284: CIRCULATION. 



the circulation in various transparent parts in the inferior 

 animals, as the web of the frog's foot, the tongue of the frog, 

 the lungs of the frog and of the water-newt, the mesentery of 

 very young rats or mice, the wing of the bat, etc. The most 

 convenient situation is the tongue or the web of the frog. 

 Here may be studied, not only the movement of the 

 blood in the true capillaries, but the circulation in the small- 

 est arteries and veins ; the variations in caliber of these ves- 

 sels, especially the arterioles, by the action of their muscular 

 tunic; and indeed the action of vessels of considerable 

 size. This has been a most valuable means of studying the 

 circulation in the capillaries, as contrasted with the small 

 arteries and veins ; the only one, indeed, which could give us 

 any definite idea of the action of these vessels. 



Before taking up the causes of the capillary circulation, 

 and the various physical or vital laws which are involved, we 

 will describe the phenomena which are observed with, the aid 

 of the microscope. 



Phenomena of the Capillary Circulation. The magnifi- 

 cent spectacle of the capillary circulation, first observed by 

 Malpighi, in the lungs, and afterwards by Leeuwenhoek, 

 Spallanzani, Haller, Cowper, and others, in other parts, has 

 ever since been the delight of the physiologist. We see 

 the great arterial rivers, in which the blood flows with won- 

 derful rapidity, branching and subdividing, until the blood is 

 brought to the superb network of fine capillaries, where the 

 corpuscles dart along one by one ; the fluid being then col- 

 lected by the veins, and carried in great currents to the heart. 

 This exhibition, to the student of Nature, is of inexpressible 

 grandeur; and our admiration is not diminished when we 

 come to study the phenomena in detail. We find here a 

 subject as interesting as was the action of the heart when first 

 seen by Harvey, involving some of the most important phe- 

 nomena of the circulation. It can be seen how the arterioles 

 regulate the supply of blood to the tissues ; how the blood 



