288 COMPARATIVE ANATOMY 



regulated by special muscles supplied by the abducent nerve, and 

 known in Reptiles and Birds as the quadratics (bursalis) and 

 pyramidalis, which are genetically related to the retractor bulbi. 



The nictitating membrane, which may enclose a cartilage, is 

 situated within the lower eyelid, or it may lie more towards the 

 anterior angle of the eye. The former condition is seen, e.g., in 

 Anurans and the latter in Birds, in which a third eyelid is so largely 

 developed as to be capable of covering the whole freely exposed 

 portion of the eye-ball. In Mammals also it is situated in the 

 anterior angle of the eye, and in Primates becomes reduced to a 

 small, half-rnoon-shaped fold (plica semilunaris). 



(c) GLANDS. 



The glands in connection with the eye are : (1) the lacrymal, 

 (2) the Hardtridn, or gland of the nictitating membrane, and (3) 

 the Meibomian glands. 



The secretions of all these serve to keep the free surface of the 

 eye-ball moist, and to wash away foreign bodies. In Fishes 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 development of a secretory apparatus in 

 connection with the eye. 



Thus in Urodeles a glandular organ is developed from the con- 

 junctival epithelium along the whole length of the lower eyelid ; in 

 Anurans and Reptiles this becomes more developed in the region 

 of the anterior, and in many Reptiles also of the posterior, angle 

 of the eye, the original connecting bridge gradually disappearing: 

 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, 1 

 which is situated at the anterior angle of the eye, surrounding 

 to a great or less extent the antero-ventral portion of the eyeball, 

 while the other gives rise to the lacrymal gland (Fig. 210). The 

 latter retains throughout life its primitive position at the posterior 

 angle of the eye, and even in Birds lies in the region of the 

 lower eyelid, and is supplied by the second division of the tri- 

 geminal. In Mammals it becomes gradually further subdivided, 

 and extends into the region of the upper eyelid, so that its ducts 

 open above the eye into the upper conjunctival sac. Neverthe- 

 less, even in the Primates, in which it consists of two parts, more 

 or fewer ducts are present which open into the lower conjuctival 

 sac, and thus the primitive position of the lacrymal gland is 

 indicated. 



The secretion usually passes by several ducts into the con- 



1 In Crocodiles, Snakes, and Hatteria the lacrymal gland is wanting, while 

 in Chelone it is extremely large. 



