16 LABORATORY STUDIES IN ZOOLOGY 



the calf of the leg is selected for study. Its name means stomach 

 plus leg, which is a reference to the enlarged belly of this muscle. 

 Lay out the frog with its dorsal side upward. Pull the right 

 foot caudad so as to straighten the leg. The gastrocnemius will 

 be found to occupy the whole inner border of the shank between 

 knee and heel. With an orange stick or blunt instrument sepa- 

 rate it from the adjacent muscles. The origin of the gastro- 

 cnemius is by two tendinous heads, one, a short clearly visible 

 tendon extending to a broad sheetlike tendon spreading over the 

 distal end of the femur and knee joint. This is the equivalent 

 mechanically to an attachment upon the end of the femur itself. 

 The other head is atta.ched by a short tendon exposed by 

 rotating the shank slightly and exposing the "hollow " of the knee. 

 Here the tendon of this head joins other tendons which are 

 attached to the end of the femur. Thus the two heads of the 

 gastrocnemius are seen to have their origin virtually upon the 

 femur. This muscle tapers toward the heel where a strong 

 tendon extends over the ankle joint to become inserted upon the 

 sole of the foot. 



In the origin, extent, and insertion of the gastrocnemius muscle 

 one sees provision made for its action which is that of flexing the 

 leg at the knee and extending the foot. In addition to its double 

 function one sees that it extends over two joints (knee and ankle) 

 instead of one. Muscles which extend over two joints and possess 

 a double function act in conjunction with other muscles, all of 

 which increases their mechanical advantage. This is to be cor- 

 related with providing the power for leaping. 



Label this muscle and its parts upon the plate showing the 

 muscles of the dorsum. Upon the plate point out the reasons 

 for regarding the gastrocnemius as a two -joint muscle with a 

 complex action. Label all parts to which reference is made. 



The action of a muscle is the movement or movements in the 

 body resulting from its contraction; that is, what the muscle 

 does is spoken of as its action. When a part of the body bends, 

 as, for example, the leg bends at the knee, it is said to be flexed 

 and the muscle or muscles concerned with the flexing are called 

 flexors. When a part of the body is straightened it is said to be 

 extended and the muscle or muscles participating in such move- 

 ments are called e.rtensors. With the proper attachments a 

 given muscle may serve as a flexor for one part and at the 

 same time as an extensor for another. Muscles may, there- 



