314 THE MECHANICAL FUNCTIONS. 



raise themselves without difficulty to the tops of the highest 

 trees, and escape to their hiding places with a quickness 

 which eludes observation and baffles the efforts of their pur- 

 suers. 



The solution of this enigma is to be sought for partly in 

 the structure of the skin, which, in almost every species, is 

 covered with numerous scales: and partly in the peculiar 

 conformation of the ribs. The edges of the scales form rough 

 projections, which are directed backwards, so as to catch the 

 surfaces of the bodies to which they are appUed, and to pre- 

 vent any retrograde motion. In some species, the integu- 

 ment is formed into annular plates, reminding us of the struc- 

 tures so prevalent among worms and myriapode animals. 

 Each scale is connected with a particular set of muscular 

 fibres, capable of raising or depressing it, so that, in this way, 

 it is converted into a kind of toe ; and thus the body rests 

 upon the ground by numerous fixed points of support. 



This support is farther strengthened by the connexion of 

 the ribs with the abdominal scuta, or the scales on the under 

 side of the body. The mode in which the ribs become aux- 

 iliary instruments of progressive motion was first noticed by Sir 

 Joseph Banks.* Whilst he was watching the movements of a 

 Coluber of unusual size which was exhibited in London, and 

 was moving briskly along the carpet, he thought he saw the ribs 

 come forward in succession, like the feet of a caterpillar. Sir 

 Everard Home, to wham Sir Joseph Banks pointed out this 

 circumstance, verified the fact by applying his hand below 

 the serpent, and he then distinctly felt the ends of the ribs 

 moving upon the palm, as the animal passed over it. The 

 mode in which the ribs are articulated with the spine is pe- 

 culiar, and has evidently been employed with reference to 

 this particular function of the ribs, which here stand in place 

 of the anterior and posterior extremities, possessed by most 

 vertebrated animals, and characterizing the type of their os- 

 seous fabric. In the ordinary structure, the head of each 

 rib has a convex surface, that plays either on the body of a 



♦ Philos, Trans, for 1812, p. 163. 



