ANIMAL MECHANICS. 



5 



the conditions of pressure necessary for the production of a 

 cleaved or striped structure. In the unstriped muscles, as 

 those of the bladder and uterus, of the intestinal canal and 

 scrotum, the contraction of the muscle is vermicular, and not 

 a constant strain from end to end ; hence the conditions of 

 pressure requisite to produce cleavage are wanting, and the 

 muscle and its fibres remain without this distinguishing mark 

 of uniform and continued contraction from end to end. 



It has been found, on careful microscopical examination 

 of the elementary fibres, that they possess not only a trans- 

 verse cleavage, dividing them into disks, but also a longitudinal 

 cleavage or jointage, separating the fibres into fibrillce ; so that 

 the ultimate subdivisions of muscles may be regarded as ele- 

 mentary molecules of muscular fibre, which are to be con- 

 sidered as units, or ultimate atoms of muscular tissue ; and 

 every muscle may be regarded as composed of a certain num- 

 ber of such atoms, variously distributed. 



2. On the Nature of Muscular Contraction. — The muscular 

 fibres already described are capable of contracting in a longi- 

 tudinal direction, and in so contracting exert mechanical force 

 on the points of origin and insertion. It was supposed by 

 the older writers (as Borelli and others), that there was no 

 change of volume during the contraction of muscles ; and this 

 conjecture has been confirmed in a satisfactory manner by 

 recent experiments ; and, as this law of constancy of volume 

 during change of form holds true of bodies under the influence 

 of magnetical and electrical forces, some writers have supposed 

 that it furnishes a key to the explanation of muscular vital phe- 

 nomena. 



It is certain that muscular contraction is accompanied by 

 developement of sensible heat (or loss of latent heat) ; by loss 

 of electrical statical tension ; and by the chemical transforma- 

 tion of fibrine, albumen, and the higher products of animal 

 life, into urea, carbonic acid, and water, which are the main 

 excretions of all animal tissues : and,, as it is well known that 



