248 FUNCTIONS OF NUTRITION. 



A mixture of one volume of carbonic acid with three volumes of 

 atmospheric air may be inspired without difficulty, producing a rather 

 agreeable sensation in the lungs. After about 3000 cubic centimetres 

 have been inhaled in successive inspirations, a sense of dyspnoea comes 

 on, which, however, is not particularly increased by continuing the 

 inspiration to 6000 cubic centimetres. The nervous symptoms are 

 moderate in degree, but similar to the preceding. 



On the other hand, pure nitrogen has no taste nor odor, nor does it 

 have any stimulating effect on the mucous membrane. It may be in- 

 spired to the amount of 6000 cubic .centimetres, without producing any 

 sense of dyspnoea, or any perceptible effect on the nervous system. 



These results indicate that the presence of carbonic acid in the lungs 

 acts as a stimulus to respiration by causing a sense of the want of air ; 

 and that, furthermore, its principal toxic effect, when in abnormal quan- 

 tity, is the production of more or less insensibility or anaesthesia. The 

 sense of drowsiness and inattention experienced in an imperfectly ven- 

 tilated lecture-room or theatre is probably due to this cause, especially as 

 the burning gas-lights contribute at the same time to the formation of car- 

 bonic acid. The temporary nature of these sensations, and their immedi- 

 ate relief on coming into the open air, are matters of common observation. 



The third element in the vitiation of air by the breath is the exhala- 

 tion of organic vapor. This is the least understood, but probably the 

 most deleterious ingredient of the atmosphere produced by respiration. 

 It is this which causes the offensive odor, and the sense of oppression 

 on entering any confined space, where too great a number of persons 

 have remained without sufficient renewal of the air. It is most marked 

 when continued respiration, with neglect of ventilation, has been going 

 on over night, as in a crowded dormitory or sleeping-car ; since the 

 organic emanations have then had time not only to accumulate but 

 also to pass into a state of incipient decomposition. In this condition 

 they resemble the class of animal poisons ; and there is reason to 

 believe, that when introduced into the system, they may cause dis- 

 turbances which last for a considerable time. It is certain that the 

 contagion of many febrile diseases is communicated through the air by 

 the products of respiration ; and the normal organic exhalations of the 

 pulmonary mucous membrane, when altered by concentration, the accu- 

 mulation of moisture, and an elevated temperature are perhaps capable 

 of producing analogous effects. 



All the above causes of vitiation of the atmosphere in respiration, 

 notwithstanding 'the differences in their nature and effects, are to be 

 obviated by the same means; that is, a sufficient renewal of the air by 

 ventilation. 



Relation between the Oxygen absorbed in Respiration and the Carbonic 

 Acid given off. It has been seen that, in man, with each respiration, on 

 the average, 16 cubic centimetres of oxygen are absorbed, and 13 cubic cen- 

 timetres of carbonic acid given off. As the oxygen thus taken in weighs 

 rather less than .023 gramme while the carbonic acid discharged weighs 



