38 EXPERIMENTAL PHYSIOLOGY. 



edge of the binding post on the key. This can be done so quickly 

 that one shock is effective. Follow directions as in the original 

 description. 



RED AND PALE MUSCLE FIBRES. 



Voluntary muscle fibres may be of two kinds, one pale in aspect, 

 the other red. The red fibres are thinner and possess a larger 

 amount of sarcoplasm. They possess a greater number of nuclei 

 and a larger blood supply. Many of the capillaries to red fibres 

 are furnished with dilatations. 



Red fibres contract more slowly and fatigue less readily than 

 do pale fibres. The former appear to be adapted for heavy work, 

 while the latter are for rapidity of action. 



Some muscles are composed largely of red fibers, such as the 

 soleus in the rabbit. The gastrocnemius on the other hand con- 

 tains mostly pale fibres. Quite often both types of fibres are 

 present in a muscle, but one predominating. 



HEAT PRODUCTION. 



A muscle releases a considerable amount of energy in the form 

 of heat when it contracts. Everyone is familiar with the increased 

 amount of heat after vigorous exercise. Sustained contraction in a 

 large muscle may raise the temperature 1 C. or more, but due to 

 the circulation and the control mechanism, excess heat is distributed 

 and eventually lost. 



It has been estimated that CO to 75 per cent, of the energy used 

 in contraction produces heat, the balance being converted into 

 work. Unpractised movements result in a greater proportion of 

 heat production. Training, therefore, is highly important in 

 getting the greatest work from a muscle. 



There are two stages of heat production. The first is immedi- 

 ate or "explosive" in character and will take place in the absence 

 of free oxygen. For example, a muscle which contracts in Ringer's 

 solution from which free oxygen has been removed or in nitrogen, 

 releases heat of the first stage. The second stage develops slowly 

 and is postponed if oxygen is absent. The evolution of heat in 

 this stage continues for a long time after the mechanical response. 

 The amount of heat evolved is as great as that of the first stage. 



