138 



PISCES. 



Cyclostomes "alone, where it is represented by the vagus and by fibres 

 of the spinal nerves. The spinal cord, the mass of which is con- 



-Fa 



FIG. 588. Brain and anterior part of the spinal cord and nerves of Hexanchus grisews (after 

 Gegenbaur). The nerves are dissected out on the right side ; the right eye removed. 

 A, Anterior cavity of the skull ; N, nasal capsule ; Vh, fore-brain (cerebral hemispheres) ; 

 Mh, mid-brain (optic lobes); Ce, cerebellum ; Mo, medulla oblongata; Bo, olfactory 

 bulb ; tr, trochlear nerve (fourth nerve) ; Tr' , first (ophthalmic) branch of the trigeminal 

 or fifth ; a, terminal branches of the same in the ethmoid region; Tr", second branch 

 Tr'", third branch ; Fa, facial (seventh); Gp, glossopharyngeal (ninth); Vg, vagus 

 (tenth) ; L, lateral branch of vagus (to lateral line) ; J, intestinal branch ; Os, superior 

 oblique muscle of eye ; Hi, internal rectus muscle ; Se, external rectus ; Rs, superior 

 rectus ; S, spiracle ; Pq, palatoquadrate ; Hm, hyomandibular ; jR, branchial rays ; 

 / VI, branchial arches ; Br, branchife ; P, spinal nerves. 



siderably greater than that of the brain, extends tolerably uniformly 

 throughout the whole length of the neural canal, and usually does 

 not form a so-called cauda equina. Rarely its upper part presents 



