296 PHILOSOPHY OF ZOOLOGY. 



and Sir E. HOME have since attended to their structure 

 more particularly *. 



The osseous system of serpents is chiefly characterised 

 by the articulation of the vertebrae, the number of ribs, and 

 the absence of the extremities. The head is connected 

 with the atlas by a single tubercle, having three articular 

 surfaces. The posterior articular surface of each vertebra 

 is in the form of a rounded eminence, which is received in- 

 to a corresponding depression in the anterior end of the 

 one which follows. There is thus throughout the vertebral 

 column a series of ball and socket joints, admitting a great 

 extent of flexure, in some cases restrained by the spinous 

 or articular processes. The ribs in some species amount 

 to nearly three hundred pairs. The dorsal extremities of 

 the ribs are enlarged, with a terminal depression, which re- 

 ceives a rounded protuberance of the vertebra. Each verte- 

 bra, therefore, has a rib on each side, limited by a modified 

 ball and socket-joint. These ribs are not continued beyond 

 the anus. They embrace a great part of the sides, and 



* The last of these observers, .when considering their use, offers the fol- 

 lowing remarks : As amphibious animals, in general, have no glands to 

 supply the skin with moisture from within, but receive it by coming in con- 

 tact with moist substances, it is possible the bags, in the snake, may be sup- 

 plied in that manner, and the more so, as the cuticular lining appears per- 

 fect. Another peculiarity is remarkable in snakes furnished with the bags 

 described above, namely, an oval cavity, situated between the bag and the 

 eye, the opening into which is within the inner angle of the eyelid, and di- 

 rected towards the cornea. In this opening there are two rows of projec- 

 tions, which appear to form an orifice capable of dilation and contraction. 

 From the situation of these ova! cavities, they must be considered as reser- 

 voirs for a fluid, which is occasionally to be spread over the cornea; and 

 they may be filled by the falling of the dew, or the moisture shaken from 

 jfche grass, through which the snake passes." Phil. Trans. 1804, p. 74. 



