64 CIRCULATION OF THE BLOOD 



cular coat, becoming thinner and thinner, until at last they present 

 an internal structureless coat lined by endothelium with oval longitudi- 

 nal nuclei, a middle coat formed of but a single layer of circular muscular 

 fibres, and an external coat composed of a thin layer of longitudinal 

 bundles of fibrous tissue. These vessels are -%^-Q to -^TQ ^ an mc h 

 (62.5 to 125 ya) in diameter. They undoubtedly possess contractility, 

 which is particularly marked in the arterial system. Following 

 the course of the vessels, when they are reduced in size to about 

 -gfa of an inch (31 /-i), the fibrous coat is lost, and the vessel then 

 presents only the internal coat and a single layer of muscular fibres. 

 The vessels become smaller as they branch, finally lose the muscular 

 fibres and have then but a single coat. These last may be regarded 

 as the true capillary vessels. 



The single coat of the capillaries consists of a layer of fusiform or 

 polygonal nucleated endothelium of excessive tenuity. The borders of 

 the endothelial cells may be seen after staining the vessels with silver 

 nitrate. In the smallest capillaries the cells are narrow and elon- 

 gated or fusiform ; and in the larger vessels they are more polygonal, 

 with irregular borders. In staining with silver nitrate, irregular non- 

 nucleated areas frequently are brought into view ; and it has been sup- 

 posed by some that these indicate the presence of stomata, or orifices 

 in the walls of the vessels. 



The diameter of the capillaries usually is as small as that of the 

 blood-corpuscles ; or it may be smaller, so that these bodies move in a 

 single line and must become deformed in passing through the smallest 

 vessels, recovering their normal shape, however, when they pass into 

 vessels of larger size. The capillaries are smallest in the nervous and 

 muscular tissue, retina, and patches of Peyer, where they have a diameter 

 f 6~rolf to 3FOTT f an m h (4- 2 5 to 6.25 /*) In the papillary layer of 

 the skin and in the mucous membranes they are ^Q o to 2lVo ^ an 

 inch (6.25 to 10 fji) in diameter. They are largest in the glands and 

 bones, where they are 3^5^ to 2irfrff f an mc ^ (8-3 to I2 -5 /*) i n diame- 

 ter. It is only the largest vessels that allow the passage of blood-disks 

 without change in form. The average length of the capillary vessels is 

 about -^Q of an inch (o. 5 millimeter). 



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. 



