MUSCLE. 19 



sarcous element) of each sarcomere is divided into two parts by Hensen's 

 line, and pervaded with longitudinal canals which are open towards 

 Krause's membrane ; and when the muscle contracts, the clear sub- 

 stance at either end of the sarcomere ' passes into the pores of the 

 sarcous element, so that the sarcomere becomes shorter and thicker. 

 The shortening of the whole muscle is thus regarded as the result of 

 the shortening of its sarcomeres. 



When a living muscle is examined with polarised light the dim 

 segments are seen to be doubly refracting (anisotropous), while the 

 clear segments are singly refracting (isotropous). 



In certain animals, such as the rabbit, some of the muscles are pale 

 and others are red in colour. The pale muscles have the structure 

 just described, whereas the red muscle fibres contain more sarcoplasrn 

 than the pale ones, and their nuclei are scattered throughout the 

 substance of the fibres ; the capillaries also show numerous small 

 saccular dilatations. The red colour is due to the presence of haemo- 

 globin in the fibres. These muscles contract more slowly than the 

 pale muscles, but their contraction is more prolonged. In many 

 animals these two varieties of fibre are found together in the same 

 muscle. All muscle fibres are supplied with nerve fibres, some of 

 which are motor and end in the muscle fibres in end-plates, while 

 others are sensory and convey impulses from the muscle to the central 

 nervous system. 



Chemical and Physical Characters of Muscle. Muscle contains 

 about 75 per cent, of water and 25 per cent, of solid substances, of 

 which proteins form 18 to 20 per cent. The other constituents are a 

 small amount of fat, glycogen (J to 1 per cent.), inosite, and a number 

 of nitrogenous extractives including creatine, xanthine, and hypo- 

 xanthine ; the most important of these is creatine, which forms 0'2 to 

 0*4 per cent, of the muscle. 



At a variable period after death the proteins coagulate, the product 

 being called myosin, and the muscles become rigid and opaque; this 

 condition is known as rigor mortis. 



The nature of the proteins in fresh muscle can be studied if 

 coagulation is delayed by cooling the muscle. The living muscle is 

 minced at a temperature of C., and is then extracted with ice-cold 

 saline solution (0'9 per cent. NaCl) and filtered ; the filtrate contains 

 two proteins, namely paramyosinogen and myosinogen. -^The former is a 

 globulin which coagulates at 47 to 50 C., and constitutes about 20 per 

 cent, of the total protein in muscle. The remaining four-fifths consist 

 of myosinogen, which has the characters of an albumin, though it 

 coagulates at the low temperature of 56 to 60 C. When the solution 



