vi CIECULATION OF BLOOD: ITS DISCOVEKY 159 



many text-books and monographs), and should know the names 

 of the men who have participated in its preparation or fulfilment. 

 In reviewing this interesting history we shall have an opportunity 

 of bringing forward those fundamental principles relating to the 

 Circulation, which must necessarily precede a more detailed treat- 

 ment of the subject. 



II. The story of the discovery of the circulation begins with 

 Galen (125-201 A.D.), who in his vivisections perceived the error 

 of the Alexandrian school. Headed by Erasistratus (300 B.C.), 

 they taught that the left heart and arteries are empty of blood, 

 and connected with the small bronchi by means of the arteria 

 aspera (trachea), which serve to carry the vital spirits (pneuma) to* 

 the different parts of the body, to animate them ; hence the veins 

 alone would contain the blood destined to provide the whole body 

 with nutriment. 



Galen showed that on puncturing any artery or the left heart 

 in a living being, the blood gushes forth, and, unlike that of the 

 veins, is pure, thin, and vaporous, due, that is, to a mixture of 

 blood with the air obtained through the lungs, " mix turn quid ex 

 ambobus." 



According to Galen, the arterial centre is the left heart, which 

 drives the blood endowed with vital spirits (sang ids spiritosus) 

 through all the organs to invigorate them. The centre for the 

 veins, on the other hand, is the liver, from which the nutritive 

 blood (sanauis nutritious) is conducted by a kind of attractive and 

 selective force to every part of the body. The blood of the right 

 heart, supplied by the vena cava inferior, passes mainly through 

 the pores of the septum (which Galen accepts, although he declares 

 them invisible), becomes spirituous by admixture with the pneuma, 

 and is then distributed by the aorta throughout the body. A 

 lesser portion of the blood contained in the right ventricle passes, 

 however, through the vena arteriosa (pulmonary artery) and returns 

 by way of the arteria venosa (pulmonary vein) to the left ventricle. 



Thus Galen had an idea, however rudimentary, of the pul- 

 monary circulation, and knew that the venous vessels anastomose 

 with the arterial, since he had observed that an animal could bleed 

 to death from one artery. One point, indeed, in his doctrine led 

 certain critics astray in their interpretation of the text. Galen 

 assumed that the blood of the arteria venosa (pulmonary vein) 

 flowed back to the lungs at each systole (by a sort of physiological 

 insufficiency of the mitral valve), in order to expel by expiration 

 the fuliginous vapours formed in the blood. He thus allotted to 

 the pulmonary vein a double and 'opposite task, i.e. that of first 

 carrying the arterial blood from the lungs to the heart, and then 

 returning a portion of the same with the vapours from the heart 

 to the lungs. Galen also assigned a double function to the portal 

 vein, and assumed that during digestion it carried the chyle to the 



