218 



COMPARATIVE ANATOMY 



FIG. 173. HARDERIAN 

 GLAND (H, H l ) AND 

 LACHRYMAL GLAND (Th) 

 OF Anguis fragilis. 



M, muscle of jaw ; B, eye- 

 ball. 



Dipnoans, 1 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 Reptiles this becomes more 

 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 differentiated 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 lachrymal gland 2 



(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- 



lit 



Fro. 174. DIAGRAMMATIC TRANSVERSE 

 VERTICAL SECTION THROUGH THE 

 EYE OF A MAMMAL. 



junctival sac, and thus the primi- 

 tive position of the lachrymal 

 gland is indicated. 



A well-differentiated Har- 

 derian gland is present from the 



tailless Amphibia to the Mammalia, but is very rudimentary in 

 the Primates. 



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 t becomes continuous with the 

 conjunctiva ; T, the so-called tarsal 

 fibre-cartilages, in which the Meibo- 

 mian glands (MD) lie embedded, 

 the latter opening at * ; //, H, eye- 

 lashes. 



1 Comp. p. 17. 



- A lachrymal gland is absent in Crocodiles and Snakes. 



