56 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



such simple arrangement, and a study of each one is necessary to understand 

 its action. The term "muscular system" generally refers to the voluntary 

 muscles. 



For the study of the muscles frogs which have been preserved for some 

 time in formalin should be employed. Those in which the viscera have been 

 dissected are usable for this purpose. Remove the skin completely. 



1. Parts and relations of a muscle. Examine the large gastrocnemius muscle 

 on the back of the shank as an illustration of a typical muscle. Identify its 

 parts as follows: 



a) The fascia is the shining tough connective tissue membrane which incloses 

 the muscle. 



b) The tendon is the shining tough band or cord at each end of the muscle. 

 It is produced by the extension and concentration of the fascia beyond the 

 fleshy part. By means of tendons, muscles are firmly attached to bones, other 

 tendons, or other fixed structures. Tendons may be of many shapes, depending 

 upon the shape of the muscle of which they are a part. 



c) The belly is the fleshy part of the muscle. 



d) The origin is the more fixed point of attachment of the muscle, i.e., the 

 part which does not move when the muscle contracts. The gastrocnemius has 

 two points of origin, or heads, as they are often called, at the upper end of its 

 (apparently) dorsal surface. The larger of these attaches the main mass of the 

 muscle to a tendon which passes from the distal end of the femur to the upper 

 end of the tibio-fibula; the smaller is a slender tendon which joins the general 

 tendon passing over the knee. With a forceps loosen the other muscles from 

 about the heads of the gastrocnemius and verify these points. 



e) The insertion is the more movable point of attachment of the muscle, 

 i.e., the part which is moved by the action of the muscle. The lower end of 

 the gastrocnemius tapers to a strong tendon (the famous tendon of Achilles), 

 which passes over and is attached to the ankle bones and then becomes continuous 

 with the broad plantar fascia which covers the sole of the foot. 



/) The action of a muscle is a description of its function. The gastrocnemius 

 through its insertion on the ankle can bend the entire leg below the knee up 

 against the thigh (flexion of the leg) ; through its continuity with the plantar 

 fascia it can straighten the foot (extension of the foot). Test these actions by 

 pulling on the gastrocnemius. 



2. General account of the muscles of the frog (Holmes, chap, xiv, pp. 246- 

 57). We shall undertake to study only the superficial and easily identifiable 

 muscles, particularly those which are of interest in the physiology of the frog. 

 Most of the following muscles have the same names, positions, and action as 

 in man and other vertebrates, and the statements about them are hence of general 

 application. No attempt is made to give all the details of the origin and insertion . 



