204 



ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



Fis;. 107. 



Capillary net-work of Muscle. 



tremely vascular, the capillary vessels being distributed in parallel lines, united 



by transverse branches, in the minute inter- 

 spaces between the fibres (Fig. 107); so that it 

 is probable that there is no fibre, which is not in 

 close relation with a capillary. The number 

 of blood-vessels in a given space will of course 

 be greater, where the fibres and the capilla- 

 ries are both small, as in Mammals and Birds, 

 than where they are of larger diameter, as in 

 Reptiles and Fishes ; and the former condition 

 will obviously be the one most favourable to 

 the performance of active changes between the 

 blood and the muscle. These changes consist, 

 it would appear, not merely in the nutrition of 



the tissue; but in' the supply of oxygen, which is a necessary condition of 

 the excitement of its activity. We shall hereafter see, indeed, that every 

 muscular contraction probably involves the disintegration of a certain amount 

 of its substance, through the union of oxygen, supplied by arterial blood, 

 with its elements ; and that the great demand for nutrition, which is occa- 

 sioned by muscular activity, is for the purpose of repairing this loss. The 

 muscles of warm-blooded animals speedily lose their irritability, after the 

 supply of arterial blood has been suspended, either through the cessation of 

 the general circulation, or by deficient aeration of the fluid. But the muscles 

 of cold-blooded animals, which are very inferior in the energy and rapidity 

 of their action, preserve their properties for a much longer period, after the 

 deprivation of their supply of arterial blood ; in accordance with the general 

 principle, that, the lower the usual amount of vital energy, the longer is its 

 persistence, after the withdrawal of the conditions on which it is dependent. 

 The very indisposition to a change of composition, on which the less ready 

 action depends, produces a longer retention of the power of acting. 



240. The Muscles of Animal life are, of all the tissues except the skin, 

 those most copiously supplied with Nerves. These, like the blood-vessels, 

 lie on the outside of the Myolemma of the several fibres ; and their influence 

 must consequently be excited through it. The general arrangement of these 

 nerves is shown in Fig. 108. Their ultimate fibres or tubes cannot be said 



Fig. 108. 



Form of the terminating loops of the nerves in the muscles. 



to terminate anywhere in the Muscular substance; for after issuing from the 

 trunks, they form a series of loops, which return either to the same trunk, or 



