20 CIRCULATION OF THE BLOOD 



consider some of his researches which bear upon the 

 heart and the organs of respiration. 



Leonardo was the first man to question the views of 

 Galen. He made most careful first-hand investigations 

 on the bodies of men and animals, and he performed 

 many physiological experiments. It happened that he 

 was particularly interested in the heart and vascular 

 system. In the course of his researches he came to the 

 correct conclusion that the branches of the air-tubes in 

 the lungs, the bronchi, as we call them, branch and gradually 

 diminish in size and finally end blindly. He inflated the 

 lungs with air, and found that whatever force he used, 

 he was unable to drive the air from the bronchi into 

 the heart. He therefore inferred quite correctly that 

 Galen's venal artery (our pulmonary vein) did not convey 

 air to the heart as the followers of Galen believed that 

 it did. 



Leonardo then turned to the examination of the heart 

 itself. First he examined its structure and form, and pre- 

 pared more accurate drawings of it than had been made 

 by any before him (Plate III.). Then he made sections 

 and dissections of it, and examined its valves. He observed 

 and carefully figured the four cavities of the heart the 

 two ventricles and the two auricles. Galen had known 

 the ventricles, and had attached much importance to them ; 

 the auricles, as we have seen, had been almost entirely 

 neglected by him, as he considered that their only function 

 was to prevent over-distension of the ventricles. Galen 

 thus described the heart as having only two cavities. 

 Leonardo correctly described it as having four. 



Of all the bodily movements, Leonardo evidently found 

 those of the heart and vascular system at once the most 

 perplexing and the most absorbing. To the question of 

 their nature he returned again and again, expending on it 

 much of his time, his art of dissection, his keen observa- 



