Zoological Society. 139 
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 pesticus 
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 before 
they pass the ankle, which they do in the place belonging to the 
flexor longus pollicis. This compound muscle, occupying the whole 
posterior surface of the bones of the leg, so pushes round the tibialis 
posticus, that it takes the chief part of its origin from the inner side 
of the tibia, which in Mammalia generally is free from muscular 
attachment. In the Paradoxurus I found that the flexor longus 
digitorum has, in addition to its usual attachments, a point of origin 
in the head of the fibula; but then the bones are separate, and the 
flexor longus pollicis is a distinct muscle, having also origin in both 
bones, and each tendon passes the ankle in its usual place*. 
March 13.—W. Yarrell, Esq., Vice-President, in the Chair. 
The following papers were read :— 
1. NOTICE OF A PECULIARITY OF STRUCTURE OBSERVED IN THE 
AORTA OF THE WiLp Swan. By Joun Davy, M.D., F.R.S. 
L. & E., INspecror-GENERAL oF ArRMy HoOspiIrALs, ETC. 
(ComMMUNICATED By Mr. GULLIVER.) 
When engaged in examining anatomically this bird (a full-grown 
female, killed m the neighbourhood of Chatham in February 1839), 
my attention was arrested by a peculiar appearance in the inferior 
* Since writing the above I have taken opportunities of looking at the same 
muscles in a Fox and in a Monkey (Cercopithecus pygerythrus). The former animal 
differed from the Paradoxurus, and resembled the Jerboa, in the great extent of the 
flexor longus pollicis and the much-reduced size of the tibialis posticus, which here 
also terminates in a long slender tendon, showing an interesting correspondence of 
adaptive character in two animals, in which the motion of the hind-limbs is vi- 
gorous, but of one kind only. In the Monkey the flexor longus pollicis is a much 
larger muscle than the flexor longus digitorum, and has considerable attachment 
to the tibia. 
Meckel and Cuvier allude to the union of the two long flexors in the Rabbit 
before they pass the ankle, but neither author informs us at which point that takes 
place. 
