INTERNAL RESPIRATION 551 



are unicellular in the earliest stages of their development. The 

 amoeba breathes, but possesses, as far as the structure can be 

 determined by the microscope, no part especially differentiated 

 for respiration ; gaseous exchange appears to be a fundamental 

 property of the protoplasm, and appears to take place directly 

 or by the aid of ferments ; oxygen is absorbed and carbon dioxide 

 is discharged at the surface. The fertilised ovum, which will 

 ultimately develop into a man, appears to breathe in a similar 

 manner. The living cell possesses a great affinity for oxygen, and 

 absorbs it from the surrounding fluid which contains oxygen in 

 solution ; by its chemical activity it produces carbon dioxide, 

 which passes into the surrounding fluid, in which the pressure of 

 the gas is less. Such appears to be the process in the unicellular 

 organism. Does this obtain in the more complex organisms, or 

 do they acquire with the differentiation of structure special methods 

 of respiration ? Are the blood and lymph to be considered as 

 simple respiratory media, or are they the special seats of the 

 oxidation of the waste products of the tissues ? Are the lungs 

 only mechanisms for the ventilation of the internal medium, the 

 blood, by exposure to the influence of the external medium, the 

 inspired air, or do they by some special form of activity oxidise 

 waste products carried to them by the blood ? These are ques- 

 tions which need careful consideration. In the first place, the 

 question of oxidation in the blood demands attention, for Lavoisier 

 and many of the earliest investigators of the chemistry of respira- 

 tion held that the oxygen breathed into the lungs combined with 

 the carbon contained in the venous blood of the lungs to form 

 the carbon dioxide which was expired. This view was supported 

 for a time by the failures of many observers to extract any gas 

 from arterial or venous blood, results which have already been 

 shown to be due to defective methods. Blood removed from the 

 body and kept from contact with the air becomes darker in colour, 

 poorer in oxygen, and richer in carbon dioxide. These changes 

 occur at the ordinary temperature of the air, but are delayed by 

 cold and quickened by a temperature equal to that of the body ; 

 the oxidation of waste products removed from the tissues might, 

 as some thought, be the explanation, but more modern work 

 points to other causes. Blood is a tissue, and as such is the seat 

 of oxidation ; plasma and serum do not show the same oxidation. 

 Putrefaction is the most important cause ; it quickly occurs in 



