RESPIRATION. 409 



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 decomposition or cleavage in consequence of 

 cell activity. 



Nevertheless many facts from the fields of comparative physiology 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 characteristic 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 pigments 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 colorless compound, which, however, on exposure recom- 

 bines with oxygen and regains the original color. 



Though free oxygen cannot 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 improb- 

 able 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., a frog placed in an 

 atmosphere of pure nitrogen will remain active and evolve CO 2 even for 

 several hours. 



Naturally the absorption of oxygen and the discharge of carbon dioxid 

 and the changes of composition which are incident to nutrition will be most 

 marked in those tissues characterized by the greatest degree of physiologic 

 activity. Muscle-tissue exhibits these changes to a greater degree than 

 bone. Tissues with intermediate degrees of activity should exhibit corre- 

 sponding degrees of respiratory change. Experiment confirms this view. 



