THE RESPIKATOEY SYSTEM. 391 



facts or theories of chemistry or physics into a solution of a 

 complex problem. Besides, some of the experiments on which 

 the conclusions have been based are questionable, inasmuch as 

 tiey seem to induce artificial conditions in the animals oper- 

 ated upon ; and we have already insisted on the blood being 

 regarded as a living tissue, behaving differently in the body 

 and when isolated from .it, so that even in so-called blood-gas 

 experiments there may be sources of fallacy inherent in the 

 nature of the case. 



Foreign Gases and Respiration.— These are divided into: 



1. Indifferent gases, as N, H, CH,, which though not in 

 themselves injurious, are entirely useless to the economy. 



2. Poisonoics gases, fatal, no matter how abundant the nor- 

 mal respiratory food may be. They are divisible iuto: (a) those 

 that kUl by displacing oxygen, as NO, CO, HON ; (b) narcotic 

 gases, as COs, N^O, producing asphyxia when present in large 

 quantities ; (c) reducing gases, as HaS, (NH4)2S, PHa, AsHs, CjNj, 

 which rob the haemoglobin of its oxygen. 



There are probably a number of poisonous products, some 

 of them possibly gases, produced by the tissues themselves and 

 eliminated normally by the respiratory tract ; and these are 

 doubtless greatly augmented, either in number or quantity, or 

 both, when other excreting organs are disordered. 



RESPIRATION IN THE TISSUES. 



We first direct attention to certain striking facts: 

 1. An isolated (frog's) muscle will continue to contract for 

 a considerable period and to exhale carbon dioxide in the total 

 absence of oxygen, as in an atmosphere of hydrogen ; though, 

 of course, there is a limit to this, and a muscle to which either 

 no blood fiows, or only venous blood, soon shows signs of 

 fatigue. 3. In a frog, in which physiological saline solution 

 has been substituted for blood, the metabolism will continue, 

 carbonic anhydride being exhaled as usual. 3. Substances, 

 which are readily oxidized, when introduced into the blood of 

 a living animal or into that blood when withdrawn undergo 

 but little oxidative change. 4. An entire frog will respire car- 

 bonic dioxide for hours in an atmosphere of nitrogen. 



Such facts as these seem to teach certain lessons clearly. It 

 is evident, first of all, that the oxidative processes that give rise 

 to carbon dioxide occur chiefly in the tissues and not in the 



