116 THE CONTRACTILE TISSUES. 



the striation of a muscular fibre and a muscular contraction. Nearly all 

 rapidly contracting muscles are striated, and we must suppose that the 

 striation is of some use ; but it is not essential to the carrying out of a con- 

 traction, for, as we shall see, the contraction of a non-striated muscle is 

 fundamentally the same as that of a striated muscle. But whatever be the 

 exact way in which the translocation is effected, it is in some way or other 

 the result of a chemical change, of an explosive decomposition of certain 

 parts of the muscle substance. The energy which is expended in the 

 mechanical work done by the muscle has its source in the energy latent in 

 the muscle substance and set free by that explosion. Concerning the nature 

 of that explosion we only know at present that it results in the production 

 of carbonic acid and in an increase of the acid reaction, and that heat is set 

 free as well as the specific muscular energy. There is a general parallelism 

 between the extent of metabolism taking place and the amount of energy set 

 free ; the greater the development of carbonic acid, the larger is the con- 

 traction and the higher the temperature. 



It is important to remember that, as we have already urged, relaxation, 

 the return to the original length, is an essential part of the whole contraction 

 no less than shortening itself. It is true that the return to the original 

 length is assisted by the stretching exerted by the load, and in the case of 

 muscles within the living body is secured by the action of antagonistic 

 muscles or by various anatomical relations ; but the fact that the complete- 

 ness and rapidity of the return are dependent on the condition of the muscle, 

 that is, on the complex changes within the muscle making up what we call 

 its nutrition, the tired muscle relaxing much more slowly than the untired 

 muscle, shows that the relaxation is due in the main to intrinsic processes 

 going on in the muscle itself, processes which we might characterize as the 

 reverse of those of contraction. In fact, to put the matter forcibly, adopting 

 the illustration used in 57, and regarding relaxation as a change of molecules 

 from a " formation " of one hundred in two lines of fifty each to a formation 

 of ten columns each ten deep, it would be possible to support the theory that 

 the really active forces in muscle are those striving to maintain the latter 

 formation in columns, and that the falling into double lines, that is to say 

 the contraction, is the result of these forces ceasing to act; in- other words, 

 that the contracted state of the muscular fibre is what may be called the 

 natural state, that the relaxed condition is only brought about at the expense 

 of changes counteracting the natural tendencies of the fibre. Without going 

 so far as this, however, we may still recognize that both contraction and relax- 

 ation are the result of changes which, since they seem to be of a chemical 

 nature in the one case, are probably so in the other also. 



ON SOME OTHER FORMS OF CONTRACTILE TISSUE. 



Plain, Smooth or Unstriated Muscular Tissue. 



86. This, in vertebrates at all events, rarely occurs in isolated masses 

 or muscles, as does striated muscular tissue, but is usually found taking part 

 in the structure of complex organs, such for instance as the intestines; hence 

 the investigation of its properties is beset with many difficulties. 



87. As far as we know, plain muscular tissue in its chemical features 

 resembles striated muscular tissue. It contains albumin, some forms of 

 globulin, and antecedents of mvosin which upon the death of the fibres 

 become myosin ; for plain muscular tissue after death becomes rigid, losing 

 its extensibility and probably becoming acid, though the acidity is not so 

 marked as in striated muscle. Kreatin has also been found, as well as- 



