280 PHYSIOLOGY. 



the heart. If a vein is compressed the blood is driven back and 

 presses the valves inward and closes the vein. The pulmonary veins 

 contain no valves, and the same may be said for the superior and 

 inferior venae cavae, the portal vein, and most of those of the head 

 and neck. The veins of the lower extremities contain more valves 

 than the corresponding vessels of the upper extremity. In certain 

 organs channels are seen lined with an extension of the internal coat 

 of the vein, which are called venous sinuses, as in the dura mater 

 and uterus. Vasa vasorum are also distributed to the veins. In the 

 coats of both arteries and veins are lymph-spaces. 



The nerve-supply to the arteries is liberal, to the veins much 

 less so. The supply is derived chiefly from the sympathetic system, 

 with a few filaments from the cerebro-spinal system. 



THE CAPILLARIES. 



The smallest arteries suddenly divide into an extremely fine net- 

 work of hair-like tubes, the capillaries. These furnish the connect- 

 ing link between arteries and the beginnings of veins. They serve 

 as the intermediate agent in all structures, between the arteries and 

 the veins. 



Each capillary tube is from V^ooo to Y 3 ooo mc h i n diameter, 

 while it averages Vso inch m length. 



Capillaries are composed of the same kind of endothelial cells 

 as the intima of the arteries; in fact, the capillaries seem to be the 

 prolongations of the lining of the arteries. Their walls are made up 

 of a single layer of lance-shaped endothelial cells. In the wall of the 

 capillary between the cells we find the cement-substance which permits 

 the blood-corpuscles to penetrate it in diapadesis. These little vessels 

 penetrate the spaces between the cells of the tissues in such a fine 

 network that many of the cells are in contact with several vessels. So 

 closely arranged are they that the point of a very fine needle cannot 

 enter the skin without injuring some of them. 



The total capacity of the capillaries is about three hundred times 

 that of the arteries. 



THE CIRCULATION OF THE BLOOD. 



The physicians and naturalists of antiquity, even at the epoch 

 when they were permitted to get enlightenment from anatomy, 

 remained in ignorance of the circulatory movement of the blood. 

 The circulatory apparatus is not one of those the mere inspection 



