278 HAKDBOOK OF PHYSIOLOGY. 



removal of oxygen from and the addition of carbon dioxide to the blood. 

 These changes are sometimes spoken of as internal respiration. The 

 oxygen carried by the corpuscles of the blood in the form of oxyhaemo- 

 globin is given up to the tissues, as the tension of the gas within them 

 is very small. The gas thus set free is apparently seized upon by the 

 protoplasm of the tissues and built up into its molecule, and thus assists 

 in the process of anabolism, possibly uniting with some compound 

 somewhat in the same manner but more firmly than it does with haemo- 

 globin. The low oxygen pressure of the tissues thus allows a constant 

 abstraction of the gas from the blood. The process of katabolism, or 

 breaking down, is always associated with the evolution of carbon diox- 

 ide, so that as the blood passes through the tissues containing little of 

 this gas, the high tension of the gas in the tissues permits of its passage 

 into the blood. It has been proved that the process of the evolution of 

 carbon dioxide from living muscle will go on for a time in the absence 

 of a supply of free oxygen, and so it is clear that the former gas is not 

 derived directly from the combustion of the carbon in the presence of 

 the latter gas. It was at one time believed that the carbon dioxide of 

 venous blood resulted from the oxidation of substances in the blood 

 itself. It has, however, been shown that the blood itself has very slight 

 oxidizing powers, and that in the frog the whole of the blood may be 

 replaced by saline solution without producing any marked effect upon 

 the metabolism of the body. It is obviously unlikely that any but very 

 slight oxidation could go on in such a medium. It has moreover been 

 demonstrated that the tension of carbon dioxide in the tissues is con- 

 siderably greater in the tissues than it is in the venous blood. 



Special Respiratory Acts. 



It will be well here, perhaps, to explain certain special respiratory 

 acts, which appear at first sight somewhat complicated, but cease to be 

 so when the mechanism by which they are performed is clearly under- 

 stood. The diagram (fig. 215) shows that the cavity of the chest is sep- 

 arated from that of the abdomen by the diaphragm, which, when acting, 

 will lessen its curve, and thus descending, will push downward and 

 forward the abdominal viscera; while the abdominal muscles have the 

 opposite effect, and in acting will push the viscera upward and back- 

 ward, and with them the diaphragm, supposing its ascent to be not 

 from any cause interfered with. It will also be seen that the lungs 

 communicate with the exterior of the body through the trachea and 

 larynx, and further on through the mouth and nostrilsthrough either 

 of them separately, or through both at the same time, according to the 

 position of the soft palate. The stomach communicates with the ex- 

 terior of the body through the oesophagus, pharynx, and mouth; while 



