198 HAND-BOOK OF PHYSIOLOGY. 



through the pulmonary capillaries in steady streams, and slowly enough 

 to permit every minute portion of it to be for a few seconds exposed to 

 the air, with only the thin walls of the capillary vessels and the air-cells 

 intervening. The pulmonary circulation is of the simplest kind: for the 

 pulmonary artery branches regularly; its successive branches run in 

 straight lines, and do not anastomose: the capillary plexus is uniformly 

 spread over the air-cells and intercellular passages; and the veins derived 

 from it proceed in a course as simple and uniform as that of the arteries, 

 their branches converging but not anastomosing. The veins have no- 

 valves, or only small imperfect ones prolonged from their angles of junc- 

 tion, and incapable of closing the orifice of either of the veins between 

 which they are placed. The pulmonary circulation also is unaffected by 

 changes of atmospheric pressure, and is not exposed to the influence of 

 the pressure of muscles: the force by which it is accomplished, and the 

 course of the blood, are alike simple. 



Changes produced in the Blood by Respiration. The most 

 obvious change which the blood of the pulmonary artery undergoes in 

 its passage through the lungs is 1st, that of color, the dark crimson of 

 venous blood being exchanged for the bright scarlet of arterial blood; 2nd, 

 and in connection with the preceding change, it gains oxygen; 3rd, it 

 loses carbonic acid; tth, it becomes slightly cooler (p. 193); 5th, it coagu- 

 lates sooner and more firmly, and, apparently, contains more fibrin (see 

 p. 87). The oxygen absorbed into the blood from the atmospheric air 

 in the lungs is combined chemically with the haemoglobin of the red 

 blood-corpuscles. In this condition it is carried in the arterial blood to- 

 the various parts of the body, and brought into near relation or contact 

 with the tissues. In these tissues, and in the blood which circulates in, 

 them, a certain portion of the oxygen, which the arterial blood contains, 

 disappears, and a proportionate quantity of carbonic acid and water is 

 formed. The venous blood, containing the new-formed carbonic acid, 

 returns to the lungs, where a portion of the carbonic acid is exhaled, and 

 a fresh supply of oxygen is taken in. 



Mechanism of Various Respiratory Actions. It will be well 

 here, perhaps, to explain some respiratory acts, which appear at first 

 sight somewhat complicated, but cease to be so when the mechanism by 

 which they are performed is clearly understood. The accompanying dia- 

 gram (Fig. 161) shows that the cavity of the chest is separated from that 

 of the abdomen by the diaphragm, which, when acting, will lessen its 

 curve, and thus descending, will push downward and forward the ab- 

 dominal viscera; while the abdominal muscles have the opposite effect, 

 and in acting will push the viscera upward and backward, and with 

 them the diaphragm, supposing its ascent to be not from any cause inter- 

 fered with. From the same diagram it will be seen that the lungs com- 

 municate with the exterior of the body through the glottis, and further 



