THE TISSUES 55 



weight the muscle can lift, and the absolute force of a muscle 

 may be expressed by the weight which is just too great to be 

 lifted. The lifting power of a muscle depends primarily upon 

 its thickness or sectional area. The absolute force of a muscle 

 may therefore be expressed per unit of sectional area. In 

 man the absolute force per 1 sq. cm. is from 5000 to 10,000 

 grams. The force of contraction is, however, modified by so 

 many other conditions that no definite figure can be given. 



The force of contraction during different parts of the con- 

 traction period may be recorded by making the muscle pull 

 upon a strong spring, so that it can barely shorten. The 

 slight bending of the spring may be magnified and recorded 

 by a long lever, and in this way it is found that the ordinary 

 curve of contraction gives a fair representation of the varia- 

 tions in the force. 



This method of recording the force of contraction is some- 

 times called the isometric method, in distinction to the 

 isotonic method of letting the muscle act on a light lever. 

 In clinical medicine the DYNAMOMETER is used for measuring 

 the force of muscular contraction. (Practical Physiology, 

 Chap. V.) 



II. The Factors modifying the Contraction. 



1. Kind of Fibre. In skeletal muscles the pale fibres con- 

 tract more rapidly and completely than the red fibres. The 

 peculiarities of the contraction of visceral muscles will be 

 considered later (p. 66). 



2. Species of Animal. In vertebrates the contraction of 

 the muscles of warm-blooded animals is more rapid than the 

 contraction in cold-blooded animals. The most rapidly con- 

 tracting muscles are met with in insects. 



3. State of the Muscle. (1) Continued Exercise. If a 

 muscle is made to contract repeatedly, the contractions take 

 place more and more slowly. At first each contraction is greater 

 in extent, but, as the contractions go on, the extent diminishes 

 as fatigue becomes manifest, and finally stimulation fails to 

 call forth any response. This condition is probably caused 

 by the accumulation of the products of activity \\\ t.TiP TY^^IA 

 acting as poisons upon its protoplasm, for the same pheno- 



