1894.] OX THE FLEXOR MTTSCLES IX BIRDS. 495 



2. On the Perforated Flexor Muscles in some Birds. 

 By P. Chalmers Mitchell, M.A., F.Z.S. 



[Eeceived May 30, 1894.] 



However opinions may differ as to the value of muscles in 

 classification, few would dispute that the ambiens muscle of birds, 

 in the peculiarity and isolation of its position and course, and in 

 the constancy of its relations, is an anatomical character difficult to 

 overlook in classification. The ambiens, as all anatomists know 

 from the researches of Grarrod, is a slender muscle which, after 

 origin from a spine or ridge immediately in front of, or below, the 

 acetabulum, runs along the inner side of the thigh to end in a thin 

 tendon which usually crosses the knee and joins the flexor 

 perforatus digitorum. Its presence and absence are associated 

 with so many other peculiarities of structure that Grarrod divided 

 all birds into the Homalogonatce, which possess the muscle in 

 question, and the Anomalogonatce, in which the ambiens is absent. 



While taking advantage of the abundant opportunities afforded 

 by the laboratory in the Society's Gardens, by the kindness of my 

 friend Mr. F. E. Beddard, the Society's Prosector, I have dissected 

 the leg- aud thigh-muscles in the following birds : — 



Balearica chrgsopelargus. 

 Haliaetus leucogaster. 

 Psoplila leucoptera. 

 Thaumalea amherstice. 

 Ftdiea leucoptera. 

 Leptoptfclus crumeniferus. 

 Palamedea cornuta. 



A ramicles gpecaha. 

 Hcematopus ostralegus. 

 Nyeticorax garden i. 

 Eclectus roratus. 

 Corvus capellanus. 

 Bubo maximus. 



In the first nine of these the ambiens is present, and the relation 

 of its tendon to the flexor perforatus digitorum is constant. In 

 these, as in other birds which I have dissected, the perforated 

 flexors lie immediately under the two " perforated and perforating" 

 flexors, those of the second and third digits. Fig. 1 (p. 496), which 

 I have drawn from a dissection of the Cape Crowned Crane, shows 

 an arrangement which is, in the main, typical of the other eight 

 birds. Distally, the three tendons pass respectively to the second, 

 third, and fourth toes. These tendons arise from a mass of 

 muscle innervated by that branch of the ischiadic nerve that also 

 supplies the middle head of the gastrocnemius muscle. The mass 

 of muscle has three distinct origins — an inner head, which arises 

 from the intercondylar notch very close to, and sometimes in 

 common with, the head of the flexor long as hallucis; an outer 

 head, from the outer condyle of the femur under and partly in 

 common with the origins of fche flexores perfarati et perforantes, and 

 from which a strong fibrous connection, sometimes double, runs to 

 the short arm of the biceps sling; and an ambiens head, sometimes 

 fleshy, sometimes tendinous, from the tendon of the ambiens. From 



