CONTRACTILE TISSUES 461 



The intimate structure of muscle fibre is very difficult to investigate. The 

 dark bands seen in cross-striated muscle are doubly refracting. In the state of 

 contraction, the dark and light bands appear to change places ; but the position of 

 the doubly refracting part does not change, so that it is the light band which is 

 doubly refracting in this state. According to Engelmann, in contraction the 

 doubly refracting part increases in volume at the expense of fluid derived from 

 the singly refracting part. The property of double refraction is, according to 

 Engelmann, associated with the power of contractility as a general rule in the 

 animal kingdom. 



The production of tension is the essential point in the mechanics of muscular 

 contraction. The muscle changes its properties from those of an unstretched steel 

 spring to those of a stretched one, without necessarily changing its length. 



Various modes of contraction may be obtained from muscle, according to 

 whether it is allowed to shorten or not, or the phase of the contraction at which 

 the muscle is allowed to shorten, or at which the load is applied or removed. 



An isolated muscle can be made to do external work by raising a weight, 

 which is prevented from falling again. This is done by a mechanism known as 

 the "work collector." The work done by an animal is measured by some form of 

 " ergometer." 



A stimulus applied before the effect of a previous one has disappeared produces 

 a contraction which is itself less than the previous one, but takes its origin from 

 a shorter state of the muscle. Since the effect of each is less than that of its 

 predecessor, a stage is reached beyond which no further shortening takes place. 

 If the stimuli succeed one another at a rate such that the muscle has not com- 

 menced to relax before the next stimulus arrives, we have a smooth continuous 

 curve. The phenomenon described is known as the " summation of contractions," 

 producing "tetanus." 



This tetanus is also the condition of skeletal muscle when excited by impulses 

 from the cells in the nerve centres. The rate at which these impulses are sent out 

 is, in man, from forty-seven to fifty- eight per second. In the tortoise, the rate is 

 a linear function of temperature between 4 and 40, like that of t"he mammalian 

 heart between 27 and 40. 



A resting excitable muscle is a physico-chemical system possessing potential 

 energy. When stimulated, this potential energy is converted into energy of 

 tension, which can then be used for the performance of work, or allowed to 

 become degraded into heat. 



This contractile process itself is associated with the splitting off of lactic acid, 

 but there is neither consumption of oxygen nor evolution of carbon dioxide. It is 

 not an oxidation process. 



To restore the potential energy which the system has lost in contracting,* 

 energy is supplied by another reaction of a chemical nature, which succeeds the 

 contractile stage. The lactic acid is put back into its original place in the course 

 of this second process. 



The energy required for the second process is afforded by a reaction in which 

 some substance, carbohydrate or fat, is oxidised. Much oxygen is used, and 

 carbon dioxide given off. 



The energy developed in contraction, as measured by the heat into which it is 

 converted, is directly proportional to the tension produced. It is proportional to 

 the length of the fibres at the time the contractile process takes place and not to 

 their volume. It is, therefore, a surface phenomenon. Osmotic energy may, 

 nevertheless, intervene as a further step, being controlled by the products of the 

 change in surface energy. 



The fact that the reaction by which the energy of the contractile system is 

 restored is one having, apparently, no chemical component in common with the 

 contractile system itself, indicates that this latter is not a chemical system, but 



