CHAP. IX.] THE CONTRACTILE TISSUES. 341 



ments of small masses, where they occur, such as in the fission of 

 nuclei and of fibres, constitute a third form. A fourth, if entirely 

 hypothetical, deserves to be mentioned. It is not indeed another form 

 of actual energy, like heat or mechanical motion, into which the energy 

 of chemical affinity is converted ; but rather a re-distribution of 

 the original potential energy. The elaboration of the intermediate 

 product in the above series of tissue-changes is entirely unknown to us 

 in its nature ; but it is at least conceivable that it is not altogether 

 a process in which stronger affinities are satisfied at every step. It 

 may, in part, be a dissociation ; in which case some of the energy set 

 free in the chemical changes of the final stage may again at once 

 become potential. 



The intensity of the chemical and physical processes of resting 

 muscle depends upon temperature, the supply of the necessary sub- 

 stances, and what may be called the nutritive instinct, or inherent 

 capacity, of the tissue for the changes. Both the supply of matter 

 and the capacity of the tissue for the changes in question, are exalted 

 in the active state of muscle. 



Muscle in Action. 



General The phenomena of resting muscle thus consist of 



Phenomena of two parallel and associated series of transformations, 

 Contracting one of matter, the other of energy. The same dualism 

 is seen in active muscle. The whole phenomenon of 

 contraction comprises (1) a sudden acceleration and extension of 

 chemical decompositions, and (2) a sudden and extensive conversion 

 of the potential energy of chemical affinity into actual energy of 

 various forms. The manifestations of actual energy in the case of 

 contracting muscle are pronounced and admit of a careful study; 

 they assume the form of heat, electrical inequality, and mechanical 

 motion. 



Special Contraction may be started either by the normal 



Phenomena of stimulus proceeding from a nerve ; or by electrical, 

 Contraction. chemical, thermal, or mechanical stimuli applied to 

 muscle even in the absence of nerves. In its mechanical aspect con- 

 traction is a shortening and thickening both of the whole muscle and 

 of its individual fibres, associated in the case of entire muscles with a 

 small reduction of bulk. Contracted muscle is less elastic and more 

 extensile than resting muscle. 



Micro- Viewed under the microscope, the act of contraction 



scopic ap- falls into well-marked stages. In the first, the bands 



pearances. draw near together as the muscular fibre shortens ; and 

 the dark and light bands approximate in tint, until the whole fibre 

 is evenly dark with little or no striation: this is called the homo- 

 geneous stage. As contraction proceeds, striae again appear, but this 



