458 



CRANIAL NERVES. 



This compound nerve together with the glossopharyngeal soon attains a 

 very complicated structure, and presents several remarkable features. There 

 are present five branches (fig. 271 B), viz. the glossopharyngeal (gl} and 

 four branches of the vagus, the latter probably arising by a considerably 

 greater number of strands from the brain 1 . All the strands from the 

 brain are united together by a thin commissure (fig. 271 B, vg~), continuous 

 with the commissure of the posterior roots of the spinal nerves, and from 

 this commissure the five branches are continued obliquely ventralwards and 

 backwards, and each of them dilates into a ganglionic swelling. They all 

 become again united together by a second thick commissure, which is 

 continued backwards as the intestinal branch of the vagus nerve. The 

 nerves, however, are continued ventralwards each to its respective arch. 



FlG. 271. VlEWS OK THE HEAD OF ELASMOBRANCH EMBRYOS AT TWO STAGES 

 AS TRANSPARENT OBJECTS. 



A. Pristiurus embryo of the same stage as fig. 28 F. 



B. Somewhat older Scyllium embryo. 



///. third nerve ; V. fifth nerve ; VII. seventh nerve ; au.n. auditory nerve ; gl. 

 glossopharyngeal nerve ; Vg. vagus nerve ; fb. fore-brain ; pti. pineal gland ; mb. mid- 

 brain ; hb. hind-brain ; iv.v. fourth ventricle ; cb. cerebellum ; ol. olfactory pit ; op. 

 eye; au.V. auditory vesicle; m. mesoblast at base of brain; ch. notochord; ht. heart; 

 Vc. visceral clefts; eg. external gills; //. sections of body cavity in the head. 



1 " Ueber d. Kopfnerven von Hexanchus," etc., Jenaische Zeitschrift, Vol. vi. 1871. 



