396 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



conditions of the pulmonary blood- stream. In the opened chest these cannot 

 be entirely restored by artificial respiration. The thinness of the wall of the 

 pulmonary artery, however, indicates that it has much less pressure to support 

 than that of the aorta, which fact also is indicated by such roughly approxi- 

 mate results as have been obtained with the manometer after opening the 

 chest. 



As the pulmonary artery and veins lie wholly within the chest, but outside 

 the lungs, their trunks and larger branches all tend to be dilated continuously 

 by the elastic pull of the lungs a pull which increases at each inspiration. 

 On the other hand, the pulmonary capillaries lie so close to the surface of each 

 lung that they are exposed to the same pressure, practically, as that surface, 

 and the full weight of the atmosphere may act upon them. These conditions 

 all tend to unload the capillaries and the pulmonary veins, but to weaken the 

 unloading of the pulmonary artery. The two effects can hardly balance one 

 another, however. The wall of the pulmonary artery is so much stiffer than 

 that of the vein, that the actual results should be favorable to the flow. The 

 elasticity of the lungs and the contractions of the muscles of inspiration thus 

 lighten, probably, the work of the right ventricle as well as of the left. The 

 right ventricle, however, like the left, can accomplish its work without assist- 

 ance; for the entire circulation, including, of course, the flow through the 

 lungs, continues after the chest has been opened, if artificial respiration be 

 maintained. 



G. THE PULSE-VOLUME AND THE WORK DONE BY THE VENTRICLES 



OF THE HEART. 



The Cardiac Cycle. It is assumed that the anatomy of the heart is known 

 to the reader. 



The general nature and effects of the heart's beat have been sketched 

 already. Each beat has been seen to comprise a number of phenomena, which 

 occur in regular order, and which recur in the same order during each of the 

 succeeding beats. Each beat is therefore a cycle ; and the phrase " cardiac 

 cycle " has become a technical expression for " beat/' as it conveys, in a word, 

 the idea of a regular order of events. As each of the four chambers of the 

 heart has its own systole and diastole, there are eight events to be studied in 

 connection with each cycle. The systoles of the two auricles, however, are 

 exactly simultaneous, as are their diastoles ; and the same is true of the sys- 

 toles and of the diastoles of the two ventricles. We may, therefore, without 

 confusion, speak of the auricular systole and diastole, and of the ventric- 

 ular systole and diastole, as of four events, each involving the narrowing or 

 widening of two chambers, a right and a left. The heart of the mammal 

 or bird consists essentially of a pair of pumps, the ventricles, each of which 

 acts alternately as a powerful force-pump and as a very feeble suction- 

 pump. To each ventricle is superadded a contractile appendage, the auricle, 

 through which, and to some extent by the agency of which, blood enters the 

 ventricle. 



