XXXI. MUSCLES 



Muscles and Movement. — We are all aware that motion in any 

 of the higher animals is caused by the action of the muscles. 

 These contract and expand, thus giving the required amount of 



movement. In man and the other vertebrate 

 animals the muscles are fastened to bones, which, 

 acting as levers, give wide range of motion. 

 Study of the muscles in the leg of a frog will 

 help us to a better understanding of the subject. 



Study of the Muscles of a Frog. — (Material pre- 

 served in 70 per cent alcohol or 4 per cent formol is 

 best for this purpose.) Remove the skin from the leg 

 of a frog by stripping it downward as you would take 

 off a glove. Notice the yellowish-white muscles which 

 form the "meat" of the frog's leg. Notice that they 

 are more or less separated into bundles each of which 

 tapers at the end. I^ook for dark-colored blood vessels 

 and tiny white nerves which lead into the bundle of 

 muscles. The wide part of the muscle is called the belly. 

 The glistening white part which attaches the muscle 

 to the bone is called the tendon. Notice the large 

 muscle forming the calf of the leg (the gastrocnemius). 

 Try to make out what movements are performed by 

 means of this muscle. (Pull it and note the resulting 

 movements.) 



Arrangement of Voluntary Muscles in the 



Muscles of the left leg 

 of the frog; b, M. 



biceps; g, M. gas- Humau Body. — Muscles are usually placed in 



trocnemius; sw, M, . " 



semimembrano- pairs; oue. Called the exteusor, serv^es to straighten 



sus; <r. M. triceps. ^^^ .^.^^. ^^^ ^^^^^^ ^j^^ ^^^^^^ ^^^^^^ ^^^ j^-^^^ 



Try to find examples of flexor and extensor muscles in the leg of 

 the frog. Locate, by means of feeling the muscles when expanded 

 and contracted, the extensors and flexors in your own arm. 

 This paired arrangement of muscles is of obvious importance, a 

 flexor muscle balancing the action of an extensor on the other 

 side of the joint. The end of the muscle that has the wider 

 movement in a contraction is called the insertion; the part th^-t 



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