OH AFTER XXII. 



THE CIRCULATION. 



The phenomenon of the circulation i;i of so interesting and 

 remai'kable a character, and its condition has such an influence 

 upon the health, that I think it advisable, as an introduc- 

 tion t(j the Medical Department, to make some reference to it. 

 There are so many diseases and difficulties of a serious character 

 which are the result of derangements of circulation, that it cer- 

 tainly seems necessary to give some explanation of it, that the 

 reader may be impressed the better with the necessity for such 

 prudence and care as would prevent its disturbance. The writer 

 thinks it also advisable, instead of giving a labored description, 

 which may be easily obtained from any physiology, and which 

 l:)ut few would take the trouble to read, to do this mainly by the 

 aid of illustrations, a variety of which liave been included at con- 

 siderable expense. 



General Plan of the Circulation. 



The blood is circulated through the body for the; purpose of 

 nutrition and secretion, by means of one forcmg pump, and 

 througii the lungs, for its proper aeration, by another ; the two 

 beino- united to form the heart. This organ is therefore a com- 

 pound machine, though the two pumps are joiaed together, so as 

 to appear to the casual observer to be one single organ. (For 

 general outlines see Fig. 625.) In common language, the heart of 

 the mammalia is said to have two sides, each of which is a forcing 

 pump ; but the blood before it passes fi-otn one side to the other, 

 has to circulate through one or the other set of vessels found in 

 the general organs of the body, and in the lungs, as the case may 

 be. This is shown at Figs. 622 and 623, where the blood, commenc- 

 ing with the capillaries on the general surface at a (Fig. 623), passes 

 (732) 



