412 LECTURE XVIII. 



a long time without oxygen, and produce carbon dioxide; and, on the other 

 hand, we have cited the experiments of Fick and Wislicenus, 1 who showed 

 that the energy set free in the cleavage processes was altogether insuffi- 

 cient to account for the work which these authors were capable of 

 accomplishing. We came to the conclusion then, that under some circum- 

 stances the muscular cells, in order to satisfy the demands placed upon 

 them, must utilize all the chemical energy available from the food 

 materials it receives. 



On the other hand, the assumption that the cells in general are satisfied 

 to accomplish their work with the energy resulting from cleavage processes, 

 is supported by the fact that there are unicellular organisms which not 

 only do not require oxygen, but on which in fact this gas even acts as a 

 poison. These are the anaerobic bacteria. All sorts of varieties of bacteria 

 are known, ranging from those to which oxygen is indispensable to those 

 which get along best without it. There are, in fact, bacteria which are 

 temporarily anaerobic; i.e., they can get along without oxygen for a time. 

 It is characteristic of all these bacteria that they eliminate carbon 

 dioxide, no matter whether they take up oxygen directly from the air or 

 not. The bacteria which are wholly anaerobic, and those which are tem- 

 porarily so, must be able to obtain oxygen from the nutriment upon which 

 they subsist. In the latter case, the assumption might be made that they 

 store up compounds rich in oxygen, which they consume during the anaero- 

 bic period, just as the muscular cells are evidently capable of storing up 

 oxygen, when they are in a state of rest, which they require when the 

 muscles are being used. 



We shall later on 2 go more into detail concerning the significance of 

 this progressive breaking down by stages of the nutriment on the part of 

 the cells in our body, and shall find that by means of this alternate simple 

 decomposition and oxidation it is possible to obtain energy from the food 

 as it is required. At all events, all our observations indicate that each 

 individual cell in the body must have the possibility of obtaining oxygen 

 for oxidation processes, and for the regulation of its internal economy. 

 We shall soon become acquainted with facts which compel us to accept 

 this assumption. 



Let us now attempt to trace the course of the oxygen from the time it 

 is taken up by the lungs till it is given up to the cells of the individual 

 tissues. The blood plays an intermediate part in the process. It takes 

 the oxygen from the lungs and gives it up to the tissues. The first gas- 

 exchange is commonly spoken of as external respiration, and the latter as 

 internal respiration. The question that interests us first of all is how does 

 the oxygen circulate in the blood. There are two possibilities to be con- 



1 Lecture IV, p. 69. 



2 See Lecture XIX. 



