132 THE BLOOD AND ITS CIRCULATION. 



distinguished from the latter by the thinness and flaccidity 

 of their coats (Fig. 37, p). 4. Between these two systems 

 is the capillary system (beginning in the arteries and ending 

 in the veins), a collection of very fine vessels, arranged like 

 the string in a net (Fig. 37, C P), the smallest having gen- 

 erally the same diameter as the blood globules) ; their calibre 

 is even less sometimes, but the globules being elastic can 

 become so long and thin that they can traverse tubes much 

 smaller than themselves. 



The whole of the circulatory system may thus be divided 

 into a central organ, the heart, and a number of peripheral 

 organs, the vessels (arteries, capillaries, veins). 



The blood circulates in a system of vessels, because at the 

 beginning of this system (origin of the aorta) is found one 

 of the cavities of the heart, which possesses the property of 

 producing a strong pressure (the ventricle), while at the 

 other extremity (vena cava) is found another cavity of the 

 heart (the auricle), whose property it is to diminish the pres- 

 sure, or at least to allow a free passage to the blood which it 

 receives, in order to transmit this fluid to the ventricle ; by 

 this double antagonism between the two cavities of the heart 

 the circulation is produced. 



In short, the circulation of the blood is caused by the in- 

 equality of pressure in the different parts of the vascular 

 circuit, and the use of the heart, taken as a whole (auricles 

 and ventricles), is to keep up this inequality of pressure, 

 which makes the blood pass from the arteries where the 

 pressure is strong into the veins where it becomes gradually 

 weaker. 



The ideas entertained by the ancients as to the circulation 

 of the blood were false and incomplete. Galen supposed the 

 blood to be formed in the liver, and that, on leaving this 

 organ, it spread through the lower part of the body by 

 means of the inferior vena cava, and through the upper part' 

 by means of the superior vena cava: that, as a portion of 

 this latter blood reached the heart, and filtered through the 

 interventricular partition, it acquired new properties, by 

 means of which it circulated through the arteries under the 

 name of vital spirits. Galen had thus no suspicion of the ex- 

 istence of the pulmonary circulation (see farther on, p. 142). 



The idea of pulmonary circulation was first suggested by 

 Michel Servet, in 1553. Fabrice, of Acquapendente, first 

 demonstrated the arrangement of the venous valves, which 

 contradicted the theory of circulation, as conceived by Galen. 



