88 OF THE BLOOD 



volume of their contents, and which convey the blood from 

 the centre towards the surface, distributing it to every 

 point of the body ; 2. The veins, of a thin, mem- 

 branous structure, furnished within with valves, 

 (Fig. 82, v), which aid in sustaining the column 

 of blood, and allow it to flow towards the 

 centre only. The arteries constantly subdivide 

 into smaller and smaller branches ; while the 

 veins commence in minute twigs, and are gath- 

 ered into branches and larger trunks to unite final- 

 ly at the centre. 



234. The extremities of the arteries and veins are con- 



r +. nected by a net-work of extremely 

 iff minute and delicate vessels, called 

 capillary vessels (Fig. 83). They 

 77 pervade every portion of the body, 

 so that almost no point can be pricked 

 Fig. 83. from which blood will not issue. 



Notwithstanding their minuteness, the most important pro- 

 cesses of nutrition are performed by these vessels, such as 

 the removal of effete particles and the substitution of new 

 ones, and all those changes by which the bright blood of the 

 arteries becomes the dark blood of the veins ; and again, in 

 the capillaries of the respiratory organs, the dark venous 

 blood is oxygenated and restored to the bright scarlet hue of 

 the arterial blood. 



235. Where there are blood-vessels in the lowest animals 

 the blood is kept in motion by 



the occasional contraction of 



some of the principal vessels, as 



in the worms. Insects have a 



large vessel running along the 



back, furnished with valves so 



arranged that, when the vessel pig. 34. 



contracts, the blood can flow only towards the head, and 



i 



