THE STRUCTURE OF THE MOTOR GROANS. 123 



Cardiac Muscular Tissue. This consists of nucleated 

 branched cells which unite to form a network, in the inter- 

 stices of which blood-capillaries and 

 nerve-fibres run. The cells present 

 transverse striations, but riot so distinct 

 as those of the skeletal muscles, and are 

 said to have no sarcolemma. 



The Chemistry of Muscular Tissue. 

 The chemical structure of the living- 

 muscular fibre is unknown, but some 

 idea as to it may be obtained from ex- 

 amination of the substances it yields on 

 proximate analysis. Muscle contains 75 Fl ^._ C ardiacmuscu- 

 per cent of water; and, among other Jg; ^^gpT'^S 



inorganic Constituents, phosphates and boundaries and cell-nuclei 



' r -t are indicated only in the 



Chlorides Of potassium, SOdium, and right-hand portion of the 



* ' . figure. 



magnesium. When at rest a living 

 muscle is feebly alkaline, but after hard work, or when dying, 

 this reaction is reversed through the formation of sarcolactic 

 acid (C 3 H 6 3 ). Muscles contain small quantities of grape- 

 sugar and glycogen, and of organic nitrogenous crystalline 

 compounds, especially kreatin (C 4 H 9 N 3 O a ). As in the case of 

 all other physiologically active tissues, however, the main 

 post-mortem constituents of the muscular fibres are proteid 

 substances, and it is probable that like protoplasm itself (p. 

 27) the essential contractile part of the tissue consists of a 

 complex body containing proteid, carbohydrate and fatty 

 residues; and that during muscular work this is broken up, 

 yielding proteids, carbon dioxide, sarcolactic acid, and prob- 

 ably other things. 



During life and for a certain time after general death the 

 muscles are soft, translucent, extensible and elastic, and 

 neutral or feebly alkaline in reaction; after a period which in 

 warm-blooded animals is brief (varying from a few minutes 

 to three or four hours) they gradually become harder, more 

 opaque, less extensible and less elastic, and distinctly acid in 

 reaction. The result of these changes is the well-known 

 cadaveric rigidity or rigor mortis. The rigid condition lasts 

 for a day or longer and then it gradually and finally disappears 

 and more marked decomposition changes commence. Until 

 a short time before the commencement of rigor the muscles 

 remain contractile and can be thrown into activity by various 



