162 RESPIRATION. 



From this fact alone, it is more than probable that respiration is inseparably connected 

 with the general act of nutrition. Its processes must be studied, therefore, as they take 

 place in the tissues and organs of the body. In the present state of the science, the 

 questions which naturally arise in connection with the essential processes of respiration 

 are the following : 



1. In what way is oxygen consumed in the system? 



2. How is carbonic acid produced by the system? 



3. What is the nature of the processes which take place between the disappearance 

 of oxygen and the evolution of carbonic acid ? 



"When these questions are satisfactorily answered, we shall understand the essence of 

 respiration ; but, in reasoning on this subject, we must not fall into the error of assimilat- 

 ing the respiratory phenomena too closely to those with which we are acquainted as 

 they occur in inorganic bodies. It must be remembered that in the organism we are 

 dealing with principles which have the remarkable property of self-regeneration, and 

 which, as a simple condition of normal existence, consume oxygen, when it is presented 

 to them, and exhale carbonic acid. Without a proper supply of oxygen, the tissues die, 

 lose these peculiar properties, and finally disappear by putrefactive decomposition. This 

 consumption of oxygen cannot be regarded in any other light than as the appropriation, 

 by a living part, of an element necessary to supply waste, in the same way as those ma- 

 terials which are ordinarily called nutritive are appropriated. That waste is continually 

 going on there can be no doubt; and, as the production of urea, creatine, creatinine, 

 cholesterine, etc., is, to a certain extent, independent of the absorption of food, so the 

 production of carbonic acid is in a certain degree independent of the absorption of oxy- 

 gen. How different are these phenomena from those which attend the combinations and 

 decompositions of inorganic matters ! As an example, let oxygen be brought in contact, 

 under proper conditions, with iron. Under these circumstances, a union of iron and 

 oxygen takes place, and a new substance, oxide of iron, is formed, which has peculiar 

 and distinct properties. In the ame - way, carbonic acid may be disengaged from its 

 combinations by the action of a stronger acid, which unites with the base and forms a 

 new substance in no way resembling the original salt. To make the contrast still more 

 striking, let fat be heated in oxygen or in the air until it undergoes combustion; it is 

 then changed into carbonic acid and water, by a definite chemical reaction, and is utterly 

 destroyed as fat. 



In the living body the organic nitrogenized principles are in a condition of continual 

 change, breaking down and forming various excrementitious principles, at the head of 

 which may be placed carbonic acid. It is essential to life that these principles be main- 

 tained in their chemical integrity, which requires a supply of fresh matter as food, and, 

 above all, a supply of oxygen. We put ourselves in the position of ignoring well-estab- 

 lished facts and principles when we assimilate without reserve the process of the con- 

 sumption of oxygen and production of carbonic acid by living organic bodies, to simple 

 combustion of sugar or fat. The ancients saw that the breath was warmer than the sur- 

 rounding air, that in the lungs the air took heat from the body, and, as they knew of no 

 other changes in the air produced by respiration, they assumed that its object was simply 

 to cool the blood. Lavoisier discovered that the air, containing oxygen, lost a portion 

 of this principle in respiration and gained carbonic acid and watery vapor. He saw that 

 this might be imitated by the combustion of hydro-carbons, such as exist in the blood. 

 He called respiration a slow combustion and regarded as its principal office the mainte- 

 nance of animal temperature. When it was shown by analyses of the blood for gases, 

 that oxygen is not consumed in the lungs, but is taken up by the circulating fluid and 

 carried all over the body, and that carbonic acid is brought from all parts by the blood 

 to the lungs, these facts, taken in connection with the fact that the tissues have the prop- 

 erty of consuming oxygen and exhaling carbonic acid, led physiologists to change the 

 location of the combustive process from the lungs to the tissues. 



