ELEMENTARY EXPERIMENTAL PHYSIOLOGY 29 



ment, have fallacies inseparable from the method of recording them, it 

 is possible to make two rough deductions from them : 



(1) The amount of actual shortening a muscle undergoes during con- 

 traction can be calculated by measuring the vertical height of the top of 

 the curve above the base line and dividing it by the magnification ; in 

 Fig. 32 the height is 20 mm., and the magnification 5, therefore the 

 muscle became shorter by 4 mm. The length of the resting muscle 

 when loaded by lever and weight was 25 mm., consequently the 

 muscle during contraction became shorter by 4 x -^, i.e. nearly a 

 sixth of its original length. 



(2) The amount of work done by the muscle during its contraction is 

 the product of the load and the height to which it was raised, W = L x 

 H. In Fig. 32 the actual load which the muscle raised was not the 

 whole of the 30 grams, hung near the axis of the lever, but a proportion 

 of it, calculated by multiplying by the distance from the axis of the point 

 of the suspension of the weight, and dividing by the distance from the 

 axis of the point of attachment of the muscle ; this fraction was J, and 

 the actual load lifted 6 grams. The height to which it was raised wa& 

 4 mm. ; consequently the work performed was 24 gramme millimetres. 



CHAPTER IV. 



THE CONDITIONS WHICH AFFECT SINGLE MUSCULAR 

 CONTRACTIONS. 



(a) Different Muscles, (b) Veratrine. The curve produced by the 

 contraction of a muscle may be altered not only by such influences as 

 temperature, load, fatigue, and drugs, but also by the differences in 

 structure of various muscles. The muscular fibres of the frog are 

 found to present two varieties, clear and granular, which differ both in 

 structure and in physiological properties. The gastrocnemius may be 

 taken as an example of a muscle whose fibres consist largely of the 

 clear variety, and the hyoglossus of the granular variety, i.e. a muscle in 

 which the majority of muscle-fibres contain more nuclei and are rela- 

 tively richer in undifferentiated living material, the sarcoplasm. The 

 chief physiological difference between granular and clear muscles are,. 

 that granular muscles have a slower and more prolonged contraction,, 

 are less excitable, more easily tetanised, and less readily fatigued. 



In mammals the same differences between red and white muscles can 

 be shown to exist. Red muscles, such as the masseter or soleus of the 

 rabbit, differ structurally in having more sarcoplasm and nuclei in their 

 fibres, and are redder in colour owing to a much richer capillary net- 



