DISTRIBUTION OF ARTERIES AND VEINS. 217 



stem of a tree ; these branches again subdivide into others 

 more numerous but smaller, and these again into twigs still 

 more numerous and more minute ; until almost every portion 

 of the body is so penetrated with them, that not even a 

 trifling scratch, cut, or prick, can be made, without wounding 

 some one of these small divisions (fig. 120). The Venous 

 system presents a corresponding distribution, but it is destined 

 for an opposite purpose ; and we must regard it as commencing 

 in the tissues by the minuter canals, which run together like the 

 I 



Fig. 120. DISTRIBUTION OF THE SMALLER BLOOD-VESSELS IN THE MEMBRANE 



BETWEEN TWO OF THE TOES OF THE HIND FOOT OF THE COMMON FROG; a O, 



veins ; b b, arteries. 



little rivulets that form the origin of a mighty river, or like the 

 smallest fibres of which the roots of a tree are made up. The 

 larger canals thus formed gradually unite with each other as 

 they approach the heart, towards which they all tend, just as 

 the various tributary streams pour their contents into one prin- 

 cipal channel : and at last all the veins empty into the heart, 

 by one or two large trunks, the blood which they have conveyed 

 from the several parts of the body ; just as all the tributaries 

 which have arisen over a wide extent of country, pour into 

 the ocean the water they have collected, by one mouth which 

 is thus common to all of them. 



247. Although the number of the Arterial branches increases 



