SENSORY ORGANS 



465 



are only present in Vertebrates which possess lungs. 

 The sacs are lined by the olfactory epithelium, which 

 is supplied by the olfactory nerves and is raised up into 

 ridges so as to increase the surface. 



The structure of the eye, as well as of the accessory 

 apparatus in connection with it, is in all essential respects 

 the same as in the frog (p. 181), except for the differences 

 in the eyelids (pp. 5 and 432), the absence of a 

 lacrymal apparatus (p. 186), and for the fact that the 

 four recti muscles 

 (Fig. 126) do not en- 

 sheath the optic 

 nerve, which emerges 

 into the orbit* a short * 

 distance in front 

 of their point of ff . r 

 origin. 



The membranous 

 labyrinth of the ear 

 (compare Fig. 59, p. 

 187) is also very 

 similar to that of the 

 frog, but being larger, 

 and the auditory cap- 

 sules being composed 

 entirely of cartilage, 

 it can be dissected 



out with comparative ease by slicing away the capsule 

 with a knife. 



A tube given off from the sacculus, called the endolym- 

 phatic duct, which in the frog communicates with the 

 lymphatic system, opens to the exterior on the top of the 

 head in the dogfish, and thus the endolymph is in free com- 

 munication with the surrounding sea-water. 



As we have seen, the membranous labyrinth is the 

 essential part of the ear, and it, together with its enclosing 



FIG. 126. Semidiagrammatic figure of the eye- 

 muscles and their nerves of an Elasmobranch. 

 ///. oculomotor, IV. pathetic, and VI. abducent 

 nerve ; e. r. posterior rectus muscle ; i. o. in- 

 ferior oblique ; in. r. inferior rectus ; t. r. 

 anterior rectus ; or. wall of orbit ; s. o. superior 

 oblique ; s. r. superior rectus. (From Parker 

 and Haswell's Zoology.) 



