84 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



forms the apparatus of accommodation necessary for viewing 

 objects at different distances. 



In the lower vertebrates accessory glands connected with 

 the eyes are but slightly developed, but with the assumption of 

 a terrestrial life (amphibia) lachrymal glands for lubricating 

 the surface appear. These arise as inpushings of the epider- 

 mis near the lids. In the lower amphibia the glands are on the 

 lower side of the eye and form a continuous series ; but higher 

 this becomes divided into two, a Harder's gland near the 

 inner angle, a true lachrymal gland at the outer. In reptiles 1 

 and birds these remain on the lower side of the eye, but in 

 mammals the lachrymal gland passes to the upper lid. The 

 Harderian gland, which has for its function the lubrication of 

 the nictitating membrane, becomes reduced in the mammalia. 

 The lachrymal duct has already been mentioned (p. 76). 



The eye is provided with muscles which move it as a whole. 

 Some of these are remarkably constant through the whole verte- 

 brate series. There are four rectus 

 muscles, known from their position as 

 the superior, inferior, external, and in- 

 ternal. These arise from the bottom 

 of the orbit near the foramen for the 

 optic nerve, and are inserted at about 

 equal distances around the ball. The 

 two oblique muscles (superior and in- 

 ferior) arise in front of the rectus mus- 

 cles, and are inserted on the ball above 

 and below the internal rectus. Besides, 

 there may be a well-developed retractor 

 bulbi attached near the optic nerve, 

 and serving to pull the eye back into 

 its socket. In the sauropsida are also 

 muscles connected with the nictitating 

 membrane, but these are reduced or ab- 

 sorbed in the mammals. 



Epiphysial Structures. Several structures which are con- 

 nected with that part of the primitive fore brain which subse- 



1 An interesting fact is the absence of lachrymal glands in crocodiles. 



FIG. 89. Eye muscles 

 and related nerves in shark. 

 a, abducens nerve; oi y infe- 

 rior oblique ; os, superior 

 oblique; oni, oculomotor 

 nerve; re, rectus externus; 

 ri, r. internus ; rif, r. infe- 

 rior ; rs, r. superior muscles. 



