72 LABORATORY DIRECTIONS 



of cartilage until all parts are exposed. A good dissection of 

 the shark ear requires patience and technique. 



Both ends of each semi-circular canal join the vestibule. 

 On one end of each is a small bulb, the ampulla. Observe 

 that the semi-circular canals are at right angles to one another. 

 The vestibule is divided into two regions which are not easily 

 distinguished. The larger dorsal part is the utriculus. The 

 semi-circular canals arise from this. The ventral portion is the 

 sacculus to which the endolymph duct is joined. 



The ear of the shark, as in other vertebrates, functions for 

 the reception of sound vibrations and as an organ of equilibrium. 

 In fishes the ear consists of an inner ear only; there is no middle 

 ear or external ear such as may be found in higher forms. 



Eye of the Shark 



Muscles of the Eye. — Remove the tissues from around 

 the eye of the shark on the same side of the head as the dissection 

 of the ear. Cut away the cartilage from above the eye, being 

 careful not to injure the brain or the nerves. If the dissection is 

 carefully made, six slender muscles that function in moving 

 the eyeball in various directions may be seen. Four of these 

 muscles originate from the posterior part of the orbit. These 

 are the rectus muscles. The two originating from the anterior 

 part of the orbit are the oblique muscles. The names of the 

 individual muscles indicate their position. 



a. The superior oblique is the more anterior of the two 

 muscles inserting on the dorsal side of the eye. It is supplied 

 by the fourth cranial nerve. 



b. The superior rectus inserts near the superior oblique on 

 the dorsal side of the eye. It is supplied by the third cranial 

 nerve. 



c. The inferior oblique Inserts on the ventral side of the eye 

 and may be seen by moving the eye backward and medially, 

 or by cutting the superior oblique. It is innervated by the third 

 cranial nerve. 



d. The external rectus Is Inserted on the posterior surface 

 of the eyeball. It Is supplied by the sixth cranial nerve. 



e. The internal rectus Inserts on the anterior end of the eye- 

 ball and Is Innervated by the third cranial nerve. 



