220 RESPIRATION. 



they are in intimate connection for another purpose. For 

 the nutrition of the rest of the lungs, including the pleura, 

 interlobular tissue, bronchial tubes and glands, and the 

 walls of the larger blood-vessels, a special supply of arterial 

 blood is furnished through one or two bronchial arteries, 

 the branches of which ramify in all these parts. The blood 

 of the bronchial artery, when, having served for the nutri- 

 tion of these parts, it has become venous, is carried partly 

 into the branches of the bronchial vein, distributed in the 

 parts about the root of the lung, and partly into the small 

 branches of the pulmonary artery, or, more directly, into 

 the pulmonary capillaries, whence, being with the rest of 

 the blood arterialized, it is carried to the pulmonary veins 

 and left side of the heart. 



Changes of the Air in Respiration. 



By their contact in the lungs the composition of both air 

 and blood is changed. The alterations of the former being 

 manifest, simpler than those of the latter, and in some 

 degree illustrative of them, may be considered first. 



The atmosphere we breathe has, in every situation in 

 which it has been examined in its natural state, a nearly 

 uniform composition. It is a mixture of oxygen, nitrogen, 

 carbonic acid, and watery vapour, with, commonly, traces 

 of other gases, as ammonia, sulphuretted hydrogen, etc. 

 Of every i oo volumes of pure atmospheric air, 79 volumes 

 (on an average) consist of nitrogen, the remaining 21 of 

 oxygen. The proportion of carbonic acid is extremely 

 small ; 10,000 volumes of atmospheric air contain only 

 about 4 or 5 of carbonic acid. 



The quantity of watery vapour varies greatly, according 

 to the temperature and other circumstances, but the at- 

 mosphere is never without some. In this country, the 

 average quantity of watery vapour in the atmosphere is 

 i -40 per cent. 



