298 FUNCTIONS OF NUTRITION. 



in the glands and the osseous tissue, where they reach the diameter 

 of 15 mmm. ; while the smallest, in the muscles, the nerves, and the 

 retina, are 4.5 mmm., that is, almost exactly the size of the smallest of 

 the red globules of the blood. 



As the arterial ramifications approach the capillary system, they 

 diminish in size, and lose their external coat of connective tissue. 

 Their middle coat is, at the same time, reduced to a single layer of 

 fusiform muscular fibres, which become gradually less numerous, and 

 at last disappear altogether. The vascular canal is thus finally com- 

 posed only of a single tunic continuous with the internal coat of the 

 arterial ramifications. 



The capillary blood-vessel, in its recent condition, when extracted 

 from any soft vascular tissue, appears to consist of a simple, nearly 

 homogeneous tubular membrane, with flattened oval nuclei placed at 

 short distances from each other, and projecting slightly into its cavity, 

 p 76 But if the vessel be treated with a 



weak solution of silver nitrate, its 

 inner surface becomes marked off into 

 regular spaces, each of which includes 

 a nucleus ; indicating that its appar- 

 ently homogeneous tunic is com- 

 posed of flattened epithelium-like 

 cells, united with each other at their 

 adjacent edges by an intervening 

 cement. It is this intervening sub- 

 stance which becomes darkened by 

 silver nitrate, bringing into view the 

 outlines of the epithelium cells of 

 the vascular wall. 



The form of the cells varies in dif- 



SMALL ARTERY with its muscular tunic (a), ferent glOUB and in Capillaries of 

 breaking up into capillaries. From the different calibre. According to Kol- 



P iamater > liter, in the smaller capillary blood- 



vessels, from 4.5 to f mmm. in diameter, they are narrow, elongated, 

 and fusiform, as in Fig. T7 ; often curled from side to side, so as to 

 form each a half cylinder, two of them joining at their edges to com- 

 plete the capillary tube, and intercalated at their ends between the 

 adjacent cells. In the larger capillaries, from 8 to 15 mmm. in diam- 

 eter, the cells are shorter and wider, like those of ordinary pave- 

 ment epithelium. Owing to this structure of the capillary blood-vessels, 

 the vascular system, in the opinion of some histologists, is to be 

 regarded as a series of intercellular canals, provided, in different 

 regions, with additional layers of muscular, elastic, or connective 

 tissue. 



The capillary blood-vessels are distinguished by their frequent inos- 

 culation. The arteries divide and subdivide, as they pass from within 

 outward, while the veins as constantly unite with each other, to form 



