27 
consequence of their being dried and mounted, the little appendage, 
which is concealed by the hair, was not to be perceived; but in the 
Alactaga, as well as the same circumstances would permit, I could see 
that a little horny process existed, but was rough and blunt. 
In the dissection of an animal whose only mode of progression con- 
sists of leaping with the hinder extremities, and which differs from 
the other jumping Mammalia in the circumstance, that in the position 
of rest the extremity only of the metatarsus is applied to the ground, 
the muscles of the leg may be expected to afford some points of in- 
terest. The most striking of these are, that none of the muscles situ- 
ated upon the tibia remain fleshy for more than about half the length 
of that bone, each terminating in a long tendon; and that upon the 
foot itself there are no muscles whatever, the actions of the flexors 
of the toes being relieved by a strong ligament, which arises from the 
os calcis, and divides into five, giving one to the middle toe, two small 
sesamoid bones being developed in it ; and two divisions to each of the 
other toes, the index and the annularis, each of which has also its 
sesamoid bones, those furthest from the axis of the foot being rather 
largely developed, extending some distance over the sides of the arti- 
culation. The ligament near its origin contains three little supernu- 
merary bones, one on the outer, two on the inner side ; the latter are 
grooved for the passage of the tendon of the flexor perforans. On 
the homology of this tendon I have next to remark. It might very 
naturally be expected, that in animals having no thumb on the hinder 
extremity, and in which the fibula is in great part wanting, the flexor 
longus pollicis, which in man has its origin in the fibula, would be 
either much reduced or absent ; but so far from such being the case, 
it will be seen, on reference to any work on the comparative anatomy 
of the muscular system, that this muscle exists, and that its tendon 
becomes entirely confluent with that of the flexor longus digitorum. 
But further, I think it will appear that in those lower Mammalia, in 
which the thumb or the fibula, or both, are wanting or imperfectly 
developed, it is the flexor longus digitorum that is reduced in size, and 
the flexor longus pollicis that becomes the principal muscle acting on 
the toes. The dissection of the Jerboa made this homology very 
evident. The large flexor muscle which gives the perforating tendons 
to the toes arises, as may be expected, partly from the tibia as well 
as from the fibula; but it is distinctly shown to be the flexor longus 
pollicis, from the fact that its tendon passes through a distinct sheath, 
separate from and posterior to that which contains the tendons of the 
other two muscles, namely the flexor longus digitorum and the tibi- 
alis posticus. Of these, which are both very small, the former shows 
its homology most clearly, by arising from the surface of the tibia, 
immediately below the insertion of the popliteus. The tibialis posticus 
is an extremely minute and delicate muscle, arising only from the 
tibia. 
In the Rabbit the two perforating flexors form a single muscle, 
having the proper origins of both ; lower down they become to a cer- 
tain extent separable, but the tendons are completely reunited defore 
they pass the ankle, which they do in the place belonging to the 
