224 VETERINARY PHYSIOLOGY 



measured by finding what weight the muscle can Uft 

 (Practical Physiology). 



The absolute force of a muscle may be expressed by the 

 weight which is just too great to be lifted by it. The lifting 

 power of a muscle depends — (1) upon its thickness or 

 sectional area, i.e. on the number of fibres. The absolute 

 force of a muscle may therefore be expressed per unit of 

 sectional area. In mammals the absolute force per 1 sq. cm. 

 is probably 5000 to 10,000 grams. 



(2) Upon the length of the muscle tuhe7i stirtiulated. 

 In the body a muscle may be in different conditions of 

 lenofth between its origin and insertion. The force of con- 

 traction or tension is greatest when the muscle is extended 

 slightly beyond its greatest normal length, and rapidly 



Fig. 110. — To show the relationship of tension developed to the length of 

 the muscle. The abscissa represents the length of the muscle, | is the 

 normal length in the body. The curve represents the tension developed. 



decreases if it is shorter, e.g. when the brachialis anticus is 

 stimulated in partial flexion of the elbow. 



The relationship of length to the tension developed is 

 shown in fig. 110. This relationship is of importance, for 

 it indicates that the development of tension is a surface 

 tension phenomenon. 



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 somewhat untrue representation 

 of the force, inasmuch as the lever, from its inertia, is carried 

 too high and seems to represent a continuance of tension 



