138 ON THE CIRCULATION OF THE BLOOD, 



fess our ignorance than attempt to be wise upon the basis ot conceit. All that 

 we do know is, that bodies of every kind are reducible to a few elementary 

 principles, which appear to be unchangeable, and are certainly invisible ; an'd 

 that from different combinations and modifications of these proceeds every 

 concrete and visible form: hence, air itself, and water; hence mineral, vege- 

 table, and animal substances. Air, therefore, and water, or either separately, 

 may contain the rudimental materials of all the rest. We behold metallic 

 stones, and of large magnitude, fall from the air, and we suppose them to be 

 formed there : we behold plants suspended in the atmosphere, and still, year 

 after year, thriving and blooming, and diffusing odours : we behold insects 

 apparently sustained from the same source ; and worms, fishes, and occa- 

 sionally man himself, supported from the one or the other, or from both. 

 These are facts, and as facts alone we must receive them, for we have at 

 present no means of reasoning upon them. There are innumerable mysteries 

 in matter as well as in mind ; and we are not yet acquainted with the nature 

 of those elementary principles from which every compound proceeds, and to 

 which every thing is reducible. We are equally ignorant of their shapes, 

 their weight, or their measure. 



LECTURE XIII. 



ON THE CIRCULATION OF THE BLOOD, RESPIRATION, AND ANIMALIZATIOJN. 



THE progress of science is slow, and often imperceptible ; and though in a 

 few instances it has been quickened by an accidental discovery or an acci- 

 dental idea, that has given a new turn, or a new elasticity to the chain of 

 our reasoning, still have we been compelled in every instance to follow up 

 the chain, link after link, and series after series, and have never leaped for- 

 ward through an intermediate space without endangering our security, or 

 being obliged to retrace our career by a painful and laborious reinvestigation. 



It required a period of three thousand six hundred years to render the 

 doctrine of a vacuum probable, and of five thousand six hundred to establish 

 it upon a solid foundation. For its probability we are indebted to Epicurus, 

 for its certainty to Sir Isaac Newton. The present theory of the solar sys- 

 tem was commenced by Pythagoras and his disciples five centuries before 

 Christ, and only completed by Copernicus fifteen centuries after Christ. 

 Archimedes was the first who invented the celebrated problem for squaring 

 the parabola, which was upwards of two hundred years before the Christian 

 era ; yet an exact problem for squaring the circle is a desideratum in the 

 present day. The simple knowledge of the magnet was familiar to the Ro- 

 mans, Greeks, and some of the oriental nations while in their infancy ; it has 

 been employed by the mariner for nearly six centuries in Europe, and for a 

 much longer period by the Chinese, in their own seas ; yet at this moment 

 we are acquainted with only a very few of its laws, and have never been able 

 to appropriate it to any other purpose than that of the compass. 



The circulation of the blood in the animal system is our subject of study 

 for the present lecture, and it is a subject which has laboured under the same 

 difficulties, and has required as long a period of time as almost any of the 

 preceding sciences, for its complete illustration and establishment. Hippo- 

 crates guessed at it; Aristotle believed it; Servetus, who was burnt as a 

 heretic in 1553, taught it; and Harvey, a century afterward, demonstrated it. 



I shall not here enter into the various steps by which this wonderful dis- 

 covery was at length effected ; the difficulty can be only fairly appreciated 

 by those who are acquainted with the infinitely minute tubes into which the 

 distributive arteries branch out, and from which the collective veins arise ; 

 but every one is interested in the important fact itself, for it has done more 



