I 



SEAT OF THE OXIDATION PROCESS. 839 



these two gases, and that both, therefore, circulate in the two kinds of 

 blood ; nor was he aware, that frogs made to respire nitrogen or hy- 

 drogen, that is, an atmosphere destitute of oxygen, continue for a time 

 to exhale carbonic acid. The existence of both carbonic acid and 

 oxygen, in solution, in the entire blood, shows that the combination of 

 oxygen with some carbonaceous compound in the venous blood derived 

 from the disintegrated tissues, does not take place in the lungs only; 

 but that this union must occur in some other part of the body. More- 

 over, if this moist combustion took place entirely in the lungs, those 

 organs should be very much warmer than any other part of the system ; 

 but, though some authorities, as already mentioned, maintain that the 

 blood in the left ventricle, just returned from the lungs, is warmer than 

 the blood in the right ventricle, the alleged increase of temperature 

 has never been stated to be more than 2 ; whilst equally competent 

 observers testify to an exactly opposite condition, as regards the tem- 

 perature of the venous and arterial blood, before, and after, it has 

 passed through the lungs. 



According to another and more plausible view, the oxygen in the 

 aerated blood, partly dissolved in the serum, but chiefly in loose chem- 

 ical combination with the colored substance of the red corpuscles, and 

 perhaps with the fibrin, is conveyed in the arterial blood to the sys- 

 temic capillaries, where a certain loss of oxygen, and a nearly propor- 

 tionate addition of carbonic acid, occur, the blood then becoming 

 venous. On this supposition, the process of oxidation, or respiratory 

 combustion, takes place not in the lungs, nor in the pulmonary capilla- 

 ries, but in the system, in or near the systemic capillaries. This 

 opinion is in harmony with the fact, that the arterial blood once hav- 

 ing acquired, in passing through the pulmonary capillaries, its special 

 characters, amongst others its bright color, retains that color, and, by 

 presumption, its other qualities also, along the whole arterial system, 

 only losing them as it passes through the systemic capillaries. It is 

 also consistent with other facts, already'mentioned, viz., that, whereas 

 arterial blood contains more oxygen than venous blood, venous blood 

 contains more carbonic acid. If, as is asserted, the sugar found in the 

 venous blood in the right side of the heart, is absent in the arterial 

 blood in the left side of that organ, then a small portion of the oxygen 

 absorbed in the lungs, or else some of that previously contained in the 

 blood, must have united with that carbhydrate, and so have given rise 

 to a certain amount of carbonic acid; but by far the larger proportion 

 of the oxygen probably passes on unchanged in the arterial blood- 

 current. 



How much of the systemic process of oxidation which then takes 

 place, occurs in the blood of the systemic capillaries, or in the tissues 

 traversed by those vessels, is yet unknown. But there is reason to 

 believe, that it happens in both situations. In the functional activity 

 of all the tissues and glands, both secreting and ductless, but especially 

 of the muscular and nervous tissues, constant nutritive changes are in 

 progress; their life never stands still. Disintegration and renewal are 

 unceasing; and the former always implies retrograde chemical meta- 

 morphoses, of which partial or complete oxidation is a characteristic 



