THE SKATE. 81 



The vestibule is a rounded sac with two small diverticula, containing 

 an otolithic mass which consists of minute calcareous particles bound 

 together by gelatinous connective tissue : from its dorsal surface pro- 

 ceeds a canal, the aqueductus vestibuli (aq.f), which passes through 

 the roof of the auditory capsule, and dilates between the latter and the 

 external integument into a small sac, lined with strongly pigmented 

 epithelium ; this opens on the surface of the head by a minute aperture. 

 The canals form nearly complete circles, the posterior being the largest of 

 the three. Each is dilated at one part of its course into an ampulla (a). 

 The posterior canal communicates with the vestibule in an unusual 

 manner, namely by a short distinct tube (x). 



_p-*e 



FiG. 28. Raja batis. The right membranous labyrinth, seen from 

 the outer side (nat. size). 



v, vestibule : ag.f, aqueductus vestibuli : a.s.c, anterior, p.s.c, 

 posterior, and h.s.c, horizontal semicircular canals : a,a,a, ampullae : 

 x, side tubulure , by which the posterior canal communicates with the 

 vestibule : ,,, branches of auditory nerve. 



XLVII. Clear away the skin and muscles from the 

 dorsal surface of the anterior vertebral plate and 

 from that of the gills. Dissect out 



214. The vagus or tenth nerve (Fig. 22, X], which, after 

 leaving the skull by the vagus foramen, passes directly back- 

 wards between the inner boundaries of the gills and the 

 lateral ridge of the anterior vertebral plate, giving off the 

 branchial nerves as it goes, to the front edge of the pro- 

 pterygium, where it divides into two chief branches, a ventral 

 branch (X,v\ to the stomach and heart, and a lateral branch 

 , which passes between the peritoneum and dorsal 



G 



