1876'.] ANATOMY OF CHAUNA DERBIANA. 195 



2. The presence or absence of a special muscular slip from the 

 Biceps humeri to the Patagium. 



The biceps humeri, the maiu flexor of the arm, arises from the 

 upper end of the coracoid hone, and from the upper portion of the 

 flexor surface of the humerus. In certain birds this muscle sends 

 off from its upper end a slender fusiform belly, which runs through 

 the proximal portion of the patagium to join its marginal tendon 

 near the middle of its course (Plate XIV. fig. 2). The presence or 

 absence of this muscular fasciculus is a very constant character among 

 closely allied birds. In the Table (p. 199) are recorded the names 

 of all those birds in which, according to my experience, it is to be 

 found. The only Anomalogonatous birds in which I have seen it 

 are the Caprimulgidce. 



3. The Area of Origin of the Obturator internus. 



It is not my intention on the present occasion to enter into the 

 consideration of whether the muscle here called obturator internus is 

 homologous with the same-named muscle in Mammalia ; suffice it to 

 say that it arises from the pelvic surface of the pubis and ischium, 

 and ends by a tendon which is inserted into the outer surface of the 

 head of the femur. 



In a large number of birds^ on looking at the pelvic view of this 

 muscle when undisturbed, its shape is seen to be an elongated oval, 

 occupying the obturator fossa, and covering the line of junction of 

 the ischium and pubis. In another large number of birds, instead 

 of being oval it is triangular, its posterior fibres expanding in such a 

 way as to cover most of the pelvic surface of the ischium. There 

 are a few birds in which an intermediate condition is found ; they 

 are, however, very few. In most there is not the least difficulty in 

 deciding whether the obturator internus is oval or triangular (oom- 

 pare Plate XV. figs. 1 and 2). From the Table (p. 199) the ar- 

 rangement existing in most birds can be found. 



4. The degree of Development of the Tensor-cruris fascice. 

 To this point I have referred in my paper on the muscles ot 

 Birds*, where its relations are explained. "It is the superficial 

 muscle of the outside of the thigh, covering the femur. It is flat 

 and triangular in shape, and arises as a membranous expansion 

 which covers the gluteus ii., from the lower two thirds of the pos- 

 terior border of the iliac fossa in which that muscle is situated, and 

 from the fibrous septum which separates that muscle from the 

 gluteus iii. Further down it has origin also from the whole length 

 of the ridge which separates the postacetabular area from the ex- 

 ternal lateral surface of the ischium, and which may be termed the 

 postacetabular ridge, as well as from the posterior border of the 

 ischium, as far forwards as its junction with the pubis, being here 

 slightly overlapped by the semitendinosus. The fibres converge 

 towards the knee ; and the deep portion of the muscle blends in its 

 course with the vastus externus, together with which it continues 

 * P.Z. s. is;:;. ,,. 628 



13* 



