SENSORY ORGANS 



465 



2,7- 



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 re- 

 spects 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 dis- 

 tance 'in front of their 

 point of origin. 



The "membranous 

 labyrinth of the ear 

 (compare Fig. 59, p. 

 187) is also very simi- 

 lar to that of the frog 

 but being larger, and 

 the auditory capsules 

 being composed en- 

 tirely of cartilage, it 

 can be dissected out 

 with comparative ease 

 a knife. 



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 ; i. r. 

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

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

 and Haswell's Zoology.') 



by slicing away the capsule with 



A tube given off from the sacculus, called the endolymphatic duct 

 (Fig. 59), 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 communication 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 capsule, is often spoken of 



PRACT. ZOOL. H H 



