598 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



ox}'gen is less than that of the arterial blood. As a consequence, the 

 oxy haemoglobin is broken up and yields a large part of its oxygen to the 

 tissues, passing through the walls of the capillaries by diffusion, to be 

 used up in the tissues in the processes of oxidation which are there con- 

 tinually taking place. One of the principal results of such oxidation 

 processes is the formation of carbon dioxide, and here, again, we have 

 phenomena of diffusion taking place between the tissues and capillaries. 

 In the tissues the gaseous tension of carbon dioxide is higher than in the 

 capillaries, and, as a consequence, by diffusion this gas leaves the tissues 

 to enter the blood-serum. This process of gaseous interchange in the 

 systemic capillaries may, therefore, be spoken of as internal respiration, 

 and varies in intensity in different conditions of the system. Thus the 

 interchange is more rapid and extensive in muscles during contraction 

 than in muscles at rest. It is more active in secreting glands than in 

 quiescent glands. 



As to the subsequent fate of the oxygen after leaving the blood- 

 vessels, but little can be positively said. It is probable that processes of 

 oxidation occur in the tissues, and that these result in the formation of 

 carbon dioxide, but it does not follow that the oxygen as it leaves the 

 blood directly unites with the organic tissue-constituents, and is thus 

 directly returned to the blood as carbon dioxide ; for we know that 

 various tissues removed from the body and placed in an atmosphere 

 entirely devoid of oxygen will still produce carbon dioxide. It is, there- 

 fore, possible that the oxygen that leaves the blood-vessels is stored up 

 in some combination or other in the tissues, and is then drawn upon as' 

 the needs of the tissues demand, finally to be returned to the blood as 

 carbon dioxide. 



5. The Nervous Mechanism of Respiration. — Although the muscles 

 of respiration are red, striped, voluntary muscles, and although through 

 an effort of will the rhythm, number, and depth of the respiratory move- 

 ments may be greatly modified and changed, the act of respiration is to he 

 regarded as an involuntary action. Although the breath may for a cer- 

 tain time be held and the movement of respiration completely interrupted, 

 after but a very short interval, probably not more than between two and 

 three minutes, in spite of the strongest effort-of the will, an inspiration must 

 be made. The fact that respiration persists in the 'absence of mental 

 effort points further to the fact of the independence of respiration of the 

 will. Thus, during sleep and in states of unconsciousness, in various 

 diseases and as an effect of various drugs, respiration may be perfectly 

 performed. So, also, in the lower animals the brain may be exposed and 

 may be gradually removed from the cerebrum down to the base of the 

 brain, and still respiration be unaffected, provided loss of blood or some 

 other accident does not interfere with the lower portions of .the central 



