IKRITABILITY 



101 



sage of a fluid from one part of the cell to another part 

 so that its shape is changed. This would be a satis- 

 factory explanation were it not for the fact that an ap- 

 preciable time is necessary for the passage of the fluid, 

 and the muscles in the wings of 

 insects move far too fast to per- 

 mit it to occur. 



Stimuli. — A muscle contracts 

 in response to some outside 

 stimulus. This stimulus may be 

 physical, e.g. a blow, or a change 

 of temperature ; or chemical, e.g. 

 an acid or other irritating sub- 

 stance. Some muscles seem to 

 work automatically without the 

 intervention of an outside stim- 

 ulus, but their spontaneity is 

 only apparent. They are really 

 stimulated by some substance 

 present in the blood. The heart 

 is probably the most remark- 

 able of these muscles, for it con- 

 tracts rhythmically as long as there is life, and so long 

 as it is provided with adequate nourishment it seems 

 to thrive under the constant work. 



Rhythmical Contraction. — Muscles which normally 

 do not show rhythmical contraction will contract rhyth- 

 mically if excised and placed in a balanced solution of 

 sodium chloride and calcium chloride. These salts are 

 present in the blood in such proportion that the blood 

 is the balanced solution for the heart muscle. So long 



Fig. 61— Diagram showing ar- 

 rangement of muscle fibers in 

 relation to their tendons. A, 

 in muscles contracting 

 through a considerable dis- 

 tance; B, in muscles of shorter 

 and more powerful contrac- 

 tion; C, in muscles of very 

 short and powerful contrac- 

 tion. (From Fitz.) 



