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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 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 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 bott 

 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 Wild Swan. By John Davy, M.D., F.R.S. 



L. & E., Inspector-General of Army Hospitals, etc. 



(Communicated by Mr. Gulliver.) 



When engaged in examining anatomically this bird (a full-grown 



female, killed in the neighbourhood of Chatham' in February 1 839), 



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 pygerythrxis). 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. 



