THE MUSCLES. 87 



CHAPTER IX. 



OF THE MUSCLES. 



Muscl::s, with tlieir tendons, are the instruments by 

 which animal motion is performed. It will be our business 

 to point out instances in wliich, and properties with respect 

 to which, the disposition of these muscles is as strictly me- 

 chanical as that of the wires and strings of a puppet. 



I. We may observe, what I believe is universal, an exact 

 relation between the joint and the muscles which move it. 

 Whatever motion the joint by its mechanical construction 

 is capable of performing, that motion the annexed muscles 

 by their position are capable of producing. For example, 

 if there be, as at the knee and elbow, a hinge-joint, capable 

 of motion only in the same plane, the leaders, as they are 

 called, that is, the muscular tendons, are placed in direc 

 tions parallel to the bone, so as, by the contraction or relax- 

 ation of the muscles to which they belong, to produce that 

 motion and no other. If these joints were capable of a freer 

 motion, there are no muscles to produce it. Whereas, at the 

 shoulder and the hip, where the ball-and-socket joint nllo\x& 

 by its construction of a rotary or sweeping motion, tendons 

 are placed in such a position, and pull in such a direction, 

 as to produce the motion of which the joint admits. For 

 instance, the sartorius or tailor's muscle,* rising from the 

 spine, running diagonally across the thigh, and taking hold 

 of the inside of the main b&ne of the leg a little below the 

 knee, enables us, by its contraction, to throw one leg and 

 thigh over the other, giving effect at the same time to the 

 ball-and-socket joint at the hip, and the hinge-joint at the 

 knee. There is, as we have seen, a specific mechanism in 



■* Plate III., Fig. 1. The sartorius^ a, is the longest muscle of 

 the human system. It is extended obliquely across the thig}^ from the 

 fore part of the hip to the inner side of the tibia. Its office is to bend 

 the knee and bring the leg inwards. 



