VI CAPILLARIES 95 



directly observing the flow became possible only after 

 the invention of the microscope. This instrument, by 

 furnishing a sufficiently high magnifying power, allows 

 us to see for ourselves the actual movement of the 

 blood in an animal or organ of sufficient transparency ; 

 and at the same time clears up the question, previously 

 insolvable, 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 enough 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 convenient, 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 wonderful 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 (v) 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 are in connection, by minute branches, with 



