PHYSIOLOGICAL ANATOMY OF THE CAPILLARIES. 79 



what seems to be the most physiological view, is to regard as capillaries those 

 vessels which have but a single coat ; for in these, the blood is brought in 

 closest proximity to the tissues. Vessels which are provided, in addition, 

 with a muscular or with muscular and fibrous coats are to be regarded 

 either as small arteries or as venous radicles. This view is favored by the 

 character of the currents of blood as seen in microscopical observation of 

 the circulation in transparent parts. Here an impulse is observed with 

 each contraction of the heart, until the vessels have but one coat and are 

 so narrow as to allow the passage of but a single line of blood-corpuscles. 



Physiological Anatomy of the Capillaries. If the arteries be followed 

 out to their minutest ramifications, they will be found progressively dimin- 

 ishing in size as they branch, and their coats, especially the muscular coat, 

 becoming thinner and thinner, until at last they present an internal, struct- 

 ureless coat lined by endothelium with oval, longitudinal nuclei, a middle 

 coat formed of but a single layer of circular muscular fibres, and an external 

 coat composed of a very thin layer of longitudinal bundles of fibrous tis- 

 sue. These vessels are ^ to -%fo of an inch (62-5 to 125 /*) in diameter. 

 They become smaller as they branch, and undoubtedly possess the property 

 of contractility, which is particularly marked in the arterial system. Follow- 

 ing the course of the vessels, when they are reduced in size to about -g^ of 

 an inch (31 ft), the external, 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 will be regarded as the true capillary vessels. 



It was formerly thought that the smallest vessels, which are described as 

 the true capillaries, were composed of a single, homogeneous membrane, 

 Friinr to ^-Vo f an i ncn (1 to 10 ft) thick, with nuclei embedded in its 

 substance, but not provided with an endothelial lining; but it has been 

 shown that the membrane is homogeneous, elastic, perhaps contractile, and, 

 in some parts at least, provided with fusiform or polygonal endothelium of ex- 

 cessive tenuity. The borders of the endothelial cells may be seen after stain- 

 ing the vessels with silver nitrate. In the smallest capillaries the cells are 

 narrow and elongated or fusiform ; and in the larger vessels they are more 

 polygonal, with very irregular borders. The nuclei in the walls of the vessels 

 belong to this layer of endothelium. By the same process of staining with 

 , silver nitrate, irregular, non-nucleated areas are frequently brought into view ; 

 and it has been supposed by some that these indicate the presence of 

 stomata, or orifices in the walls of the vessels. 



The diameter of the capillaries is generally as small as that of the blood- 

 corpuscles, or it may be smaller ; so that these bodies always 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 of -^-$ to ^oW ^ an 

 inch (4-25 to 6-25 ft). In the papillary layer of the skin and in the mucous 

 membranes, they are ^gVrr to ^OT of an i ncn (6 '25 to 10 ft) in diameter. 



