DISCOVERY OF THE CIRCULATION. 29 



During coagulation, fibrin assumes a filamentous form, presenting, under 

 the microscope, the appearance of rectilinear fibrillae. These fibrillae 

 gradually increase in number, and as contraction of the clot occurs, they be- 

 come irregularly crossed. They are always straight, however, and never 

 assume the wavy appearance characteristic of true fibrous tissue. 



The blood of the renal and hepatic veins, capillary blood and the blood 

 which passes from the capillary system into the veins after death generally 

 does not coagulate or coagulates very imperfectly ; in other words, these varie- 

 ties of blood do not readily form fibrin. The reason of this peculiarity is not 

 known ; but the fact affords a partial explanation 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 losing its coagulability in its pas- 

 sage 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 coagulation can not take 

 place while the normal circulation is maintained and while the blood is 

 undergoing the constant changes incident to general 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 the movements of the heart Force of the heart Action 

 of the valves Sounds of the heart Causes of the sounds of the heart Frequency of the heart's action 

 Influence of age and sex Influence of digestion Influence of posture and muscular exertion In- 

 fluence of exercise etc. Influence of temperature Influence of respiration on the action of the heart 

 Cause of the rhythmical contractions of the heart Accelerator nerves Direct inhibition of the heart 

 Reflex inhibition of the heart Summary of certain causes of arrest of the action of the heart. 



HAEVEY "set forth for the first time his discovery of the circulation," 

 in his public lectures in 1616, and in 1628 published the " Exercitatio Ana- 

 tomica de Motu Cordis et Sanguinis in Animalibus." This discovery, from 

 the isolated facts bearing upon it which were observed by anatomists to its 

 culmination in the experiments of Harvey, so fully illustrates the gradual 

 development of most physiological truths, that it does not seem out of place 

 to begin the study of the circulation with a brief sketch of its history. 



The facts bearing upon the circulation developed before the time of 

 Harvey 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 physi- 

 ologists 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 

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