164 



contained only a few drops of biood which passed through small hole&> 

 which, Galen says, perforate the septum of the ventricles. 



The blood, however, continually urged on by the columns which fol- 

 low each other in succession, by the action of the veins whose parietes 

 become gradually stronger, and by the compression which these vessels 

 experience from the viscera, during the motion of respiration, reaches 

 the heart, and enters the auricles with the greater facility, as the 

 orifices of the cavae not being directly opposed to each other, the co- 

 lumns of blood which they convey, do not meet, and do not oppose each 

 other. 



LXVI. The blood continually carried to all parts of the body by the 

 arteries, returns, therefore to the heart, by a motion which can never be 

 interrupted, without considerable danger of life. We know that the cir- 

 culation is thus effected, from the direction of the valves of the heart, of 

 the arteries and veins; by what happens, when these vessels are opened, 

 compressed, or tied, or when a fluid is injected into them. When an 

 artery is wounded, the blood comes from the part of the vessel nearest 

 the heart: it comes, on the contrary, from towards the extremities, if it 

 is a vein that has been opened. By compressing or tying an artery, the 

 course of the blood is suspended below the ligature, and the vessel 

 swells above. The veins, on the contrary, when tied or compressed, di- 

 late below. Lastly, when an acid fluid is injected into a vein, the blood 

 is seen to coagulate in the direction of the heart. By the help of the mi- 

 croscope, we may see in the semi-transparent vessels of frogs and other 

 cold-blooded animals, the blood flowing from the heart into the arteries, 

 and from these into the veins, which return it to the heart. It was on 

 the strength of these convincing proofs, that William Harvey established, 

 towards the middle of the seventeenth century, the theory of the circula- 

 tion qf the blood. Its mechanism had rather been guessed at, than un- 

 derstood, by several authors. Servetus and Cesalpinus appear to have- 

 been acquainted with it*; but no one has more clearly explained it than 

 the English physiologist, who is justly considered the author of that im- 

 mortal discovery. 



LXVII. The theory of Harvey, such as it is laid down in his work, 

 entitled, De sanguinis circuitu^ extrcitationes anatomicx, does not appear 

 to roe entirely admissible. He considers the heart as the only agent 

 which sets the blood in motion, and does not take into account the action 

 of the veins and arteries, which he considers as completely inert tubes, 

 while every thing tends to prove that the arteries and veins assist the mo- 

 tion of the blood by an action peculiar to themselvesf. He admits, that 

 the blood flows in every part of the circulatory system, with an uniform 

 degree of speed; an opinion so manifestly contradicted by reasoning and 

 experience, which prove that the velocity of its course diminishes, the 

 greater its distance from the heart, from the influence of a great number 

 of circumstances, which it would be useless to repeat (LV1I.) This 

 doctrine has yet, however, several abettors, and among the moderns, 

 Spallanzani has endeavoured to support it, by a number of experiments 



* Their " acquaintance" with the circulation was nothing better than mere guessing. 

 The passages of their works referring to this subject, maybe seen in the introduction 

 to Harvey Godman. 



j See APPENDIX* Notes A, B, S, T. 



