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 



/V 



-la 



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

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

 A, Anterior cavity of the skull ; 2V, 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) ; 2V, first (ophthalmic) branch of the trigeminal 

 or fifth; a, terminal branches of the same in the ethmoid region ; Tr", sepond 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 ; Bi, internal rectus muscle ; He, external rectus ; Rs, superior 

 rectus; S, spiracle; Pg, palatoquadrate ; Hm, hyomandibular ; E, branchial rays; 

 I VI, branchial arches ; Br, branchiae ; 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 



