Chap. X] 



BLOOD VASCULAR SYSTEM 



171 



eye/ are traversed by these networks of capillary vessels. Their 

 diameter is so small that the blood-corpuscles must pass through 

 them in single file and very frequently the corpuscle is bigger than 

 the calibre of the vessel, and has to be squeezed to enable it to pass 

 through. In many parts they lie so closely together that a pin's 

 point cannot be inserted between them. They are most abundant, 

 and form the finest networks, in those organs where the blood is 

 needed for other purposes than local nutrition, such as, for example, 

 secretion or absorption. 



Function. In the glandular organs the capillaries supply the 

 substances requisite for secretion ; in the alimentary canal they 

 take up some of the elements of digested food ; in the lungs they 

 absorb oxj^gen and give up carbon dioxide ; in the kidneys they 

 discharge the w^aste products collected from other parts ; all the 

 time, everywhere through their walls, that interchange is going 

 on which is essential to the renovation and life of the whole body. 

 It is in the capillaries, then, that the chief work of the blood is 

 done ; and the object of the vascular 

 mechanism is to cause the blood to 

 flow through these vessels in the man- 

 ner best adapted for accomplishing 

 this work. 



VEINS 

 The veins have three coats and on 

 the whole resemble the arteries in 

 structure. They differ from them in 

 having: (1) much thinner walls (see 

 Fig. 107) ; (2) they contain less elastic 

 tissue, more white fibrous tissue, and 

 because of this are not so elastic or 

 contractile as the arteries ; (3) many 

 of the veins are provided with valves. 

 Valves. — The valves are semilu- 

 nar folds of the internal coat of the 

 veins ; and usually consist of two 

 flaps, rarely one or three. 



The convex border is attached to 



w 



Fig. 107. Th.\xsvkrse Section' 

 THROUGH A Sm.\ll Artery .\ND Veix, 



SHOWING THE ReL.^TIVE DiFFER- 

 EXCE IN THE THICKNESS OF THEIR 



W.\LLS. In the vein (T-O the outer 

 coat is thickest, in the artery (A) the 

 contractiJe and elastic middle coat is 

 thickest. (Klein and Noble Smith.) 



^ These parts not penetrated by the blood-vessels imbibe nutritive matter from 

 adjacent tissues, and are just as dependent on the blood as all the other tissues. 



