OF THE RUDIMENTS OF POSTERIOR EXTREMITIES. 39 



the internal piece, the most developed of all, has more or 

 less the figure of .S, may be compared to the tibia, and 

 extends with a free end just within the abdominal cavity. 

 The middle piece, on the other hand, which represents the 

 tarsus, is thick, short, a little arched, and is completely 

 concealed in the flesh. The muscles vrhich move this ap- 

 paratus we are describing, have a very simple structure : 

 the principal are an extensor, with its antagonist a flexor : 

 both are inserted near the upper end of the tibia, and are 

 attached to the tarsus ; the extensor on the anterior face 

 near the nail, the flexor at an apophysis jDlaced in the 

 middle of the posterior face of that bone. Two other 

 muscles less marked, suspended from the flesh, and at- 

 tached to the two little accessory pieces of the tarsus, 

 produce the lateral movement : the adductor, Avhich is 

 directed towards the abdomen, is a good deal less than 

 the abductor, the fibres of which are prolonged on the 

 sides of the back. We find in the Boa, besides these 

 muscles, a second very small flexor, placed at the internal 

 surface of the tibia and tarsus, and contributing to the 

 movements of these two pieces. The apparatus represent- 

 ing the posterior extremities in other serpents provided 

 with them, with some modifications excepted, are precisely 

 on the model of that in the Boa. 



We are still ignorant of the use of these vestiges of ex- 

 tremities. Their small size forbids us to suppose that 

 they can contribute to locomotion. It has been stated 

 that they might aid copulation : an opinion to which we 

 can only oppose the fact, that the two sexes are equally 

 provided with them. Other naturalists have maintained 

 that they serve to cling more closely to objects which the 

 wi-eaths of the tail or of the trunk embrace ; and this, per- 

 haps, is the most probable opinion. 



or THE MOVEMENTS. 



After having described the organs of locomotion, we 

 shall speak of the divers movements which serpents per- 

 form. In complete repose, these reptiles love to roll the 

 bodies in a spiral, in such a manner that the head alone. 



