40 THE SKATE 



the choroid coat to the front of the eye where it separates 

 from the cornea, forming a black curtain, the iris, which 

 may be seen through the transparent cornea and in the 

 center of which may be seen the opening or pupil. 



The iris divides the cavity of the eyeball into two cavities, 

 an external cavity the anterior chamber between the iris 

 and the cornea and the vitreous humor cavity between the 

 lens and the retina. The anterior chamber is filled with 

 the aqueous humor while the vitreous humor cavity con- 

 tains in life a gelatinous vitreous humor. In life the lens 

 is attached to the margins of the pupil and the margins 

 of the retina, leaving a small space between these two points 

 of attachment called the posterior chamber. 



Drawing 22. Make a diagram of the cross-section of the eye. 



After the eyeball is removed from the orbit note (1) the 

 origin of the six eye muscles, (2) the optic pedicle for 

 support of the eyeball which is a cartilaginous stalk, 

 situated among the rectus muscles, and (3) the optic 

 nerve, a stout white nerve in front of the rectus muscles. 

 On the floor of the orbit is the infraorbital nerve. 



C. The Ear. 



The internal ear of the skate is located between the spira- 

 cle and the mid-dorsal line where there is an elevation of 

 the chondocranium. A pair of small holes is present in the 

 median line between these elevations. If the internal ear 

 has been destroyed on one side during the dissection of 

 the eye use the other side, being careful not to injure the 

 brain. 



Remove the skin from the region of these holes and trace 

 the entrance of the endolymphatic ducts which pass 

 through the fossae in the chondocranium. These ducts con- 

 nect the cavity of the internal ear with the exterior. 



