412 PROPERTIES ANB FUNCTIONS 



tricle, from which proceeds a vessel that conveys it to the 

 gills. In the gills the blood undergoes the same changes 

 as in the lungs of men. After the change in the blood 

 is effected, instead of returning to the heart it is taken up 

 by small vessels and conveyed directly to all parts of the 

 body. Insects have no real heart, but they are supplied 

 with a small, simple, alternately contracting vessel, which 

 answers the purpose of a heart. Plants have a circulatory 

 system which consists of vessels, by means of which the 

 nutriment is conveyed from the earth to the leaves, and 

 from the leaves after it is assimilated to all the organs of 

 which they are composed. This system in plants is as 

 simple as it would be in man, if the food which enters 

 the mouth pas'sed into the stomach, and there after it had 

 undergone the process of assimilation, it was taken up by 

 small vessels and distributed to all parts of the body with- 

 out being compelled to pass through a heart or lungs. 



Theopinionof the circulation of the blood through the 

 body was loosely started by some of the early writers ; 

 but to obtain sufficient proof to support the doctrine was 

 so difficult that it was abandoned almost as soon as con- 

 ceived. Serveto, who lived in the 16th century, imper- 

 fectly taught it by pointing out the smaller circulation, or 

 that through the lun-s; but the illustrious Harvey, in the 

 17th century, established it by the most satisfactory ex- 

 periments. He, like all great discoverers, had to combat 

 the prejudices of mankind, but he had the happiness be- 

 fore his death to know that his doctrine was almost uni- 

 versally acknowledged. We rely at the present day upon 

 the same proofs that Harvey did for the support of this 

 doctrine. The proofs are deduced from the disposition 

 of the heart and blood vessels, and from the following ex- 

 periments. ' If we open an artery or a vessel that carries 

 the blood from the heart, the blood which comes from the 

 puncture flows in a direction from the heart, but if we 

 open a vein or a vessel that conveys the blood to the 

 heart the blood flows in an opposite direction. If we 

 examine with a microscope the almost transparent vessels 

 of frogs or other cold blooded animals, we see the blood 

 flowing from the heart into the arteries from these into 

 the veins, and from the veins back again to the heart 

 thus completing its circular career.' 



