CAPILLARIES 



143 



diminishes until in the arterioles it disappears com- 

 pletely. The proportion of muscle fibre, on the contrary, 

 increases until the arterioles are reached, when it in turn 

 decreases until none is left. 



Capillaries. The smallest branches of all, the capil- 

 laries, which are only about half a millimeter in average 

 length, have neither elastic tissue nor muscular fibres. 

 They are the smallest, 

 the most delicate, as 

 well as the most nu- 

 merous of all the blood- 

 vessels. The finest nee- 

 dle point can penetrate 

 scarcely any part of the 

 body without severing 

 some of them. Their 

 walls consist of an ex- 

 ceedingly thin mem- 

 brane, which is made up 

 of flat cells placed edge 

 to edge to form a con- 

 tinuous tube. Their thin- 

 ness permits the blood to come into close contact with 

 the tissue cells, without, however, allowing the red 

 blood corpuscles to escape. The required amount of 

 oxygen freely passes from the red corpuscles to the 

 tissue cells, and the plasma of the blood containing the 

 food material in solution also oozes through in sufficient 

 quantity to nourish them. The blood in the capillaries 

 receives in exchange from the cells their waste products, 

 including carbon dioxide, which pass readily through 

 their walls into the blood stream. 



Veins. The capillaries join together at their farther 

 ends to form a second system of larger tubes, the veins. 



FIG. 84. A, diagram of branching capilla- 

 ries, showing how they are made up of a 

 single layer of cells, the edges of which 

 are cemented together to form tubes. 

 B. Cross section of same, showing nuclei 

 in walls. 



