424 ADAPTATIONS TO MEDIA AND SUBSTRATES 



Snakes — The snakes show a maximum of modifications, of which the 

 spectacle (Fig, 154, p. 456) is the most conspicuous — the others being 

 consequences of its presence. The lacrimal gland has disappeared, and the 

 enormous Harderian gland lies beneath and behind the eyeball. Its duct 

 opens directly into the nasolacrimal canal, which has a single aperture 

 (spectacled lizards have two) in the conjunctival sac, at the nasal side. 

 As in the lizards, the distal end of the nasolacrimal duct opens within 

 the nasal cavity, inside the accessory olfactory Vomeronasal organ' (of 

 Jacobson). The Harderian secretion then proceeds to the mouth cavity 

 and contributes substantially to the saliva, which in snakes must lubricate 

 the prey thoroughly for swallowing. Rudimentary-eyed snakes such as 

 Typhlops and Rhinophis have even lost the connection of the nasolacri- 

 mal duct with the conjunctival sac, and the Harderian duct opens into 

 the mouth independently of Jacobson's organ, to facilitate still further 

 the strange function of the Harderian gland as an accessory salivary 

 organ. 



Birds — In birds, the lid opening reveals only the small cornea, so that 

 one is easily misled as to the true size of the eyeball, and receives 

 quite a shock upon skinning a bird for the first time! In this class of ver- 

 tebrates, mobility of the upper lid reappears, in nearly half of all species. 

 Most of these are in the higher orders, the ostrich being a conspicuous 

 exception. The lower lid has a fibrous tarsus (except in parrots) , but the 

 nictitans has none, and is more perfectly transparent than in reptiles 

 (except crocodiles, where its exceptional clarity would seem to go with 

 nocturnality) , In the owls and dippers, however, the nictitans is cloudy. 

 Its inner surface is always covered by an epithelium whose surface cells 

 are built like unicellular feathers, which improve its cleansing action. 

 These, incidentally, are imitated in some lizards by peculiar epithelial 

 papillae; and the lizards have produced imitation hairs as well as feathers, 

 for some (e. g., Eublepharus, Coleonyx) have 'eyelashes', manufactured 

 from scales. 



A large bursalis is present but the pyramidalis has been retained, not 

 abandoned as by the lizards — if indeed they ever had one (Figs, 142 and 

 143f). The nictitans- tendon may have a very long path, because of the 

 breadth of the globe equatorially, to reach the muscles which operate it. 

 In the owls the eye is so long, and the orbit so snug, that the tendon 

 courses along a groove, and over a pulley, on the surface of the eyeball 

 (Fig, 144). As in most reptiles, the lids can be closed without the eye 



