BLOOD. 123 



not merely by the heart's action, but also by a certain electric 

 atti'action of the organs, by the influence of the nerves, and 

 by a motive power inherent in the blood itself. Since the 

 heart's action is occasioned by the irritation exercised upon 

 that organ by the living blood, there can be no doubt that 

 the reciprocating action of the organs and of the blood must 

 influence the circulation. Schultz evidently undervalues the 

 influence of the rhythmic motion on the circulation, when he 

 limits the functions of the heart to the conveyance of arterial 

 blood to the peripheral system, and to the conduction of 

 venous blood back again, and regards the blood in the peri- 

 pheral system as moving entirely independent of it. 



The circulation is usually divided into the greater and 

 the lesser. There is however, in fact, but one circulation ; and 

 this is di\dded into the greater coui'se, which proceeds from 

 the left heart through the arteries of the body, and through the 

 veins to the right heart, and into the lesser course, which recon- 

 ducts the blood through the lungs from the right to the left heart. 



On the pi'ocess of respiration. 



Respiration takes place through lungs, gills, tracheae, or the 

 integument. 



Oxygen is indispensable for the process, although pure 

 oxygen is less conducive to health than a mixture of oxygen 

 with a gas not detrimental to life, as nitrogen or hydrogen. 



The proportions of oxygen and nitrogen that occur in atmo- 

 spheric air are doubtless the most suitable for the respiration 

 of the higher animals ; \az. 21 parts of the former, and 79 of 

 the latter gas. In an atmosphere of pure hydrogen or nitro- 

 gen, a man would run the risk of suff'ocation in a very few 

 seconds, not because these gases are themselves poisonous, 

 but simply from the absence of oxygen. 



Many gases produce a directly poisonous efi"ect, and cannot 

 be breathed even when mixed with oxygen ; as, for instance, 

 arseniuretted hydrogen, sulphuretted hydrogen, phosphoretted 

 hydrogen, carburetted hydrogen, carbonic oxide, cyanogen, 

 chlorine, ammonia, and many others. 



As a consequence of the process of respiration, the blood 

 becomes chemically changed; this change is almost entirely 

 confined to the blood-corpuscles, which in this independent 



