CHAP. IX.] THE CONTRACTILE TISSUES. 321 



The facts then that (1) a large portion of the contents of the 

 sarcolemma can be expelled from it in the condition of a liquid, 

 and (2) that living bodies move in the interior of the living fibre, as 

 in a liquid holding solid bodies in suspension, appear to us to settle 

 definitely the great problem of the physical condition of the 

 doubly refracting and isotropous elements of muscular fibre. The 

 most weighty consequences follow the conclusion to which we are led. 

 We cannot, for instance, for one moment suppose that a liquid 

 can change its form in consequence of internal forces acting within it, 

 unless these lead to its becoming solid; we are therefore led on theo- 

 retical grounds to the conclusion that the sarcous elements must be 

 the structures which are directly concerned in the change of shape of 

 the fibre. Engelmann's observations, which in all respects are the 

 most consistent and satisfactory which have been advanced, since the 

 earlier classical investigations of Bowmann and Brticke were pub- 

 lished, seem to shew that in contraction the sarcous elements undergo 

 a change in form and in volume, increasing in bulk at the expense of 

 the isotropous substance, so that the combined volume of the contents 

 remains sensibly the same during contraction and during rest. 



Were it possible, we should wish, in the first place, to study 

 the chemical history of the various structural elements which make 

 up the muscular fibre ; but this ideal aim can but most imperfectly 

 be realized ; so far as possible, we feel however bound to attempt the 

 task. 



Chemical characters of the Sarcolemma. 



The delicate transparent sheath which in voluntary muscle encloses, 

 as in a sac, the contractile matter which forms the chief substance of 

 the muscular fibre, was formerly supposed to be of the same nature as 

 elastic tissue; like the latter, it is unacted upon by acetic acid, and 

 resists long boiling with water, though it is ultimately dissolved. 

 It differs, also, from elastin in being slowly dissolved when heated in 

 dilute solutions of acids and alkalies. The fact that it is dissolved 

 gradually at the temperature of the body by the ferments of the 

 stomach and pancreas has also been adduced as proving that the 

 sarcolemma is not identical with elastic tissue ; in point of fact even 

 elastic tissue is slowly dissolved by these ferments, and particularly 

 by pepsin. 



Chemical nature of the doubly-refracting elements of voluntary muscle. 



The doubly refracting (anisotropous) matter of voluntary muscular 

 fibre is, during life, as after death, of solid consistence. It loses its 

 peculiar optical properties when the fibre containing it is subjected to 

 the action of either acids or alkalies, or when it is heated to boiling. 

 For these reasons it has been surmised that this matter is proteid 

 in nature. It has, however, been remarked that neither alcohol 



G. 21 



