CIRCULATION. 137 



been shown to have arisen from transfusing the blood of one species 

 into the body of another, in which the globules are of a different 

 size or shape. The blood of a sheep, for example, transfused into 

 a cat or rabbit, causes death in a short time ; and instantaneous 

 death follows the transfusion of blood with circular globules into an 

 animal which has these elliptical. 



If other substances are mingled with the blood, equally serious 

 effects follow. Farriers produce instantaneous death in horses by 

 blowing air into their veins ; and a person in Paris, a few years ago, 

 who was having an operation performed in which a large vein in 

 the neck had to be cut, from the entrance of air fell over and ex- 

 pired. No less than about thirty other cases have been published, 

 in which it is probable death occurred from the same cause. 



Having made these observations regarding the blood, we must 

 now explain the means employed for its circulation ; and in doing 

 so, we shall first describe this as it takes place in man and in the 

 other Mammalia ; for though their circulating system is really the 

 most complex, a knowledge of it forms a key to all the modifica- 

 tions which it sustains in the other classes. 



The course of the circulation of the blood was unknown until 

 the reign of James I., when it was discovered by Dr. Harvey. The 

 ancients knew of the existence of the veins and arteries, but 

 thought that the blood was moved backwards and forwards in the 

 veins, and that the arteries were filled with air. The name of the 

 latter, indeed, is derived from arteria, or air-tube. Harvey publicly 

 taught his new doctrines as early as 1616 ; but, with a caution 

 worthy of one whose fame was to be coeval with our race, spent no 

 less than twenty-six years in amassing materials for his immortal 

 work on the circulation. The reception it met with, when pub- 

 lished, is instructive. Derided by his own profession as a quack, 

 he was looked upon by the vulgar as crack-brained, as is always 

 the case with the ignorant, prejudiced, and jealous; and in a letter 

 written to a friend at this period, he complains that his practice had 

 suffered seriously since the publication of his book. To the honor 

 of mankind, however, it must be said that he lived long enough to 

 ^see his system taught in every university in the world. 



The circulating system in the Mammalia may be said to consist 

 of four principal parts first, the heart, which is the centre of the 

 whole; second, the arteries, which receive the blood from the 

 heart ; third, the veins, which return tie blood to the heart ; and. 



