80 GENERAL ZOOLOGY 



the hindlimb forward in a ventral direction. The rectus internus 

 major {gracilis major) is a large muscle occupying most of the 

 inner half of the ventral surface of the thigh, marked by an 

 oblique tendinous insertion across its middle. It arises from the 

 posterior border of the ischium and is inserted at two points in 

 the head of the tibiofibula. Its contraction draws the thigh 

 backward and may flex or extend the crus, depending upon the 

 degree of bend in the knee joint at the time of contraction. Thus, 

 if the crus is partially extended, contraction of the rectus major 

 internus will extend it still more. If the crus is at an angle of 

 less than a right angle, contraction of the muscle will flex the 

 crus. The rectus internus minor (gracilis minor) is a long narrow 

 muscle at the inner edge of rectus internus major, having its 

 origin in the ischium and its insertion in one of the tendons of the 

 rectus internus major. The actions of the two internal recti 

 muscles are similar (Fig. 47). 



Between the sartorius and the rectus internus major a portion 

 of the adductor magnus can be seen (left side of Fig. 47). If the 

 sartorius and rectus muscles are removed, the adductor magnus 

 is fully exposed (right side of Fig. 47). Just distal to its origin 

 in a tendon attached to the pubis and ischium, this muscle 

 divides into two parts, between which passes the tendon of one 

 of the heads of the semitendinosus muscle. Distally the two 

 parts of the adductor magnus reunite and join with a small slip 

 from the semitendinosus. The adductor magnus is inserted in 

 the distal end of the femur and it acts either as an abductor or 

 adductor, depending upon the position of the thigh at the begin- 

 ning of contraction. If the centers of its origin and insertion lie 

 in a line behind the head of the femur, contraction of the muscle 

 adducts the thigh, i.e., draws it back to the mid-line. If this 

 line lies in front of the head of the femur, contraction draws the 

 thigh forward, i.e., away from the mid-line, and ventrally. 

 The semitendinosus, located to the inner side of the adductor 

 magnus, arises by two distinct heads from the pubis, the anterior 

 head dividing the adductor magnus near the head of the latter. 

 Its insertion is a tendon which, with that of the sartorius, forms 

 a connective tissue arch joined with the fascia of the crus, beneath 

 which the tendons of the internal recti muscles pass. 



The adductor longus arises from the ventral part of the ilium 

 and is inserted with the adductor magnus in the middle of the 



