GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 107 



Place the subject in a sitting position, make the suspended leg immovable 

 by suitable clamps, strap a board to the bottom of the foot, and connect the 

 toe end of the board with a weight by means of a cord passing over a pulley. 

 As the weight is increased or decreased the foot is more or less flexed, and 

 the gastrocnemius muscle is stretched more or less. A pointer fastened to 

 the foot-board moves over a scale and indicates the amount the muscle 

 changes its length when subjected to various weights. Mosso reports that 

 though the curve of elasticity has about the same character as that of isolated 

 frog muscle, the curve of extensibility is different, each of the added weights 

 causing greater amount of stretching. This is probably due to the fact that 

 a muscle within the body is always being influenced by the central nervous 

 system. Its length at any given moment is due not only to its elasticity as 

 compared with that of its antagonist, but also to the strength of the nervous 

 impulses, reflex and voluntary (often unintentional), coming to it. The sub- 

 ject would have to be under an anaesthetic or in very deep sleep for such an 

 experiment to give a true picture of its elasticity. Mosso describes, in fact, 

 movements of the foot aeeompanying the respirations, due to variations in 

 the tonus impulses coming to the muscles in inspiration and expiration. In 

 spite of the innate difficulties of such an experiment, we can ascertain that 

 in general the conclusions arrived at by studying the isolated muscles of a 

 frog apply to the muscles when in the living body. 



The elasticity of a muscle within the normal body suffices to preserve the ten- 

 sion of the muscle under all ordinary conditions. The muscles are attached to 

 the bones under elastic tension, as is shown by the separation of the ends in case 

 a muscle be cut. This elastic tension is very favorable to the action of the 

 muscle, as it takes up the slack and ensures that at the instant the muscle 

 begins to shorten the effect of the change shall be quickly imparted to the 

 bones which it is its function to move. The extensibility of the muscle is 

 a great protection, lessening the danger of rupture of the muscle-fibres and 

 ligaments, and the injury of joints when the muscles contract suddenly and 

 vigorously, or when they are subjected to sudden strains by external forces. 

 The importance of extensibility and elasticity to muscles which act as antag- 

 onists is evident. When a muscle suddenly contracts against a resisting force 

 such as the inertia of a heavy weight, the energy of contraction, which puts the 

 muscle on the stretch, is temporarily stored in it as elastic force, and as the 

 weight yields to the strain, is given out again; thus the effect of the contrac- 

 tion force is tempered, the application of the suddenly developed energy being 

 prolonged and softened. Elasticity is very important to the function of the 

 non-striated muscles of the blood-vessels, bladder, intestine, etc. This is 

 especially true of the sphincter muscles, for it is an important factor in 

 securing the continued tension characteristic of their action. 



4. Influences which Affect the Activity and Character of the Con- 

 traction. — («) The Character of the Mused-. — Attention has been called to 

 the fact that irritability and conductivity may be different not only in different 

 kinds of muscle-tissue, and in muscles of different animals, but even in similar 



