VI CAPILLARIES . 95 



insoluble, of how the blood, having reached a given part or 

 organ by the arteries, finds its way into the veins to begin 

 its return journey. 



The Circulation in the Frog's Web.— There are three 

 parts in the frog transparent ejiough to allow of the blood- 

 flow being seen in them — the web of the foot, the tongue, 

 and the mesentery. Of these the web is the most con- 

 venient, and can be examined under the microscope without 

 any injury to the animal. 



The Capillaries. — If you have the makings of a naturalist, 

 you will acknowledge the sight to be one of the most won- 

 derful you ever saw. In the thickness of the web is an 

 irregular network of minute blood-vessels, called capillaries 

 (Fig. 24), and through them the blood is seen to flow with 

 great rapidity, its course being made especially evident by 

 the minute particles or corpuscles it contains, the structure 

 of which we shall study later on. You will also notice much 

 larger vessels, the smallest arteries and veins. The arteries 

 (a) are distinguished by the fact that the blood in them 

 flows in the direction from the leg towards the margin of the 

 web, while in the veins (») it takes the opposite direction. 

 You must remember, however, that under the microscope 

 everything is reversed ; right appears left and left right, and 

 a current actually flowing towards the observer appears to 

 go in the opposite direction. 



By careful examination you will see that both arteries and 

 veins arfe in connection, by minute branches, with the 

 capillary network, and will be able to trace the blood from 

 an artery, through the capillaries, into a vein. 



The same thing can be seen in other transparent organs ; 

 and by injecting the vascular system with a fluid injection- 

 mass, such as gelatine suitably coloured, it can be proved 

 that all parts of the body are permeated with a capillary 



