344 



THE CIRCULATION. 



Fig. 121. 



indirect contact with the substance of the tissues. They are continuous 

 with the terminal ramifications of the arteries on the one hand, and with 

 the commencing rootlets of the veins on the other. They vary some- 

 what in size in the different organs and tissues, their average diameter 

 in the human subject being about 10 mmm., or T ^ of a millimetre. The 

 largest capillaries, according to Kolliker, in the glands and the osseous 

 tissue, may 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 confines of the capillary 

 system they diminish gradually in size, and lose first their external 

 coat of connective tissue. Their middle coat at the same time becomes 

 reduced to a single layer of fusiform muscular fibres, which become 

 in turn less numerous, and lastly disappear altogether. The vascular 

 canal is thus finally composed only of a single tunic continuous with 

 the internal coat of the arterial ramifications. 



The capillary bloodvessel, examined in its recent condition, as ex- 

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



nearly homogeneous tubular mem- 

 brane, provided with flattened oval 

 nuclei placed at more or less regular 

 distances from each other, and pro- 

 jecting slightly into the cavity of the 

 vessel. 



It has been found, however, that if 

 a capillary bloodvessel 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 appa- 

 rently 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 thin layer of in- 

 tervening substance which becomes 

 darkened by the action of the silver 



nitrate and thus brings into view the outlines of the cells forming the 

 vascular wall. 



The form of the cells constituting the vascular membrane varies in 

 different regions and in capillaries of different calibre. According to 

 Kolliker, in the smallest capillary bloodvessels, measuring from 4.5 

 to 7 mmm. in diameter, the cells are narrow, elongated, and fusiform, 

 as in Fig. 122; often curled from side to side, so as to form each a 

 half cylinder, two of them joining at their edges to complete the 

 capillary tube, and alternating longitudinally, the pointed extremity of 

 one cell being intercalated between those of the two following cells. In 



AKTERY, with its muscular 

 tunic (a) breaking up into capillaries. 

 From the pia mater. 



