402 THE TISSUES. 



ence imparted by a nerve distributed among them in the muscle. 

 Electricity, galvanism, and a variety of other agents, will, however, 

 excite it in muscles in the living body, or in portions removed from 

 the same. In fact, the only way in which a muscular fibre can 

 react vitally when acted upon by any external agent, is by con- 

 tracting, since this is its peculiar vital endowment. The shortening 

 also occurs instantaneously. 



The chemical changes which attend the contraction of a fibre 

 are not all understood. There is, however, reason to believe that 

 the substance of the fibrillae the musculine alone is concerned 

 actively in the contraction, and alone undergoes chemical changes 

 during it. It is also certain that the contact of oxygen is necessary 

 to the Contraction of a fibre, and that during it carbonic acid gas is 

 formed within the fibres, and not in the bloodvessels distributed 

 among them; while the temperature of the muscle also rises two or 

 three degrees. (Becquerel and Breschet.) Hence the contractility of 

 a set of muscles is lost if their supply of blood is cut off. 



The extent to which a fibre may become shortened during con- 

 traction seldom if ever exceeds two-thirds of its length; or reduces 

 the fibre to one-third of its original length. Some give one-third 

 as the usual amount of shortening, the fibre then being two-thirds 

 its original length. Kolliker states that the average shortening is 

 three-fourths (i. e. down to one-fourth), and in powerful muscles even 

 five-sixths (or down to one-sixth), of the original length. Hassall 

 estimates the shortening at one-third to one-half only, of the original 

 length. 



In ordinary circumstances, not all the fibres in a muscle contract 

 simultaneously; but each contracts for an instant and relaxes, while 

 of the rest some are contracting at the same time, and others follow 

 these in their turn. It is probably only when the most powerful 

 muscular efforts are made that all the fibres of a muscle contract at 

 once. . 



The absolute extent of motion (or shortening) produced by a 

 striated fibre will, therefore, depend on its length; while its con- 

 tractile force or strength will depend on its size, it being stronger 

 in proportion to the area of the transverse section of the musculine 

 (and of the fibrillse) within its myolemma. 



So long as a muscular fibre is in a state of perfect nutrition, it also 

 manifests a slight but constantly exerted tension, called tone or ton i- 

 city, and in regard to the nature of which very diverse opinions 



