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 vfhich 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 placed 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, which 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 
wreaths of the tail or of the trunk embrace ; and this, per- 
haps, is the most probable opinion. 
OF 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, 
