xm 



PHYLUM CHORDATA 



111 



chamber, the utriculus (Fig. 798, u.), and, usually, three tubes, 

 the semicircular canals. Of these two, the anterior (ca.) and 

 posterior (cp.) canals, are vertical in position and have their adjacent 

 limbs united, so that the two canals have only three openings 

 between them into the utriculus : the third or external canal (ce.) 

 is horizontal, and opens into the utriculus at either end. Each 

 canal is dilated at one of its ends into an ampulla (aa., ae, ap.), 

 placed anteriorly in the anterior and external canals, posteriorly 

 in the posterior 

 canal. 



The ventral 

 compartment o f 

 the auditory sac is 

 called the sacculus 

 (s.) : it gives off 

 posteriorly a blind 



considerable di- 

 mensions in the 

 higher classes ; 

 while from its inner 

 face is given off a 

 narrow tube, the 

 endolymphatic duct 

 (de.), which either 

 ends blindly or 

 opens on the dor- 

 sal surface of the 

 head. The utricle 

 and saccule are 

 sometimes im- 

 perfectly differen- 

 tiated, and are 

 then spoken of 

 together as the 

 membranous vesti- 

 bule. 



Hi 



FlQ. 797. Section o! t 



i Sphenodon. g. blood- 



vessel ; h. cavity of eye, filled with fluid ; k. connective-tissue 

 capsule ; 1. lens ; M. molecular layer of retina ; r. layer of rods 

 and cones ; st. nerve ; x. cells in nerve. (From Wiedersheim's 

 Vertebrata, after Baldwin Spencer.) 



Patches of sensory cells (Fig. 799, ae.) elongated cells produced 

 into hair-like processes (a. h.) occur in the ampulla and in the 

 utricle and saccule : they are known as maculw acusticce and cristce 

 acusticce (c. r.), and to them the fibres of the auditory nerve (n.) 

 are distributed. A fluid, the endolymph, fills the whole of the 

 auditory organ or membranous labyrinth, and in it are formed otoliths 

 of varying size and number. There is every reason for thinking 

 that the labyrinth, like the otocysts or statocysts in the lower 

 animals, functions as an organ of equilibration as well as of hearing. 



