On the Structure and Functions of the Organs of Respiration. 285 



blood. It would appear that the oxygen absorbed in the lungs, enters into 

 some feeble combination with arterial blood, and is carried in it to the 

 capillary vessels, where part of it is converted into carbonic acid, by taking 

 up carbon. Dr. Prout has framed an ingenious hypothesis, to account for 

 the disengagement of carbon in this situation. He says that " gelatine, 

 which contains three or four per cent, less of carbon than albumen con- 

 tains, enters into the structure of every part of the animal frame, and es- 

 pecially of the skin •" and as albumen exists largely in the blood, without 

 any gelatine, he considers that " the conversion of albuminous matter into 

 gelatine, is one great source of the carbonic acid of venous blood." If this 

 be the case, there is another beautiful analogy between the effects of animal 

 and vegetable respiration ; the conversion of albumen into gelatine, by the 

 liberation of carbon, being very similar to that of fecula, and other proxi- 

 mate principles, into sugar. Besides this source, however, it is certain 

 that a large quantity of carbonic acid is disengaged in the alimentary canal, 

 during the process of digestion ; and it is more than probable, that part of 

 it is absorbed by the mucous membrane with which it is in contact. The 

 quantity of carbon exhaled from the lungs is very considerable : it has 

 been usually estimated in man, at from six to eleven ounces per day; but 

 as the least of these quantities is more than is contained in three pounds 

 of beef, it is probable that both statements must be much beyond the truth. 



With regard to the absorption and exhalation of nitrogen. Dr. Edwards 

 has shown that both the processes are constantly going on, but that their re- 

 lative activity varies in different species, and at different parts of the year. 

 It appeared that an increase in the volnme of nitrogen in the respired air 

 took place in most young animals, and during the summer months ; but 

 that in the autumn and winter, there is a considerable absorption when 

 adult animals are employed. It is a curious question, which is yet unde- 

 termined, whether herbivorous animals absorb more nitrogen from the 

 atmosphere, than those of the carnivorous orders ; for as nitrogen scarcely 

 exists in vegetables, but enters largely into the constitution of all animal 

 bodies, it docs not seem unlikely that this is the source from which it is 

 derived, when not contained in the food. It is probable that no animal 

 can exist long, if fed exclusively on unazotized principles. 



The changes j)rodo.ced by respiration on the animal oeconomy are very 

 great : the most obvious are the conversion of venous into arterial blood, 

 and the maintenance of animal heat. The blood which has been charged 

 with carbonic acid, by circulating through the system, is thereby rendered 

 unfit for the support of life; and it is absolutely necessary that this sub- 

 stance should be got rid of, and replaced by oxygen, before the blood is 

 again fit for its varied purposes. This object is eflected, as we have al- 

 ready seen, by its exposure to atmospheric air, through the medium of the 

 respiratory surface ; and the changes which then take place, are strictly 



No. 5.— Vol. 1. 2 p 



