194- THE MOTION OF THE BLOOD. 



deposited. This nursling sap has been compared to the milk elaborated for 

 the young of animals. The returning or descending sap passes through either 

 vessels or intercellular spaces, chiefly along the innermost layer of bark, and 

 some along the outermost layer of wood, where it must mix more or less 

 with the ascending sap. In cellular plants, of course, the passage cannot be 

 through vessels, and perhaps it passes through cellular tissue in all. The 

 motion of the sap both in cellular tissues and vessels is explained, according to 

 M. llaspail, by the fact of the inner surfaces of the cells and vessels of vege- 

 tables, &c. absorbing and exhaling rapidly, by which motion is given to the fluid 

 and a current is established. (1. c. p. 317. sqq.) The power propelling the 

 sap is such, that, if a piece of the stem is cut out, it entirely empties itself; and 

 the sap has been found to flow from the extremity of a branch with a force suffi- 

 cient to overcome a column of water 43 feet 3^ inches in height. (Hales, 

 Statical Essays, vol. i. p. 101. ) 



It would not be right to terminate this section without a note upon the discovery 

 of the circulation of the blood ; atruth of which the ancients are thought to have re- 

 mained ignorant, from finding the arteries empty after death. But they knew tha* 

 these contained blood during life, as Galen (DeAnat. Admin, vii. 15.) relates some 

 amusing anecdotes of his pupils and some persons who promised to prove the arte- 

 ries empty. The discovery was made by our countryman, Dr. Harvey, Physician 

 to St. Bartholomew's Hospital, and promulgated by him at the age of forty-one, 

 in an anatomical and surgical course of lectures at the College of Physicians, in 

 1619. He is entitled to the glory of having made it, says Hume (History of 

 England, ch. 62.), " by reasoning alone, without any mixture of accident." He 

 informed Boyle, that he was led to it by reflecting on the arrangement of the 

 valves of the heart and veins, as exhibited by his master Fabricius. Nothing, he 

 knew, was planned in vain, and they clearly allowed a fluid to pass but one way. 

 By this argument, and the fact of a ligature upon an artery causing the blood to 

 accumulate in it on the side nearest the heart, and, upon a vein, beyond the liga- 

 ture j and that animals bleed to death by wounds in arteries or veins, he chiefly 

 established his doctrine. After his time it was demonstrated with the microscope 

 in cold-blooded animals. His immediate reward was general ridicule and abuse, 

 and SL great diminution of his practice r ; and no physician in Europe, who at the 

 time had reached forty years of age, ever, to the end of life, adopted his doctrine 

 of the circulation of the blood. (Hume, 1. c.) When the truth could be denied 

 no longer, he was pronounced a plagiarist ; the circulation was declared to have 

 been known to Plato ; nay, more, to king Solomon. (See Haller, EL Physiol. 

 t. i. p. 243.) The circulation through the lungs had certainly been taught about 



r This he laments in a letter to a friend, as may be seen in a MS. of the 

 Royal Society, referred to in the Life prefixed to the College edition of his 

 works : " Quod multo rarius solito ad aegros invisendos accersitus esset, post- 

 quain librum de motu cordis ediderit." 



