242 INTERNAL RESPIRATION OR TISSUE-RESPIRATION. 



dilation and the tissues. Those organic constituents of the tissues that 

 contain carbon are subjected during their vital activity to a process of 

 gradual oxidation, with the formation of carbon dioxid. Hence, the 

 following inferences may be drawn : 



r. The chief seat for the absorption of oxygen and the formation of 

 carbon dioxid is to be found within the tissues themselves. That the 

 oxygen rapidly penetrates from the capillary blood into the tissues is 

 shown by the fact that this blood rapidly becomes richer in carbon 

 dioxid and poorer in oxygen, while oxygenated blood, kept warm out- 

 side the body, changes much more slowly and incompletely. Further, 

 if fresh pieces of tissue be placed in defibrinated blood rich in oxygen, 

 the oxygen rapidly diminishes. Also, the circumstance that frogs de- 

 prived of their blood exhibit almost as great an interchange of gases as 

 do normal animals indicates that the gaseous interchange takes place 

 in the tissues themselves. Moreover, if the chief seat of oxidation lay, 

 not in the tissues themselves, but in the blood, then, if oxygen were 

 withheld from the blood (during suffocation), those reducing substances 

 that consume the oxygen in the process of oxidation should accumulate 

 in the blood. This is not the case, for even the blood of suffocated 

 animals contains only a trace of reducing substances. The absorption 

 of oxygen into the tissues may occur in the form of a temporary storing 

 of the gas, perhaps with the formation of intermediate lower oxida- 

 tion-products. This is followed by a period of more rapid separation 

 of carbon dioxid. Thus, the absorption of oxygen and the excretion of 

 carbon dioxid in the tissues do not necessarily proceed on parallel lines 

 and to the same extent. 



A clear picture of the development of carbon dioxid in the tissues is furnished 

 by the fact that a larger amount of this gas is found in the cavities of the body 

 and in their gases and fluids than in the blood of the capillaries. Pfliiger and 

 Strassburg found the tension of the carbon dioxid (in millimeters of mercury) 

 as follows : 



In arterial blood, 21.28 mm. In bile, 50.0 mm. 



" the peritoneal cavity, .... 58.8 ' hydrocele-fluid, 46.5 



" acid urine, 68.0 



The abundance of carbon dioxid in these fluids, as compared with that in the 

 blood, can arise only from the addition to them of the carbon dioxid generated in 

 the tissues. 



In the lymph of the thoracic duct the tension of the carbon dioxid (from 33.4 

 to 37.2 mm. of mercury) is, indeed, greater than in the arterial blood, but it is 

 still considerably less than in the venous blood. This fact does not, however, justify 

 the conclusion that only a small quantity of carbon dioxid is formed in the tissues 

 from which the lymph is collected. It rather permits the inference, either that 

 the lymph possesses less attraction for the carbon dioxid formed in the tissues 

 than does the capillary blood, where chemical forces are active in the production 

 at least of a partial combination of the gas; or that in the course of the slow 

 lymph-current the carbon dioxid is partially given back to the tissues by equaliza- 

 tion of tension; or, finally, that carbon dioxid is formed independently in the 

 blood. Furthermore, it is to be pointed out that those muscles that are known 

 to be the principal producers of carbon dioxid furnish this gas abundantly to the 

 blood, their tissues being relatively poor in lymph- vessels. 



The amount of uncombined, free carbon dioxid, capable of being pumped out, 

 in the fluids and gases mentioned indicates that the carbon dioxid passes over 

 from the tissues into the blood in an uncombined free state. However, Preyer 

 believes that the gas is carried over into the blood of the veins also in chemical 

 combination. 



The interchange of oxygen and carbon dioxid varies considerably in the differ- 

 ent tissues. In the first rank belong the muscles, which in a state of activity 



