N. IV 



Trigeminal and facial nerves Nn. V, VII 

 Fig. 400. — The brain of the dogfish, Squalus acanthias, lateral view. (From Ranson's 

 "The Anatomy of the Nervous System," courtesy of W. B. Saunders Company.) 



Little can be said concerning the sympathetic nervous system of 

 cyclostomes. The nerves of cyclostomes like those of Amphioxus, are 

 not medullated, so that this means of distinguishing sympathetic from 

 other fibers cannot be used. A number of observers claim to have 



Epiphysis 

 I 

 Paraphysis 

 Cerebral hemisphere \ 

 Olfactory tract , .''n, ! 



Olfactory bulb 



Optic lobe 



Mesoccele 

 ! Cerehellurrt 



Metacoele 



Tiiberciilum acusticum 

 Tela chorioidea 

 I Fourth ventricle 

 Visceral lobe 



Telencephalon 



Preoptic recess 

 Velum transversum 



Optic chiasma 



i i \ Metencephalon Myelencephalon 



I i Mesencephalon 



1 Saccus vasculosus 



Third ventricle 



Fig. 401. — The brain of the dogfish, Squalus acanthias, medial sagittal section. 

 (From Ranson's "The Anatomy of the Nervous System," courtesy of W. B. Saunders 

 Company.) 



found clusters of sympathetic ganglion cells associated with the vagus. 

 Also a plexus comparable to Auerbach's plexus of higher vertebrates is 

 found in the intestinal wall. Chromaffin cells, which have an origin in 

 common with sympathetic cells, are found in the trunk region and have a 

 segmental distribution as in mammals. 



