MUSCULAR ACTIVITY 



113 



ing when acidified a greater affinity for water than they show 

 when their reaction is neutral. The cylindrical colloidal sarco- 

 styles which make up the actual contractile elements of muscle 

 are surrounded by watery sarcoplasm (p. 88). If these sarco- 

 styles are suddenly acidified we can picture a rush of water into 

 them from the surrounding sarcoplasm, which would cause them 

 to swell. 



Studies of the contraction process in the individual sarcostyles 

 of insects' wing muscles (p. 85) show that during contraction they 

 present a beaded appearance. This beading 

 could be brought about by a swelling of the 

 segments of which the sarcostyles are composed, 

 provided the membranes between the segments 

 remain undistended (Fig. 48). That such a 

 swelling might cause a forcible shortening can 

 be proven with the aid of a suitable model. A 

 similar swelling of the sarcostyles appears to 

 occur during the contraction of skeletal muscle. 

 If microscopic cross-sections of relaxed and con- 

 tracted skeletal muscle are compared the sarco- 

 styles of the relaxed specimen are seen to be 



smaller and further apart than are those of the of sarcostyles of in- 

 sects wing muscles. 



contracted one. This suggests a transfer of sarco- A, relaxed; B, con- 

 plasm from the spaces between the sarcostyles t] 

 into the sarcostyles themselves. When. we recall the minute size 

 of the elements and the correspondingly short distances through 

 which any given particles of fluid would have to pass we can under- 

 stand without difficulty how the contraction is able to occur in so 

 small a fraction of a second. 



Summary of the Contraction Process. The mechanism of con- 

 traction of skeletal muscle will be more readily grasped as a whole, 

 perhaps, if summarized briefly. In the resting muscle there has 

 been some oxidation of fuel which has furnished energy for the 

 building up of a substance of high energy value, the "lactic acid 

 precursor." Stimulation of the muscle causes decomposition of 

 some of this precursor into lactic acid with the liberation of energy. 

 This energy is made available for the act of contraction through 

 the property the colloidal sarcostyles have of absorbing water 

 when acidified. The forcible absorption of water from the sur- 



B 



FIG. 48. Diagrams 



