3 68 TEXT-BOOK OF PHYSIOLOGY. 



cells whereby the potential energy of the former is liberated as kinetic 

 energy; that the carbon dioxid so evolved passes into the lymph 

 and through the wall of the capillary into the blood. 



While this is doubtless the case, the presence of free oxygen in the 

 tissues can not be demonstrated by the usual methods of gas analysis. 

 Only in the saliva and in the blood of the placental umbilical vein can 

 it be shown that oxygen has directly passed through the capillary wall. 

 For this reason it has been claimed by a few investigators that oxygen 

 does not leave the blood, but that the field of its activity as an oxidizing 

 agent is limited to the blood-current, where it meets with and oxidizes 

 easily reducible substances entering from the tissues. On this view 

 the potential energy of the food would be liberated by mere decom- 

 position or cleavage in consequence of cell activity. 



Nevertheless many facts from the fields of comparative physi- 

 ology and physiologic chemistry combine to support the view that 

 oxygen is absolutely necessary to the maintenance of the life of all 

 tissue-cells. Though they will continue to manifest their character- 

 istic activities e. g., contraction on the part of a muscle, secretion by a 

 gland, the conduction of a nerve impulse by the nerve, etc. for a 

 variable length of time after oxygen is prevented from gaining access 

 to them, nevertheless they will in due time die. 



The necessity for oxygen on the part of the tissues and the avidity 

 with which they absorb it, is shown by their power of reducing pig- 

 ments such as alizarine blue. If this pigment be injected into the 

 blood-vessels of an animal and the animal killed in about ten minutes, 

 it will be found that while the blood exhibits a deep blue color the 

 tissues present their usual colors. But after exposure to the air or to 

 free oxygen the latter also acquire the characteristic blue color. The 

 explanation offered for this fact is that the tissues in their need for 

 oxygen absolutely extract it from the pigment, reducing it to a color- 

 less compound, which, however, on exposure recombines with oxygen 

 and regains the original color. 



Though free oxygen can not be shown to be present in the tissues, 

 there are many reasons for believing that it is continually passing into 

 them by way of the lymph-stream. Its rapid disappearance would 

 indicate that it is immediately utilized for the production of carbon 

 dioxid (which is improbable on other grounds), or that the tissues 

 possess a capacity for oxygen storage, of placing it in reserve under 

 some combination or other, by which it can be securely retained 

 until required for oxidation purposes. This is rendered probable 

 from the fact that the carbon dioxid evolved at any given moment is 

 not necessarily dependent on the oxygen just absorbed, for if oxygen 

 be withheld from a nutritive fluid which is being artificially circulated 

 through a recently isolated organ, carbon dioxid will continue to be 

 discharged for some time. A muscle, or even a living animal, e. g., 



