158 VERTEBRATES, 



In the same manner put some of the vitreous humor on 

 a printed page. How does it affect the appearance of the 

 page? 



Return to the eye and observe the inside of the ball. 

 Find a filmy gray mass on a black coat. This is the expan- 

 sion of the optic nerve, called the retina, over the black cho- 

 roid coat of the eye. Do you find these two coats all over 

 the eyeball? Stick the needle from the outside through the 

 optic nerve into the inside. Where does it penetrate the 

 choroid coat? Scrape the black coating off of the interior 

 of the eye and see if there is anything left. This coat is 

 tough and durable and gives shape to the eye. It is the 

 sclerotic coat. Make a large diagram showing a section of 

 the eye from the front to the rear, including all of the parts 

 studied. 



. The Skeleton. — (i) With a scalpel cut down by the 

 side of the dorsal fin to the ribs. Remove the flesh from 

 both sides of the fish from the head to the caudal fin. 



Note the arrangement of the muscles under the skin 

 and the position of the small bones. Are the spines of the 

 dorsal fin attached to the central back bone, or skeletal axis? 

 Are the ventral and pectoral fins attached to the backbone? 



(2) The bones to which the pectoral fins are attached 

 form a ring around the body called pectoral, or shoulder 

 girdle. Are these bones attached on the dorsal side? On 

 the ventral? What is the advantage of this girdle? 



(3) Do you find a similar girdle at the ventral fins? 

 Make out its parts as before. This is the pelvic girdle. 



After cleaning away the entire skeleton remove a vertebra 

 from the spinal column about half way between the pectoral 

 and the ventral fins. 



