$4 MORPHOLOGY OP THE ORGANS OF VERTEBRATES. 



forms the apparatus of accommo iation necessarv- for \-iewing 

 objects at different distances. 



In the lower vertebrates accessory- glands connected with 

 the e\ es are but slighth- 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 n the 

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

 this becomes di\-ided into two, — a Harder's gland near the 

 inner an^ie, a true lacar)'mal giand at the juter. In reptiles ^ 

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

 mammals the lachrvraal gland passes to the upper lid. The 

 Harderian gland, which iaas for its function the lubrication of 

 the nictitating membrane, becomes reduced in the mammalia. 

 The lachrjinal 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 tineir position as 

 the superior, inferior, external, and in- 

 temaL These arise from the bottom 

 of the orbit near the ii-amen for the 

 optic ner\"e. and are inserted at ab>;ut 

 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 ner^e. 

 and ser\-ing to pull the eye back into 

 its socket. In the sauropsida are also 

 muscles connected with the ni.tititing 

 membrane, but these are reduced or ab- 

 sorbed in the mammals. 



Epiphysial Structures. — 5e . eral structures which are con- 

 nected with that part of the primitive fore brain which sub^e- 



1 An interesting fact is the absence of lachrymal jlanii ia crocodiles. 



Fic;. 89. Eye ir-.i5cles 

 and reia:ed neires in sh^rk. 

 </, abduceriS nerve; •-'/, infe- 

 rior oblique ; os. sccenor 

 obliqne : :m. ocjlomotor 

 nrrre; re, rertus extemns; 

 ri. r. intemns; rif, i. i-:e- 

 rior ; rj, r. superior muicles. 



