60 THE NECTURUS 



where they are needed, either on the exterior or in the 

 interior. The sense organs of the skin can be studied only 

 in specimens prepared for histological examination. The 

 special senses are the nose, eye, ear, together with a 

 number of other senses that are not so easily studied. 



Nose 



The nose is supplied by the olfactory or first cranial 

 nerve. The nerve spreads out fan shaped in the nose 

 region to serve this organ. The narial passage leads 

 posteriorly to the back part of the roof of the mouth and 

 external to the splenial teeth. 



Dissect out the nose and determine the shape of the 

 cavity. 



Eye 



The eye is rather small since the animal is more or less 

 of a bottom feeder. After the skin is removed from the 

 head, study the eye in position. A sheet of muscle located 

 between the eyeball and the skull protrudes the eye, while 

 a retractor bulbi draws it into the head. 



Dissect around the eye so as to see the muscles clearly. 



Drawing 21. Make a drawing showing these muscles in position. 



They may be named from their position on the eyeball. 

 Try these muscles by pulling gently to see their effects in 

 moving the eyeball. 



After the muscles have been determined in position, 

 loosen them from the skull and remove the eyeball. Turn 

 to the inner side and make a drawing large enough to 

 show the muscles in position on the eyeball. 



The muscles of the eye are six in number with an addi- 

 tional retractor bulbi. The rectus superior is located at 

 the anterior end of the eye; the rectus inferior at the 

 posterior end ; the rectus externus at the outer edge of the 

 ball; the rectus internus on the inner edge; the superior 



