116 



HUMAN BIOLOGY 



occur. Hence, the capillaries form the most important 

 portion of the blood system. 



164. Structure of the capillaries. In structure the capil- 

 laries are extremely simple (Fig. 36). At the points in the 



blood system where arteries 

 end and capillaries begin, 

 muscular and elastic tissues 

 are wanting. The walls of 

 the capillaries are formed of 

 a single layer of very thin- 

 walled cells. We have in 

 this arrangement the best 

 possible conditions for the 



A = Surface view. B= Longitudinal section. prOCCSS of OSHlOSis. Only 



FIG. 36. Structure of capillaries. the thin membrane of the 



capillary wall separates the 



blood from the surrounding tissues, and an exchange of 

 materials between the two is readily carried on. 



165. Position of the veins. 



On the back of the hand one 

 sees through the skin a branch- 

 ing system of bluish blood ves- 

 sels. These are veins. Unlike 

 the arteries, veins have no 

 pulse. Many veins, like those 

 in the hand, lie near the sur- 

 face, while most of the arteries, 

 as we have stated above, are 



A B c 



FIG. 37. Struc+nre of a vein. 



A = vein laid open to show shape of 

 valves ; B = section of vein showing valve 

 buried deeply among the Other open ; C = section of vein showing valve 



closing. 



tissues. 



166. Structure of veins. When the veins are emptied 

 of blood, they immediately collapse. This is due to the fact 



