PHYSIOLOGICAL ANATOMY OF THE CAPILLARIES. 



83 



in the nervous and muscular tissue, retina, and patches of Peyer, where they have a di- 

 ameter of from ^Vo to Woo- of an incii - I n the mucous layer of the skin and in the 

 mucous membranes, they are from 3-^- to ^y 1 ^ of an inch in diameter. They are 

 largest in the glands and bones, where they are from ^W to ^Vfr of an inch in diam- 

 eter. These measurements indicate the size of the vessels and not their caliber. Tak- 

 ing out the thickness of their walls, it is only the very largest of them that will admit of 

 the passage of a blood-disk without a change in its form. 



FIG. 28. Small artery and capillaries from the muscular coals 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 epithelium of the vessels. It is injected with nitrate of silver, stained with carmine, and 



mounted in Canada balsam. 



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

 come larger, as we follow the course of the blood, by union with each other, the capil- 

 laries form a true plexus of vessels of nearly uniform diameter, branching and inosculat- 

 ing in every direction and distributing blood to the parts as their physiological necessi- 

 ties 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. Al- 

 though their arrangement presents certain differences in different organs, the capillary 

 vessels have everywhere the same general characteristics, the most prominent of which 

 are uniform diameter and absence of any positive 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 only distended with blood during the physiological activity of these parts. In the 

 lungs, the meshes are particularly close. In other parts, the vessels are not so abundant, 

 presenting great variations in different tissues. In the muscles and nerves, in which nu- 

 trition 13 very active, the supply is much more abundant than in other parts, like fibro- 



