



THE TISSUES 53 



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 con- 

 traction is sometimes called the isometric method, in distinc- 

 tion to the isotonie 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.} 



The contraction of muscles in the body of the mammal may 

 be studied by recording their thickening by Marey's muscle 

 forceps, in which a tambour placed between the limbs of a 

 pair of forceps is pressed upon by the contraction of a muscle 

 or group of muscles lying between the opposite limbs, and 

 transmits the pressure to another tambour which carries a 

 recording lever. (Practical Physiology. ) 



II. The Factors modifying the Contraction 



1. Kind of Fibre. In skeletal muscles the pale fibres contract 

 more rapidly and completely than the red fibres, which contain 

 more sarcoplasm and nuclei. The peculiarities of the con- 

 traction of visceral muscles will be considered later (p. 70). 



2. Species of Animal. In vetebrates the contraction of the 

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

 traction in cold-blooded animals. The most rapidly contracting 

 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 sluggishly. At first each contraction is greater in 

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

 fatigue becomes manifest, and stimulation finally fails to call 



