296 ELEMENTARY PHYSIOLOGY less. 



such elementary organisms, through the intermediation of 

 which its temperature is kept constant and its supply of 

 nourishment maintained. 



The special property of a living muscular filjre, that 

 which gives it its physiological importance, is its peculiar 

 contractility. The body of a colourless blood corpuscle, 

 as we have seen, is eminently contractile, insomuch as it 

 undergoes incessant changes of form. But these changes 

 take place at all points of its surface, and have no definite 

 relation to the diameter of the corpuscle, while the 

 contractility of the muscular fibre is manifested by a 

 diminution in the length and a corresponding increase 

 in the thickness of the fibre. Moreover, under ordinary 

 circumstances, the change of form is effected very rapidly, 

 and only in consequence of the application of a stimulus. 

 When a contracting striated fibre is observed under the 

 microscope all the bands become broader (across the 

 fibre) and shorter (along the fibre) and thus more closely 

 approximated. Some observers think that the clear 

 bands are diminished in total bulk relatively to the dim 

 bands ; but this is disputed by others. When the fibre 

 relaxes again the bands return to their previous condition. 

 5. The Chemistry of Muscle. — Whilst much in the 

 chemistry of muscle is obscure the muscular contraction 

 may be compared to an explosion, the explosive material 

 for which is manufactured on the spot. The muscle may 

 therefore be regarded as consisting of (1) the stored 

 explosive material, which is relatively small in quantity, 

 (2) the machinery and fuel for its manufacture, and (3) 

 the fabric of the building which forms the factory. Of 

 these three the last is protein in nature, the other two 

 we may consider as non-nitrogenous. 



We are quite uncertain as to the actual nature of the ex- 

 plosion, i.e., the physical and chemical changes which are 

 directly associated with the change of shape in the muscle. 

 Theyappear capableof taking place in the ahsenceof oxygen 

 and under such circumstances there would be little or no 



