MUSCLES OF THE POSTERIOR LIMBS. 287 



"Relations. The gluteal aponeurosis is prolonged over the external 

 surface of the long vastus, becomes increased in thickness, and is more or less 

 elastic ; it strongly adheres to this muscle, and superiorly gives attachment 

 to a great number of its muscular fasciculi, Inwards, the long vastus 

 responds : To the aponeurosis of the superficial gluteus, which separates it, 

 for the most part, from the principal gluteus and the trochanter ; to the deep 

 layer of the fascia lata, which isolates it from the external vastus ; to the 

 anterior face of the patella, over which it glides by means of a small 

 synovial bursa, before becoming inserted ; to the external muscles of the 

 leg ; to the sciatic nerves ; to the great adductor of the thigh, and to the 

 semiinembranosus. Inferiorly, the semitendinosus is related to its internal 

 face, and more superiorly, it is in contact with the posterior border of this 

 muscle, whose sacro-ischiatic insertions it covers by its upper extremity. 



Action. The two portions of the long vastus do not act in the same 

 manner, and in this respect they are essentially distinct. The principal 

 division, pulling the patella outwards and the femur backwards, is an 

 abductor of the entire limb, and an extensor of the thigh ; while the posterior 

 portion merely determines the flexion of the leg and the tension of the tibial 

 aponeurosis. 1 



If the leg be the fixed point, this muscle, in contracting, causes the 

 pelvis to swing on the head of the femur, and in this way it plays an 

 important part in rearing. 



2. Semitendinosus. (Figs. 129, 10; 130, 14.) 



Synonyms. Ischio-tibialis medius or posticus Girard. (Posterior sacro-iseMo- 

 tibialis Leyh. Percivall describes this and the next muscle by the name of adductor 

 tibialis.) 



Situation Extent Direction. This muscle is situated behind the pre- 

 ceding, and, like it, extends from the sacral spine to the leg, describing a 

 curve whose convexity is posterior. 



Volume Form Structure. Less considerable than the long vastus, it 

 is elongated from above to below, bifid at its superior extremity, thick and 

 prismatic, but nevertheless compressed on both sides. Its muscular fibres 

 are of a pale-red colour, are parallel to each other, and follow the general 

 direction of the muscle ; they terminate, inferiorly, on an aponeurosis and on 

 a flattened tendon. 



AttacJiments.^ThiB muscle arises, above, by one of its branches from the 

 sacral spine and the sacro-ischiatic ligament, in common with the long vastus ; 

 by the other branch, which is the shortest, from the ischiatic tuberosity. 

 Its inferior aponeurosis is confounded with that of the tibia ; the tendon 

 glides over the internal surface of the tibia, and is inserted into its anterior 

 crest. 



Relations. Its sacro-ischiatic branch is covered by the gluteal aponeurosis, 

 and covers the long vastus. For the remainder of its extent, it responds : 

 posteriorly, to that aponeurosis ; anteriorly, to the sciatic nerves ; externally, 

 to the long vastus and gastrocnemius ; internally, to the semimembranosus 

 and the long adductor of the thigh. 



Action. It is a flexor of the leg, and tensor of the tibial aponeurosis, 



1 If it were wished to establish a comparison between the nature of the action of the 

 two portions of the long vastus, and their relations and attachments, their analogues in 

 Man could be easily determined. The anterior is undoubtedly a considerable portion of 

 the gluteus maximus, prolonged to the patella; the posterior represents the biceps 

 femoralis. 



