80 



CIRCULATION OF THE BLOOD IN THE VESSELS. 



They are largest in the glands and bones, where they are 

 of an inch (8'3 to 12'5 p] in diameter. These measurements indicate the 

 size of the vessels and not their caliber. Taking out the thickness of their 

 walls, it is only the very largest of them that will allow the passage of a 

 blood-disk without a change in its form. The average length of the capil- 

 lary vessels is about -^ of an inch (0-5 mm.). 



FIG. 32. Small artery and capillaries from the muscular coats of the urinary bladder of the frog ; 



magnified 400 diameters (from a photograph taken at the United States Army Medical Museum). 



This preparation shows the endothelium of the vessels. It is injected with silver nitrate, stained with 



carmine and mounted in Canada balsam. 



Unlike the arteries, which grow smaller as they branch, and the veins, 

 which become larger, in following the course of the blood, by union with 

 each other, the capillaries form a true plexus of vessels of nearly uniform 

 diameter, branching and inosculating in every direction and distributing 

 blood to the parts as their physiological necessities demand. This mode of 

 inosculation is peculiar to these vessels, and the plexus is rich in the tissues, 

 as a general rule, in proportion to the activity of their nutrition. Although 

 their arrangement presents certain differences in different organs, the capil- 

 lary vessels have everywhere the same general characteristics, the most promi- 

 nent of which are the nearly uniform diameter and an absence of any definite 

 direction. The net- work thus formed is very rich in the substance of the 

 glands and in the organs of absorption ; but the vessels are distended with 



