VII THE CONTRACTION OF MUSCLE 299 



6. The Phenomena of Muscular Contraction, —Every 

 fibre in a muscle has the property, under certain con- 

 ditions of shortening in length, while it increases corre- 

 spondingly in width, so that the volume of tlie fibre 

 remains unchanged. This property is called muscular 

 contractility, and whenever, in virtue of this property, 

 a muscle fibre contracts it tends to hruuj its two ends 

 closer together. Since a muscle is m«ade up of a collection 

 of these fibres, when the fibres contract the muscle as a 

 whole also contracts ; it becomes shorter and thicker, and 

 brings its two ends closer together, along with whatever 

 may be fastened to those ends. By this action the 

 muscles lead to the motion of the parts to which they are 

 attached and by these motions give rise to locomotion. 



The condition which ordinarily determines the con- 

 traction of a muscular fibre is, the passage along the 

 nerve fibre, which is in close anatomical connection with 

 the muscular fibre, of a nervous impulse, i.e., of a 

 particular change in the substance of the nerve which is 

 propagated from particle to particle along the fibre. The 

 nerve fibre is thence called a motor fibre, because, by 

 its influence on a muscle, it becomes the indirect means 

 of producing motion (see Lesson XI.). 



The phenomena of muscular contraction may be con- 

 veniently studied in the muscles from the calf of a frog's 

 leg, which, since the frog is a "cold-blooded" animal, 

 retains its powers of contracting for some time after it is 

 removed from the body. This muscle is called the 

 gastrocnemius and may be dissected out so as to be 

 still attached to a piece of t\\e femur near the knee and to 

 the nerve, the sciatic, wliich supplies it. This muscle 

 as thus taken out of the body is known as a muscle-nerve 

 preparation (Fig. 90). 



The muscle may now be suspended by the femur and a 

 weight hung on to the temlon at its lower end, and then 

 made to contract by stimulating the sciatic nerve (see also 

 Lesson XL). 



