xiv THE MUSCULAR SYSTEM 255 



flexors of the back of the thigh. A flexion of the crus 

 stretches the tibialis anticus longus on the anterior side of 

 the crus and thereby brings about a flexion of the foot. If 

 a muscle pulls the femur backward, the triceps femoris is put 

 on the stretch : this tends to straighten the leg, the ten- 

 dency becoming stronger the farther back the thigh is pulled. 

 Extending the leg causes the gastrocnemius to be put on a 

 strain, and the tension is increased through the pull on the 

 tendon this muscle receives from the lower end of the 

 triceps. Through the pull on the gastrocnemius the foot is 

 extended. Other muscles are of course brought into play 

 in this process, but what has been said will illustrate the 

 principle on which coordination of movements employed in 

 jumping or swimming is effected. The relative intensity of 

 the nervous impulses distributed to the various muscles is 

 an important factor in determining the kind of movement 

 the limb will make, but the basis for the unity of action of 

 the parts in the ordinary movements of the limb lies in the 

 structural arrangement of the bones and muscles. 



As an extended treatment of the remaining portions of 

 the muscular system lies beyond the scope of this work, only 

 a few of the more noteworthy muscles will be described. On 

 the ventral side of the body the large rectus abdominis ex- 

 tends from the pubis, to which its fibers converge behind, to 

 the sternum in front. Its two halves are separated in the 

 middle line by the tinea alba, and there are five connective 

 tissue septa (inscriptiones tendincz] which cross it transversely 

 and divide it into segments. 



The obtiquus externus is a large muscle covering most of 

 the sides of the body. It originates on the sides of the 

 ileum and dorsal fasciae above, and is inserted into the sides 

 of the rectus abdominis. Its fibers extend obliquely upward 

 and forward. 



