THE BLOOD. 71 



The circulation in the tongue of the frog is best seen, 

 in the first instance, by means of low powers, a larger 

 surface of the organ being thus brought under view, and 

 a more exact idea obtained of the relative size and dispo- 

 sition of its numerous constituents. The arteries may be 

 distinguished from the veins by their fewer number, smaller 

 calibre, and by the fact that, while the veins increase in 

 diameter, in the direction of the course which the blood 

 contained in them pursues, the arteries decrease in the course 

 which the current follows in them. The arteries, from their 

 origin, diminish in size and multiply in number, by the con- 

 stant giving off of secondary branches ; the veins, on the 

 contrary, become enlarged during their progress, and lessen 

 in number, by the continual addition of subsidiary veins. 

 These differences, as well as the circumstance that the velocity 

 of the blood in the arteries is greater than in the veins, are 

 abundantly sufficient to distinguish the two orders of vessels 

 from each other. If, now, a somewhat higher power be 

 applied to the objects, we shall be able to dive still further 

 into the mysteries of organisation ; we shall not merely 

 perceive the general motion of the blood, but also that nearly 

 the entire mass of that fluid consists of red globules. We 

 shall be able to recognise clearly their form, and to see the 

 different modifications of shape which they undergo in passing 

 by each other, and in escaping any impediment which presents 

 itself to impede their progress. We shall perceive, likewise, 

 that, in the smaller capillaries, the globules circulate in single 

 series, and mingled with them will be noticed occasionally 

 a colourless globule, which, in the blood of the frog, is not 

 more than half the size of the elliptical corpuscle. (See PL 

 V. fig. 2.) Furthermore, it will be remarked that the 

 circulation does not flow on in an uninterrupted stream of 

 equal velocity, but that certain arrests of its motion occur. 

 These are but momentary, and after each the current again 

 quickly flows on with the same speed as before : with each 

 action of the heart, also, a slight impulsion of the blood in 

 the capillaries may be clearly seen. 



This instructive sight of the capillary circulation may be 



G 



