xiil ANATOMICAL STRUCTURE 583 



haemapophyses on their centra for the more effective attachment 

 of the muscles. All the vertebrae, except the atlas, carry ribs. 

 These articulate by their capitular portions only, and are very 

 movable in a head- and tail-ward direction. The ribs being long, 

 and fitting with their ventral ends into the connective tissue of 

 the sides of the ventral transverse scales, are the principal agents 

 in pushing the body forwards, the posterior edges of these scales 

 being sharp and imbricating. 



The skin is covered with scales, absolutely devoid of osteo- 

 derms. "\Mien the scales are enlarged they are called shields. 

 The keel, a common feature, is caused by a slight ridge of 

 the cutaneous part of the scale. The whole skin is covered 

 with a thin layer of horny epidermis, which is shed frequently, 

 at least several times in one year ; the shedding begins at the 

 lips, and the whole outer skin is turned inside out from head to 

 tail, retaining every minute detail of the cutaneous scales ; even 

 the watch-glass-like covering of the eyes is preserved. 



The eyes are peculiar in so far as they possess no lids. The 

 latter are still present in a vestigial condition in the embryo, but 

 their place is taken by what is probably a modification of the 

 nictitating membrane, which is drawn over the eye and 

 covered with a single transparent scale of the horny skin, like a 

 watch-glass. The eyes themselves are not movable. The 

 " tears," which* of course cannot appear on the outside, are 

 drained off into the nasal cavities by the naso-lacrymal ducts. 



The ear is likewise peculiar. There is a long columellar rod 

 with a fibrous or cartilaginous pad at the outer end, which plays 

 against the middle of the shaft of the quadrate, an arrangement 

 which, we must assume, produces a thundering noise in the 

 internal ear, since every motion of the quadrate during the act 

 of swallowing conveys the vibrations directly to the fenestra 

 ovalis. The tympanic cavity, the Eustachian tubes, and the 

 tympanum are abolished, and no external traces of the ear are 

 visible. However, in spite of all this. Snakes can hear very well. 



The nose is well developed, and many Snakes, for instance 

 the Grass-Snake, are guided to their prey as much by the sense 

 of smell as by the eyes and ears. The tongue is slender, very 

 protractile and bifid, always moist, and furnished with many 

 sensory corpuscles. It acts entirely as an additional sense-organ, 

 hence the incessant play of the tongue of a snake which wants 



