142 THE HUMAN BODY. 



and thus energy is liberated and used by the muscle. In the 

 ordinary course of events the carbon dioxide is carried off by 

 blood and lymph and eliminated from the Body; the sarco- 

 lactic or other similar substance or substances are also carried 

 off and oxidized elsewhere to form carbon dioxide and water 

 and be then eliminated ; but the nitrogen-containing product 

 remains behind, and with the help of fresh oxygen and of 

 other food material brought by the blood is reconstructed into 

 the original inogen. In the excised muscle there is but 

 scant store of material for repair; carbon dioxide is given 

 off when the muscle contracts, and the sfrcolactic acid and 

 nitrogen-containing product accumulate: the latter then 

 undergoes further changes, and ultimately becomes myosin. 

 If the excised muscle be thrown into rigor quickly (as by 

 heat), then the inogen is at once broken up, forming myosin 

 and carbon dioxide and sarcolactic acid: if it be worked 

 for a time before being thrown into rigor, then some of its 

 inogen will have been already broken up, so there will be less 

 to give rise to carbon dioxide at the moment of rigor, but the 

 missing amount is found in that given off during work. If 

 some such view as this, which may be called the " inogen 

 theory," be the correct one, then the energy liberated by a 

 resting muscle passing into rigor must take some other form 

 than muscular work. As a matter of fact a good deal of heat 

 is liberated during death stiffening, but whether sufficient to 

 account for all the missing energy is by no means clear. The 

 whole subject of the immediate source of muscular work is 

 still in much need of elucidation. 



Physiology of Plain Muscular Tissue. What has hither- 

 to been said applies especially to the skeletal muscles; but 

 in the main it is true of the unstriped muscles. These also 

 are irritable and contractile, but their changes of form are 

 much more slow than those of the striated fibres. Upon 

 stimulation, a longer period of latent excitement elapses 

 before the contraction commences and when, finally, this 

 takes place it is comparatively very slow, gradually attaining 

 a maximum and gradually passing away. 



Unstriped muscular tissue has a remarkable power of 

 remaining in the contracted state for long periods : the mus- 

 cular coats of many small arteries, for example, are rarely 

 relaxed ; sometimes they may be more contracted, sometimes 

 less, but in health seldom if eyer completely at rest. There 



