III. THE MUSCULAR SYSTEM 



Much of the laboratory work for the rest of the course will 

 consist of dissection. Dissection means separating the parts so 

 that they may be more easily seen; very little cutting is required. 

 Blunt instruments and the fingers often make the best dissecting 

 tools for separating structures. Follow directions carefully. 

 Be sure that you know what you are going to do before you do it. 

 In case of doubt, ask the instructor. 



Muscles of the Shark 



A detailed comparative study of muscles is very unsatis- 

 factory because of the great variability in arrangement and 

 function, even among closely related species. However, the 

 primitive arrangement of muscles should be clearly understood. 

 In the lowest vertebrates, as in amphioxus, the greater part of the 

 external musculature consists of a series of zigzag bands on each 

 side of the body called myomeres, each separated from the next 

 by a connective tissue septum, the myocomma. Each myomere 

 is composed of many muscle fibers which run from one myo- 

 comma to the next. 



From the shark remove a narrow strip of skin from the dorsal 

 to the ventral side in the region of the pelvic fin, and observe the 

 primitive arrangement of the muscles. In the vicinity of the 

 lateral line a connective tissue septum divides the myomeres 

 into a dorsal epaxial region and a ventral hypaxial region. The 

 muscles of the two sides of the body never overlap but are 

 separated in the mid-ventral line by a connective tissue partition, 

 the linea alba. 



Where paired limbs occur some of the myomeres are modified 

 in character and position to form the musculature of the limbs. 

 Remove the skin from the pelvic girdle and expose a mass of 

 muscle originating from the myomeres of the body and inserting 

 on the fin. By cutting through this muscle mass its origin from 

 the myomeres beneath may be readily seen. In land verte- 



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