244 VERTEBRATE ZOOLOGY 



Inserted in the eyeball just back of the inferior oblique is a 

 broad muscle which passes straight back to the hinder part of the 

 orbit ; this is the external rectus muscle. Along its posterior border 

 lies the lachrymal gland ; remove the gland, and also any fat that 

 may be present, and sharply define the sides of the muscle. On 

 each side of it is one of the four heads of the retractor muscle. 



Just beneath the middle of the inferior oblique is the insertion 

 of the inferior rectus muscle, which passes straight back along the 

 ventral side to the hinder part of the orbit. 



Dorsal to the external rectus and separated from it by the head 

 of the retractor muscle just mentioned is the superior rectus 

 muscle, which arises in the hinder portion of the orbit and passes 

 straight forward to its insertion in the dorsal surface of the 

 eyeball. Passing over the outer surface of this muscle and 

 parallel with it is a narrow strand of muscle, which goes to the 

 margin of the upper eyelid ; it is called the levator muscle of the 

 upper eyelid. 



Just in front of the insertion of the superior rectus is that of 

 the superior oblique muscle. Its tendon runs at right angles to 

 that of the superior rectus toward the dorsal margin of the inner 

 side of the orbit, where it passes through a fibrous loop ; here its 

 direction is changed and it goes anteroventrally to the body 

 of the muscle that lies against the medial side of the eyeball. 

 Its origin is on the inner wall of the orbit, just in front of the 

 optic foramen. 



Cut the tendon of the superior oblique, press the eye ventrally, 

 and find, on the medial side of the eyeball, the internal rectus 

 muscle, which extends from its insertion near the dorsal margin 

 of the eyeball straight back to its origin in the inner wall of the 

 orbit. On each side of it is one of the heads of the retractor muscle. 



Exercise 35. Draw a diagram showing the eyeball and its muscles. 



Pull the eyeball forward and cut the optic nerve and the muscles. 

 Remove the eyeball from the orbit and cut away the muscles and 

 fat still attached to it. It will be seen to be a spherical structure, 

 the anterior surface of which is somewhat flattened. Its outer 

 covering is the sclerotic coat, a thick, tough membrane of con- 



