AND THEIK FUNCTIONS. 147 



or by chloroforming, the puhnonary arteries and the venae 

 cavae are distended, and dark or nearly black so as to be 

 easily seen. Aristotle says correctly that the great blood- 

 vessel is conspicuous even in very small animals. 



Here, near the end of a description having serious 

 defects, yet far surpassing in excellence the corresponding 

 descriptions of his predecessors, we see that Aristotle tried, 

 but without success, to make out the nature of the small 

 ramifications of the blood-vessels. More than this, it is 

 clear from other passages in his Vi^orks that he tried to 

 investigate the phenomena of the ultimate destination of 

 the blood passing through the very small blood-vessels. 

 His views are fairly expressed in a series of passages in his 

 P. A. iii. c. 5, 668, which are too long to be given in full. 

 In these passages he says that, just as in the conveyance of 

 water by irrigating channels or ditches the largest channels 

 persist but the smallest disappear beneath the mud, so it 

 is with the blood-vessels, for the largest persist while the 

 smallest functionally become flesh, although they are none 

 the less capable of acting as blood-vessels. When flesh is 

 cut, he proceeds to say, blood flows from it, although no 

 blood-vessels may be seen in the flesh, because of their being 

 choked up. The blood-vessels, he says, divide and become 

 smaller and smaller, until their passages are too small to 

 allow the blood to flow through them, but they still allow 

 the passage of a more liquid substance, viz., sweat. Finally, 

 he says that the blood in the very small blood-vessels gives 

 up some of its material in the form of sweat or vapour, or, 

 as stated in P. A. iii. c. 8, 611a, is diverted into feathers, 

 scales, or the like, while the remaining parts are transformed 

 into flesh. 



In this way he explains how the blood can flow away 

 from the heart and never return. Although he was aware 

 of some differences between the aorta and the great blood- 

 vessel and their ramifications, he had no conception of a 

 return of blood to the heart, nor of the existence of networks 

 of capillaries through which the blood passed before entering 

 the veins. Having imperfect means of investigation and an 

 inadequate idea of the difficulties to be overcome, it is not 

 surprising that he failed in his attempts to explain the 

 ultimate distribution and functions of the blood, but he did 

 more than any other anatomist who lived before the times of 

 Vesalius and Servetus to prepare a way for a satisfactory 

 explanation of the phenomena of the circulation of the blood. 



