DISCOVERY OF THE CIRCULATION. 31 



from the capillary system into the veins after death, does not generally coagulate or 

 coagulates very imperfectly ; in other words, these varieties of blood do not readily form 

 fibrin. The reason of this peculiarity is not known ; but the fact affords a partial ex- 

 planation of the normal fluidity of the blood ; for this fluid, passing over the- entire 

 course of the circulation in about thirty seconds, seems to be constantly IUMM^ its coagu- 

 lability in its passage through the liver, kidneys, and the general capillary system, as fast 

 as its coagulability is increased in the other parts. Taking into consideration the rapidity 

 of the circulation, it is evident that the blood cannot coagulate while the normal cir- 

 culation is maintained, and while it is undergoing the constant changes incident to gen- 

 eral nutrition. 



CHAPTER II. 



CIRCULATION OF THE BLOOD ACTION OF THE HEART. 



Discovery of the circulation Physiological anatomy of the heart Valves of the heart Movements of the heart 

 Impulse of the heart Succession of movements of the heart Force of the heart's action Action of the valves- 

 Sounds of the heart Causes of the sounds of the heart Frequency of the heart's action Influence of age 

 Influence of digestion Influence of posture and muscular exertion Influence of exercise Influence of tem- 

 perature Influence of respiration on the action of the heart Cause of the rhythmical contractions of the heart 

 Influence of the nervous system on the heart Division of the pneumogastrics Galvanization of the pneu- 

 mogastrics Causes of arrest of action of the heart Blows upon the epigastrium. 



HARVEY discovered the circulation of the blood in 1616, taught it in his public lect- 

 ures in 1619, and, in 1628, published the " Exercitatio Anatomica de Motu Cordis et 

 Sanguinis in Animalibus." 1 This momentous discovery, from the isolated facts bearing 

 upon it which were observed by anatomists, to its grand culmination with Harvey, so 

 fully illustrates the gradual development of most great physiological truths, that it does 

 not seem out of place to begin our study of the circulation with a rapid sketch of its 

 history. 



The facts bearing upon the circulation, which were developed before the time of Har- 

 vey, were chiefly anatomical. The writings of Hippocrates are very indefinite upon all 

 points connected with the circulatory system ; and no clear and positive statements are 

 to be found in ancient works before the time of Aristotle. The work of Aristotle most 

 frequently quoted by physiologists is his " History of Animals ; " and in this occurs a 

 passage which seems to indicate that he thought that air passed from the lungs to the 

 heart; but in his work, De Partibus Animalium, it is stated that there are two great 

 blood-vessels, the vena cava and aorta, arising from the heart, and that the aorta and 

 its branches carry blood. Galen, however, demonstrated experimentally the presence of 

 blood in the arteries, by including a portion of one of these vessels between two liga- 

 tures, in a living animal ; but his ideas of the communication between the arteries and 

 veins were erroneous, for he believed in the existence of small orifices in the septum be- 

 tween the ventricles of the heart, a mistake that was corrected by Vesalius, at about the 

 middle of the sixteenth century. 



In 1553, Michael Servetus, who is generally regarded as the discoverer of the pas- 

 sage of the blood through the lungs, or the pulmonary circulation, described in a work 

 on theology the course of the blood through the lungs, from the right to the left side of 

 the heart. This description, complete as it is, was merely incidental to the development 

 of a theory with regard to the formation of the soul, and the development of what were 

 called animal and vital spirits (spiritus). The same year, at the instigation of Calvin, 

 Servetus was burned alive at Geneva, and a copy of his book was also committed to the 

 flames. A few months b.'fora. a number of these books was burned at \'ienn:i, and 



