60 THE FROG CHAP. 



upwards or towards the thigh, the foot will instantly be 

 bent backwards, so as to come into a straight line with 

 the shank, the action being one of those performed by 

 the living frog when leaping. It will be seen that the 

 proximal tendon is attached to a relatively fixed point : 

 it is distinguished as the tendon of origin, or the 

 muscle is said to arise from the femur and tibio-fibula. 

 The distal tendon is attached to a relatively movable 

 part, the foot, and is called the tendon of insertion, 

 the muscle being said to be inserted into the plantar 

 fascia. 



Muscular Contraction. Obviously, however, there is 

 nothing to pull upon the muscle from outside in the living 

 frog. We must, therefore, try to form some idea as to 

 how the action of bending the foot, roughly imitated in 

 the dead subject, is performed during life. If the gastro- 

 cnemius be exposed in a recently killed frog, fhe foot 

 bent up as before, and a smart pinch be given to the 

 belly of the gastrocnemius, the foot will be bent back, 

 although no pull has been exerted on the muscle. The 

 same thing will happen if you drop on the gastrocnemius 

 a single drop of weak acid or of a strong solution of 

 common salt, or if you touch it with a hot wire, or if 

 you apply to it the electrodes from an induction coil so 

 as to pass an electric current through it. 



Careful observation shows that what happens under 

 either of these circumstances is that the belly of the 

 muscle decreases in length and at the same time increases 

 in breadth, so as to become shorter and thicker 

 (Fig. 17) . The result of this must necessarily be to cause its 

 two ends to approach one another. As the tendon of 

 origin is attached to the femur, which we suppose to be 

 fixed, it is unable to move, and the insertion is therefore 

 drawn upwards, bringing with it the movably articulated 

 foot. In fact exactly the same thing takes place as 



