CHANGES IN THE BLOOD BY RESPIRATION. 293 



tion in a crowded and ill-ventilated apartment. 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 for a time 

 without sufficient renewal of the air. It is most marked when such con- 

 tinued respiration and neglect of ventilation have 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. They are then in the condition 

 in which they belong to the class of animal poisons ; and there is reason 

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

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

 tagion of many febrile diseases, as scarlatina, meases, and smallpox, 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 accumulation of moisture, and an elevated 

 temperature, are undoubtedly capable of producing morbid effects of an 

 analogous kind. 



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. 



Changes in the Blood by Respiration. 



The blood as it circulates in the arterial system has a bright scarlet 

 color; but as it passes through the capillaries it gradually becomes 

 darker, and on arriving in the veins it is deep purple, or in some situ- 

 ations nearly black. There are, therefore, two kinds of blood in the 

 body ; arterial blood, which is of a bright color, and venous blood, which 

 is dark. The dark-colored venous blood, which has been thus altered 

 by passing through the capillaries, is incapable, in this state, of supply- 

 ing the organs with their healthy stimulus and nutrition, and has lost its^ 

 value as a circulating fluid. It is accordingly returned to the heart by 

 the veins, and is thence sent, through the pulmonary artery, to the lungs. 

 In passing through the pulmonary circulation it reassumes its scarlet 

 hue, and is again converted into arterial blood. Thus the most striking 

 physical effect produced upon the blood by respiration is its change of 

 color from venous to arterial. 



This change is accomplished by the influence of the air in the pulmo- 

 nary cavities. For if defibrinated venous blood, recently drawn from 

 the veins of the living animal, be shaken up in a glass vessel with 

 atmospheric air, it at once changes its color and acquires the bright hue 

 of arterial blood. If injected through the vessels of the lungs them- 

 selves after removal from the body, the lungs being filled with air, the 

 same change takes place. If a dog be rendered insensible by a narcotic 

 injection or other similar means, the thorax opened, and artificial re- 

 spiration kept up by the nozzle of a bellows inserted into the trachea, 

 the dark venous blood can be seen in the great veins and in the right 



