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PHYSIOLOGY. 



Names of Muscles. Some muscles are named from 

 their shape, as the deltoid on the shoulder ; from position, 

 pectoralis major ; from their supposed action, as sartorius 

 and adductor ; direction, as rectus, etc. The biceps and 

 triceps are named from their division at their origins. 



Peculiar Muscles. The diaphragm is a sheet of muscle 

 that forms a partition between the chest and the abdomen. 

 It is arched, and has a clear tendinous center. The ab- 

 dominal muscles form a wall to hold the organs of the 

 abdominal cavity. These muscles also aid in breathing, 

 especially in forced expiration, as after violent exercise 

 and in coughing. The abdominal wall consists of several 



layers of muscle. 



Heart Muscle. The fibers 

 which make up heart muscle 

 are different in appearance from 

 either the striated or smooth 

 muscle fibers. They are more 

 or less branched, as shown in 

 the accompanying figure. No 

 sheath has been found on these 

 fibers. 



The Three Kinds of Muscular 

 Fibers Compared. For the 

 sake of comparison, the striated 

 and unstriated muscle fibers are 

 here shown again, alongside the 

 tschweigger-seideu heart muscle fibers. The stri- 



The nuclei and cell-junctions are only 

 represented on the right hand side ated fibers (of the skeleton) are 



usually called "voluntary," and 



the plain fibers "involuntary." The heart muscle fibers 

 are intermediate, being striated, but involuntary in their 



