242 VERTEBRATE ANIMALS. 



The most striking of these characters of the snakes is to 

 be found in the nature of the organs of locomotion. The 

 fore-limbs are invariably altogether wanting, and there is no 

 pectoral arch nor breastbone ; and the hind-limbs are either 

 totally absent or are at best rudimentary and never exhibit 

 any outward evidence of their existence, beyond the occasion- 

 al presence of short, horny claws or spurs. In the entire ab- 

 sence, then, or rudimentary condition of the limbs, the snakes 

 progress by means of the ribs, which are always excessively 

 numerous, and, in the absence of a breastbone, are also ex- 

 tremely movable. Their free ends, in fact, are simply attached 

 by muscular fibres to the scales or " scutes," which cover the 

 lower or abdominal surface of the animal. The number of 

 ribs varies from 50 up to 320 pairs, and, by means of this 

 arrangement, the snakes are able to progress rapidly, walking, 

 as it were, upon the ends of the ribs. Their movements are 

 also much assisted by the extreme flexibility of the whole 

 spine, caused by the cup-and-ball articulation of the bodies of 

 the vertebrae, each of which is concave in front and convex 

 behind (proccelous). 



Of the other characters of the snakes, a few words may be 

 said as to the tongue, the eye, and the teeth all important 

 structures in this order. The tongue, in serpents, is probably 

 more an organ of touch than of taste, and consists of two 

 muscular cylinders, which are united toward their bases. The 

 forked organ thus formed can be protruded and retracted at 

 will, being in constant vibration when protruded, and being 

 in great part concealed by a sheath when retracted. The eye 

 of serpents (Fig. 121, A) is not protected by any eyelids, and 

 hence the peculiar stony and unwinking stare for which these 

 reptiles are celebrated. In place of eyelids, the outer layer 

 of the skin is prolonged over the eye as a continuous and 

 transparent film, behind which is a chamber formed by the 

 mucous covering of the eye, into which the tears are dis- 

 charged. The outer membrane is periodically shed along with 

 the rest of the external or epidermic layer of the integument, 

 and is again renewed. The pupil is round in most serpents, 

 but it forms a vertical slit or fissure in the venomous snakes 

 and in the Boas. 



As regards the teeth, it is to be noticed that the snakes 

 are not in the habit of chewing their prey, but of swallowing- 

 it whole, and the construction of their dental apparatus is in 

 accordance with this peculiarity. The lower jaw, as before 

 said, articulates with the skull by means of a quadrate bone 



