MUSCULAR SYSTEM OF AVES. 103 



unusual extension, like that of the preceding, with which it is 

 posteriorly continuous : it comes off aponeurotic, from the anterior 

 and superior margin or labrum of the ilium; the fibres soon 

 become fleshy, and the muscle diminishes in breadth and increases 

 in thickness as it descends ; it is inserted by short and strong 

 tendinous filaments obliquely into the anterior part of the tendon 

 of the broad rectus, and into the anterior and inner part of the 

 head of the tibia. Its insertion is partly covered by the internal 

 head of the gastrocnemius. It bends and adducts the thigh, and 

 extends the leg. 



The homologue of the biceps flexor cruris^ is a unicipital 

 muscle, corresponding with the abductor magnus, by the removal 

 of which it is exposed, in the characteristic modification of its 

 extended origin, in relation to the great antero-posterior develope- 

 ment of the pelvic bones. Orig. By a broad and thin aponeurotic 

 tendon, which at first is confluent with that of the abductor, but 

 soon becomes distinct, from the posterior prolongation of the 

 ilium : there is no second head from the femur. Ins. The fleshy 

 fibres converge as they descend along the back and outer part of 

 the thigh, and finally terminate in a strong round tendon, which 

 glides through a loop formed, as in the common Fowl, Ostrich, 

 &c., by a ligament extended from the back of the outer condyle 

 of the femur to the head of the tibia, and is inserted into the 

 process on the outside of the fibula. By means of the loop the 

 weight of the hinder parts of the body is partially transferred, 

 when the leg is bent, to the distal end of the femur ; and the 

 biceps is enabled, by the same beautiful and simple mechanism, 

 to effect a more rapid and extensive inflection of the leg than it 

 otherwise could have produced by the simple contraction of its 

 fibres. 



The semimembranosus'^ arises from the side of the caudal 

 vertebrae, and from the posterior end of the ischium ; it crosses 

 the superficial or internal side of the semitendinosus. It is in- 

 serted into the fascia covering the gastrocnemius and the inside 

 of the tibia : through the medium of the fascia it acts upon the 

 tendon of the internal gastrocnemius. 



The semitendinosus^ arises from the posterior and outer part 

 of the sacrum and the aponeurosis connecting it with the ischium : 

 it is a flattened triangular muscle, which receives the square 

 accessorius muscle from the lower and posterior part of the femur. 

 It gradually diminishes as it descends, and having passed the 



' xr. vol. iii. pis. 31, 32, k. ^ lb. pis. 32, 35, l. ^ lb. pis. 32, 3.5, m. 



