52 HUMAN PHYSIOLOGY 



the clear band seems to be indicated by the facts that it 

 retains its reactions to polarised light and staining reagents. 



"Usually the contraction of a muscle occurs simultaneously 

 in all the fibres. This is because a nerve fibre passes to every 

 muscular fibre, and sets them all in action together. When, 

 as sometimes occurs in disease, the nerve fibres become 

 implicated, the muscular fibres may not all act at once, and 

 a peculiar fibrillar twitching of the muscle may be produced. 

 ] If the muscle be directly stimulated at any point, the 

 contraction starts from that point and passes as a wave of 

 'contraction outwards along the fibres. This may be seen by 

 sharply percussing the fibres of the pectoralis major in the 

 chest of an emaciated individual. The rate at which the 

 wave of contraction travels is ascertained by finding how 

 long it takes to pass between any two points at a known 

 distance from one another. Its velocity is found to vary 

 much according to the kind of muscle and the condition of 

 the muscle. In the striped muscular fibres of a frog in good 

 condition it travels at something over three metres per second. 

 When the muscle is in bad condition the wave passes more 

 slowly, and in an exhausted muscle it may remain at the 

 point of stimulation. (Practical Physiology, Chap. IV.) 



The cause of the propagation of this wave is simply the 

 continuity of the muscle fibres. The fibres stimulated are 

 set in action, and the evolution of energy in these stimulates 

 the adjacent fibres, and so the contraction passes along the 

 muscle as a flame passes down a trail of gunpowder. 



Contraction of Muscle as a whole may best be studied 

 under the following heads : 



1st. The course of contraction. 

 2nd. The extent of contraction. 

 3rd. The force of contraction. 



1st. Course of Contraction (Fig. 21). 



By attaching the muscle (M) to a lever (L\ and allowing 

 the point of the lever to mark upon some moving surface, 

 a magnified record of the shortening of the muscle when 

 stimulated may be obtained. 



A revolving cylinder covered with a smoke-blackened, 

 glazed paper is frequently used for this purpose, and to 



