1042 DR. p. CHALMERS MITCHELL ON THE 



the tendon lias been dissected out of its synovial groove, it 

 appears too long for its purpose, but if its action be tested before 

 this dissection has been made, it is clear that the straightening of 

 the foot is carried out by the action on the sustentaculum and 

 that subsequently the third toe is bent by the special slip. 



The arrangement is odd and very striking, and it is extremely 

 difficult to believe that it has been independently developed in the 

 large number of different groups of birds in which it occurs. In 

 my opinion the disposition of the muscle and its tendons shown 

 in Gliauna can be explained best if it be regarded as the spe- 

 cialized derivative of a condition in which the perforated flexor 

 muscles and their tendons were continuous wath each other and 

 with the peroneus ; as the foot and its digits became specialized, 

 the fasciae of insertion became strengthened along the lines 

 leading to each digit, and finally by the disappearance of tiie 

 connecting fascite the continuous sheet became broken up into 

 separate tendons ; as the tibio-tarsus became longer the peroneal 

 slip to the tendon of digit III assumed its odd course. All the 

 stages in the possible differentiation of continuous fasciae into 

 highly specialized tendons still exist in the case of the alar 

 tendons of birds. The differentiation of the foot, however, must 

 have taken place before the groups of birds became separated, 

 and, as I shall show later, the various conditions of the peroneus 

 in the different groups of birds can all be explained as differ- 

 entiations of the condition in Chauna, in particular by the loss 

 of some part. 



Pej'oneus profimdifs seu hrevis (Tibialis posticus of R. W. Shu- 

 feldt, " Myology of the Raven," 1870, p. 228).— This is a much 

 simpler and more slender muscle ai'ising fleshy from the fibula 

 and the adjacent surface of the tibia on their anterior aspects, 

 beginning about the level of the insertion of the biceps tendon, 

 and extending a little beyond the distal extremity of the fibula, 

 where it passes into a stout rounded tendon (text-fig. 179, P.BEE). 

 The greater portion of the muscle is concealed by the edges of the 

 tibialis anticus and of the superficial peroneal which meet above 

 it. The tendon as it aj)proaches the end of the tibio-tarsus is 

 vei-y close to the bone and is deep of the peroneus longus tendons ; 

 it flattens out, runs through a synovial channel, crosses the tarsal 

 joint overlain by a ligament (text- fig, 179, Tar. lig.), which also 

 crosses the joint, and is inserted to the proximal edge of the tibio- 

 tarsus on its posterior, outer aspect (text-fig. 179, P.B.I). In 

 Chcntna it also has a second attachment to the antero-median end 

 of the tarsal shaft (text-fig. 179, P.B. 2). 



This muscle flexes the tarsus-metatarsus on the tibio-tarsus, and 

 at the same time slightly rotates the foot, depressing the axial, or 

 great toe side, and raising the abaxial, or fourth toe side. The 

 latter action varies with the exact position of the insertion and 

 is often very feeble, as, indeed, is the action of the muscle as a 

 whole. 



