THE CIRCULATION OF THE BLOOD 667 



the first eight lines of this passage, as given in the book referred to, reads 

 thus : 



" WH constat per fabricam cordis sanguinem 

 per pulmones in Aortam perpetuo 

 transferri, as by two clacks of a 

 water bellows to rayse water 

 constat per ligaturam transitum sanguinis 

 ab arteriis ad venas 

 unde A perpetuum sanguinis motum 

 in circulo fieri pulsu cordis." 



It is to be remembered that these notes were only meant to be used by the 

 lecturer to assist his memory, so that a translation must be somewhat free. It 

 may be given thus : " WH shows, by the way the heart is made, that the blood 

 is perpetually driven from the lungs into the aorta, ' as by two clacks of a water 

 bellows to rayse water.' He shows, by means of ligature, the passage of the blood 

 from arteries to veins. Hence it is demonstrated that the perpetual movement 

 of the blood takes place in a circle, owing to the beat of the heart." 



The more complete demonstration was given in the book published in 1628. 

 A portrait of Harvey is given in Fig. 219. 



The demonstration of the actual passage of blood from arteries to veins in 

 the peripheral parts of the circulatory system was first given by Leeuwenhoek 

 in 168G (see H. G. Plimmer, 1913, p. 130). He saw it in the tail of the tadpole by 

 the aid of the microscope which he had invented. A portrait of Leeuwenhoek 

 will be found in Fig. 220. 



Malpighi in 1661 (see Foster, 1901, p. 97) saw the capillaries in the dried lung of the frog, 

 but it was Leeuwenhoek who first detected, in the living animal, the blood actually passing 

 from artery to vein through the capillaries. 



Although the first clear presentation of the circulation of the blood was made 

 by Harvey, it is plain that Leonardo da Vinci (1452-1519) was not far from the 

 discovery. It seems evident from his descriptions in the " Quaderni d'Anatomia " 

 that he realised that the function of the heart is to drive the blood into the 

 arteries. One of his drawings of the heart and blood vessels in man is given in 

 reduced size in Fig. 221, and some sketches illustrating observations made on the 

 movements of the heart of the pig, when killed by inserting a " piercer " for wine 

 casks into the heart, in Figs. 222 and 223. In the description of these observa- 

 tions he states, " il core nella sua espulsione del sangue si racorta," " the heart 

 shortens itself during its expulsion of the blood " (" Quaderni d'Anatomia," 

 I. p. 22. Line 8 of Fig. 223). 



In Fig. 221 the curious "mirror" writing used by Leonardo will be noticed. It is 

 sometimes stated that this is a proof that the artist was left-handed. This view is confirmed 

 by the fact that his shading is always drawn from left to right downwards. Others hold that 

 it is much more probable that he could use either hand equally well, and adopted the mirror 

 writing as a protection against ecclesiastical interference, since the nature of the writing was 

 not discovered for a considerable time, and it was thought to be a cipher. 



Since the blood vessels, as they get further from the heart, divide up into 

 smaller and smaller branches, it will be clear, from the account given on page 241 

 above, that the internal friction of the blood causes considerable resistance to the 

 flow. A somewhat high pressure is thus required in the main arteries to drive the 

 blood at an adequate rate through these small vessels. It may be repeated here 

 that it is in the small arterioles that the chief resistance occurs, on account of the 

 fact that the rate of flow is great here, and the friction is proportional to the square 

 of the velocity. In the capillaries, although they are, individually, narrower than 

 the arterioles, the rate of flow is small, owing to the sectional area of the bed being 

 greatly increased by the great increase in their number. A high arterial pressure 

 is also of advantage when the arterioles of an active organ are dilated. The high 

 blood pressure enables a considerably greater flow to take place through the organ, 

 without notable diminution of that through other organs. Moreover, a much more 



