82 CIRCULATION OF THE BLOOD. 



at the point where the blood is brought near enough to the tissues to enable them to sep- 

 arate the elements necessary for their regeneration and to give up the products of their 

 physiological decay. With our present knowledge, it is impossible to assign any limit 

 where the vessels cease to be simple carriers of blood ; and it does not seem probable that 

 it will ever be known to what part of the vascular system the processes of nutrition are 

 exclusively confined. The divisions of the blood-vessels must be, to a certain extent, 

 arbitrarily defined; and we should feel at liberty to adopt the views of any reliable ob- 

 server with regard to the kind of vessels which are to be considered as capillaries. The 

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

 those vessels which have but a single tunic; 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 we come to vessels which 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 diminishing in size as they branch, 

 and their coats, especially the muscular, becoming thinner and thinner, until at last they 

 present an internal structureless coat, lined by epithelium 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 fibres of the white inelastic tissue. 

 These vessels are from ^i^ to -^ of an inch in diameter. They become smaller as they 

 branch, and undoubtedly possess the property of contractility, which is particularly 

 marked in the arterial system. Following the course of the vessels, when they are re- 

 duced in size to about -g-i^ of an inch, the external fibrous coat is lost, and the vessel then 

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

 as they branch, finally lose the muscular coat, and have then but a single tunic. These 

 last we shall consider as the true capillary vessels. 



The minute structure of the capillary vessels is of considerable importance and 

 interest and has been very closely investigated within the last few years. It was for- 

 merly thought that the smallest vessels, which we describe as the true capillaries, were 

 composed of a single, homogeneous membrane, from -^^-^ to ^-jVfr of an inch thick, with 

 nuclei embedded in its substance, but not provided with an epithelial lining. Recent 

 observations, however, have shown that the membrane is homogeneous, elastic, perhaps 

 contractile, and, in some parts at least, is provided with fusiform or polygonal epithelium 

 of excessive tenuity. The borders of the epithelial cells may be seen by staining the ves- 

 sels with nitrate of silver. 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 which have been observed in the walls of the vessels belong to this layer of 

 epithelium. By the same process of staining with nitrate of silver, we frequently observe 

 irregular, non-nucleated areas ; and it has been supposed by some that these indicate the 

 presence of stomata, or orifices in the walls of the vessels. This latter point, however, 

 has not been definitely determined. It cannot at present be stated positively whether or 

 not orifices normally exist in the walls of the blood-vessels. Most of the anatomical 

 points we have just mentioned have been developed by observations upon the vessels of 

 the frog. 



The diameter of the capillaries is generally as small as, or it may be smaller than 

 that of the blood-corpuscles ; so that these bodies always move in a single line and 

 must become deformed in passing through the smallest vessels, recovering their natural 

 shape, however, when they pass into vessels of larger size. The capillaries are smallest 



