BIRDS 425 



adjusts the eye to under-water vision as it is drawn across immediately the head 

 is immersed (Ischreyt, 1913-14) ; it thus acts as the lens of a diver's spectacle. ^ 



The lacrimal gland with its single duct is ventre -temporal in 

 location being associated, as is usual, with the more active lid ; although 

 it is well developed in most water-birds, it is absent in the fully water- 

 adapted penguins (Impennes) and also in the owl, Bubo. The harderian 

 gland in its nasal position associated with the nictitating membrane, 

 secretes a thick oily fluid ; in the cormorants it is exceptionally 

 large and the secretion abimdant, acting probably as a protection 

 against sea-water. Meibomian glands are absent (Anelli, 1936). There 

 are two slit-shaped lacrimal puncta, a larger upper and a smaller lower 

 at the nasal canthus. 



The orbits are very large to accommodate the enormous eyes and 

 occupy a considerable proportion of the entire head (Fig. 533) ; as a 

 rule they meet in the median plane, being separated from each other 

 only by a thin bony interorbital septum (Bellairs, 1949). 



The orbits are open in type ^ resembling in their general form those 

 of Reptiles, particularly the tortoises ; it is to be remembered that the 

 lack of protection to the anterior part of the globe that results from this 

 configuration is to some extent compensated by the firm ring of im- 

 bricated scleral ossicles which encircles the sclera immediately behind 

 the limbus. 



Into this orbit the globe usually fits so snugly that the extra- 

 ocular muscles must perforce be small (Fig. 534) ; a retractor bulbi is 

 absent in Birds since the globe cannot be further retracted into a 

 cavity which it already fills. In consequence, ocular movements are 

 negligible or absent. As we shall see at a later stage, ^ this immobihty 

 of the eyes is compensated by the extreme mobility of the neck and the 

 constant movements of the head. Nevertheless, although the muscles 

 are tenuous, the four recti and the two obliques are normally repre- 

 sented, each being provided with the standard nerve supply charac- 

 teristic of the vertebrate phylum. 



Abelsdorff and Wessely. Arch. Augen- Barany, Berggren and Vrabec. Brit. J. 



heilk., 64, Erg., 65 (1909). Ophthal, 41, 25 (1957). 



Anelli. Boll. Oculist., 13, 1461 (1934); 14, Bartels and Dennler. Zool. Anz., 52, 49 



499 (1935). (1921). 



Ric. Morjol.. 15, 233 (1936). Beauregard. C. R. Soc. Biol. (Paris), 27, 

 Bacsich and Gellert. v. Graejes Arch. 132 (1875). 



Ophthal., 133, 448 (1935). Bellairs. J. Linn. Soc. London, 41, 482 

 Balducci. Monit. zool. Ital., 16, 258 (1949). 



(1905). Bernd. Inaug. Diss., Bonn (1905). 



1 p. 643. 



^ To this generalization there are exceptions, such as the Australian cockatoo, 

 Cacatua roseocapella (Prince, 1956). 

 3 p. 696. 



