PROGRESSIVE MOTION IN SERPENTS. 453 



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 auxiliary instru- 

 ments 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 

 whom Sir Joseph Banks pointed out this cir- 

 cumstance, 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 pecu- 

 liar, and has evidently been employed with re- 

 ference to this particular function of the ribs, 

 which here stand in place of the anterior and 

 posterior extremities, possessed by most verte- 

 brated animals, and characterising the type of 

 their osseous fabric. In the ordinary structure^ 

 the head of each rib has a convex surface, that 

 plays either on the body of a single vertebra 

 with which it is connected, or upon the two 

 bodies of adjacent vertebrae : but in serpents the 

 extremity of the head of the rib has two slightly 

 concave articular surfaces, which play on a 



* Philos. Trans, for 1812, p. 163. 



