INFLUENCE OF ARTERIAL BLOOD. 443 



the stimulus of alcohol, nitrous oxide, &c., or of some purely mental excite- 

 ment, individuals can perform actions requiring a degree of strength, which 

 they cannot exert under any other circumstances. But it does not hence fol- 

 low, that the irritability is increased ; since the energy of the action may be 

 due solely to the power of the stimulus by which it is excited, and to the un- 

 usual number of fibres called into simultaneous contraction. It is well known 

 that stimulating agents, which thus temporarily increase Muscular power, pri- 

 marily excite the Nervous system ; as is shown by the increased mental acti- 

 vity which results from the moderate use of alcohol, nitrous oxide, opium, 

 &c. ; and it does not seem necessary, therefore, to go further in search of an 

 explanation of their effect on muscular action. It is worthy of remark that, 

 whilst the influence of general depressing causes acting through the Nervous 

 System, is primarily manifested on the muscles of Organic life, that of stimu- 

 lants chiefly shows itself in the muscles subjected to the Will. 



583. There can be no question that, in the living body, the energy of Mus- 

 cular contraction is determined (other things being equal), by the supply of 

 Arterial Blood, which the muscle receives. It is well known that, when a 

 ligature is applied to a large arterial trunk in the Human subject, there is not 

 only a deficiency of sensibility in the surface, but also a partial or complete 

 suspension of muscular power, until the collateral circulation is established. 

 The same result has been constantly attained, in experiments upon the lower 

 Animals ; the contractility of the muscle being impaired or altogether ex- 

 tinguished, when the flow of blood into it was arrested ; and being recovered 

 again, when the supply of blood was restored. The influence of this supply 

 of arterial blood is twofold ; it affords the materials for the nutrition of the 

 tissue ; and it furnishes (what is perhaps more immediately necessary) the 

 supply of oxygen required for that metamorphosis of the tissue, which seems 

 to be an essential condition of the generation of its contractile force. As this 

 oxygen is taken in through the lungs, and as the greater part of it is thrown 

 off when united with carbon into carbonic acid by the same channel, we 

 should expect to find a very close correspondence between the amount of 

 muscular power developed in an animal, and the quantity of oxygen consumed 

 in its Respiration : and this is in reality the case. We find, for example, that 

 in Birds and Insects, whose respiration is the highest, the muscular power is 

 greater in proportion to their size, than in any other animals. In the Mam- 

 malia, and certain Fishes that might be almost called warm-blooded, it is only 

 in a degree inferior. But in the cold-blooded Reptiles, Fishes, and Mollusca, 

 the muscular power is comparatively feeble ; though even here we trace gra- 

 dations, which accord well with the relative quantities of oxygen consumed. 

 But in proportion to the feebleness of the power, do we usually find its dura- 

 tion greater ( 578) ; so that it is not so immediately dependent upon the 

 supply of oxygen, in cold-blooded, as in warm-blooded animals. Thus, it is 

 found that Frogs are still capable of voluntary movement, after the heart has 

 been cut out ; they can move limbs which are connected with the trunk by 

 the nerves alone : and that this power is not altogether due to the blood which 

 may remain in the capillary vessels, is shown by the experiment of Muller, 

 who found the muscles still contractile, after he had expelled all the blood, 

 by forcing a current of water into an artery, until it escaped from the divided 

 veins. 



584. It seems probable that the Muscles of Organic life are less dependent 

 upon a supply of arterialized blood, than are those of Animal life ; for the 

 Heart will continue to contract, when the blood in its vessels is entirely ven- 

 ous, and when the circulation in it has come to a stand. Still the dependence 

 of its action upon a constant supply of arterial blood, is very close ; and in all 

 animals, however different the plans of their circulation, we find a provision 



