210 THE PHYSIOLOGY OF MUSCULAR WORK 



tissue. In muscle the power of contraction, which is present in 

 the primitive cell, has been especially developed in the process of 

 division of labour and differentiation of structure. Three kinds 

 of muscle are recognised voluntary, cardiac, and involuntary. 

 Their minute structure, which has given rise to much con- 

 troversy, is strictly a question of anatomy, and will not be 

 discussed here. There are not only differences in the structure, 

 composition, and properties of voluntary muscles in different 

 animals, but also in the same animal. The best known example 

 is the red and the white muscle of the rabbit ( 1 ). Simple inspec- 

 tion of the muscles of a rabbit directly its skin is removed de- 

 tects a marked contrast in the colour of the different muscles ; 

 the masseters and some of the muscles of the hind limbs, such as 

 the soleus and semi-membranosus, are red in colour, but most of 

 them are pale. This difference is not due solely to variations in 

 the vascular supply, for even after the blood has been removed a 

 contrast remains. The red muscle fibre contains haemoglobin and 

 myohaematm within its substance, and these pigments are pro- 

 bably of some importance in the process of internal respiration. 

 The capillary blood vessels have dilatations, which are not pre- 

 sent in the case of the pale muscles. The red fibres are thin with 

 nuclei in their substance as well as under the sarcolemma, and the 

 transverse striation is less regular. Functional differences can be 

 easily demonstrated. The red muscle contracts and relaxes slowly, 

 in marked contrast to the rapid wave of contraction in white 

 muscle ; red muscle is more easily tetanised, and does not pass into 

 rigor mortis so rapidly. 



THE PHYSICAL AND CHEMICAL PROPERTIES OF MUSCLE 



One of the most important characteristics of muscle is its 

 elasticity. A muscle fibre regains its original length after it has 

 been stretched within certain limits ; it possesses perfect elas- 

 ticity and conforms to Hooke's law ; the successive increments in 

 length produced by equal increments of weight are equal. It is 

 only when the muscle has been extended beyond the limits to 

 which it is exposed in the living body that its perfect elasticity 

 is impaired ( 2 ). 



In the normal condition muscles are stretched between their 

 points of attachment and by the action of antagonistic muscles ; 



