300 OP THE PRIMARY TISSUES OF THE HUMAN BODY. 



this to their function; since, as already shown, the function of each separate ele- 

 ment of the organ, of which that of the entire organ is the aggregate, is due to 

 its own inherent vital powers the supply of blood being only required as fur- 

 nishing the material on which these are to be exercised. 



294. It has been rendered highly probable, by the observations of Schwann 

 and other Histologists, that the Capillaries of Animals originate in cells, like 

 the straight and anastomosing Ducts of Plants. Bodies having the appearance 

 of cell-nuclei may frequently be seen in the walls of the capillaries of embryos 

 and of tadpoles; and these are too wide apart to warrant the idea, that they are 

 the nuclei of epithelial cells, such as those which line the larger vessels. Simi- 

 lar nuclei may be brought into view in the capillaries of adult animals, by treat- 

 ing them with acetic acid ; and they are particularly well seen in the Pia Mater, 

 which consists almost entirely of a congeries of bloodvessels (Fig. 88). The 

 accompanying figure shows the contrast between the long oval nuclei 6, 6, im- 

 bedded at intervals in the walls of the true capillaries, and rather projecting on 

 their exterior; and the nuclei of the epithelium-cells,/,/, lining the interior of 



Fig. 88. 



Capillary Bloodvessels from Pia Mater: a, caliber of the tube, partly occupied by oval nuclei, alternately 

 arranged lengthways, and epithelial in their character ; b, b, b, nuclei projecting on the exterior of the tube ; 

 c, c, walls, and d, caliber, of a large branch;/,/, oval nuclei, arranged transversely. Magnified 410 di- 

 ameters. 



a larger branch, which last are more numerous and of less regular form, and are 

 sometimes placed transversely to the direction of the tube. The first formation 

 of the Capillary bloodvessels in the vascular area in the Bird's egg, seems to be 

 effected entirely by the coalescence of cells, which send off prolongations in 

 various directions, in the manner of stellate pigment-cells, such as those seen at 

 c, c, Fig. 87. By the junction of these prolongations, a network of tubes is 

 formed, which is at first very irregular in its character; the greatest diameter 

 of the tubes being in the situation of the centres or bodies of the original cells ; 

 whilst between these, at the points where their prolongations coalesced, they are 

 much contracted. The caliber of the vessels, however, gradually becomes 

 equalized, and the network becomes connected with the larger trunks, and bears 

 a part in the general circulation. Appearances indicative of a similar process 



