6o 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 moveable 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, the 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 circurristances 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 moveably articulated foot. In fact exactly the same 



