NERVOUS SYSTEM 



339 



are traditionally described as a single series of paired nerves 

 numbered from one to ten (or, in some animals, twelve). Their 

 segmental arrangement, which has been known since the work 

 of F. M. Balfour, A. M. Marshall, and J. W. van Wighe in the 



'TLdTTt. 



Vrrur 

 Vmd- 







VIIp 

 Vlfhm 



Fig. 256. — A dissection of the nervous system and sense organs of a dogfish. 



On the left : nerves labelled as in Figs. 254-255 and 257-258. ....,.„ 4 



On the right : a.c.s., anterior cardinal sinus ; a.v.s., a.m.p., h.s., p.v.s., utr., parts of labyTinth, labelled as 



in Fig. 305 ; au.c, auditory capsule ; cer., olf.o., olf.L, sp.c, as in Fig. 254 ; n.am., neuromast ampulI^E ; 



o.s.,r.e., r.int.,r.s.., s.p., as in Fig. 258. .. if^u j -^ •■ » 



The eye and part of the auditory capsule, which have been removed from the left-hand side remain in stlu 



on the right. The cartilage of the skull and vertebrae is dotted, and the nerves are seen passmg through 



the foramina shown in Fig. 239. 



eighteen-seventies and early eighties, is described in Chapter 

 27 ; the old numbering, which is still convenient in dissection, 

 is shown in Table IV. The chiasma in the optic nerve is a crossing- 

 over of the fibres from the eye of one side to the optic lobe of 

 the other. The distinction of fibres as visceral or somatic is ex- 

 plained in Chapter 27. 



Each of the olfactory organs of the dogfish is a sac enclosed 



