218 



COMPARATIVE ANATOMY 



Fig. 173. — H.4kdbkiax 

 Gland (H, H^) .and 

 Lachrymal Glax'd [Th) 

 OF Anguis fraij'dis. 



M, muscle of jaw ; B, eye- 

 ball. 



M]}- 



Dipnoans,^ the outer medium appears to suffice for this purpose, 

 but the first attempt of a Vertebrate to exchange an aquatic for an 

 aerial existence necessitated the develop- 

 ment of a secretory apparatus in connection 

 with the eye. 



Thus in Urodeles a glandular organ is 

 developed from the conjunctival epithelium 

 along the whole length of the lower eye- 

 lid ; in Keptiles this becomes mote 

 developed in the region of the anterior 

 and posterior angles of the eye, and the 

 original connecting bridge gradually dis- 

 appears : thus two glands are developed 

 from the primitively single one, each of 

 which becomes further ditferentiated both 

 histologically and physiologically. From 

 one is formed the Harderian gland, which always lies at the 

 anterior angle of the eye, sur- 

 rounding to a greater or less ex- 

 tent the antero-ventral portion of 

 the eyeball, while the other gives 

 rise to the laclirymal gland ' 

 (Figs. 173 and 175). The latter 

 retains throughout life its primi- 

 tive position at the posterior 

 angle of the eye, and even 

 in Birds lies in the region of 

 the lower eyelid ; it is supplied 

 by the second division of the 

 trigeminal. In Mammals it be- 

 comes gradually further sub- 

 divided, and extends into the 

 region of the upper eyelid, so 

 that its ducts open above the 

 eye into the upper conjunctival 

 sac (Fig. 175, A & b). Never- 

 theless, even in the , Primates, 

 more or fewer ducts are present 

 which open into the lower con- 

 junctival sac, and thus the jDrimi- 

 tive position of the lachrymal 

 gland is indicated. 



A well-differentiated Har- 



'•ij}kOp 



Fk;. 174. — DiagramjMatic Transverse 

 Vertical Section through the 

 Eye of a JIammal. 



Op, optic nerve ; B, eyeball ; Fo, Fo, 

 upper and lower conjunctival sac ; 

 LH, LH, outer skin of the eyelids, 

 which at the free edges of the latter 

 at + becomes continuous with the 

 conjunctiva ; T, the so-called tarsal 

 fibro-cartilages, in -Mhich the Meibo- 

 mian glands (MD) lie embedded, 

 the latter opening at *" ; H, H, 'eye- 

 lashes. 



derian gland is present from the 



tailless Amphibia to the Mammalia, but is very rudimentary in 



the Primates. 



^ Comp. p. 17. 



" A lachrymal gland is absent in Crocodiles and Snakes. 



