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LECTURE XIIL 



ON THE CIRCULATION OF THE BLOOD, RESPIKATION, AND ANIMALIZATIOlf . 



The progress of science is slow, arid often imperceptible ; and though 

 in a few instances it has been quickened by an accidental discovery or an 

 accidental 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 nev^r leaped 

 forward through an intermediate space without endangering our security, 

 or being obliged to retrace our career by a painful and laborious re- 

 investigation. 



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 probabiJity we are indebted to Epicurus, 

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

 system 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 de- 

 sideratum in the present day. The simple knowledge of the magnet was 

 familiar to the Romans, 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 sybject of gtudy 

 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. 

 Hippocrates guessed at it ; Aristotle believed it ; Seivetus, who was burnt 

 as a heretic in 1553, taught it ; and Harvey, a century afterwards, demon- 

 strated it. 



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

 discovery was at length effected ; the difficulty can be only fairly appre- 

 ciated 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 towards establishing the healing art upon a rational basis, 

 and subjecting the different diseases of mankind to a successful mode of 

 practice, than any other discovery that has emblazoned the annals of 

 medicine. 



In our last lecture we traced the action of the digestive organs ; we be- 

 held the Tood first comminuted by rtjeans of jaws, teeth, or peculiar mus- 

 cles or tneiiibranes ; nexi cOiiverted into a pulpy mass, and afterwards into 

 a milky liquid ; and in this state drunk up by the mouths of innumerable 

 minute vessels, that progressively unite into one conmion trunk, and con- 

 vey it to the heart as the chief organ of the system, for the use and benefit 

 of the whole, 



