220 VETERINARY PHYSIOLOGY 



marked in the dim band. The clear band also slightly shortens, 

 and at the same time appears to become darker. These 

 appearances may best be explained on the assumption that 

 the fibrils are the part of the fibre which shorten and 

 thicken, that these fibrils chiefly change in the dim 

 band, and that, by the contraction of the fibrils in the clear 

 band, adjacent dim bands are pulled nearer to one another, 

 and so cast a shadow over the clear band. 



That no fundamental chemical change takes place in 

 either band is indicated by the fact that they retain their 

 reaction to polarised light and to staining reagents (p. 204). 



Various theories have been advanced as to how these 

 changes are produced, and it will be shown later that the 

 change is of the nature of an increase in surface tension. 

 Suppose a fibril to be made up of elongated potential 

 spheres in the dim bands, an increase in the surface tension 

 of each would cause it to assume a more spherical form, and 

 this change occurring all along the fibril would lead to a 

 shortening (fig. 102). 



It has been maintained that the sarcoplasm is also 

 capable of contraction, and that the tonic contraction of 

 muscle is due to this (p. 211). 



Usually, in a maximum contraction, all the fibres contract 

 simultaneously. In a lesser contraction fewer fibres are 

 involved but they contract simultaneously. This is because 

 a nerve fibre passes to every muscle fibre. When, as 

 sometimes occurs in disease, the nervous mechanism acts 

 abnormally, the nmscular fibres may not all act at once, and 

 a ipQCuli&r fibrillar twitching, a jerky contraction of one set 

 of fibres apart from the other, may be produced, a condition 

 which most people have experienced in the orbicularis oculi 

 or in some other muscle. 



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 



