190 



ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



not altogether exact, particularly as tubes of the smallest of the above sizes 

 would not admit ordinary blood-corpuscles. The dimensions of the individual 

 vessels, indeed, are by no means constant ; as may be seen by watching the 

 Circulation in any transparent part, for some little time. Putting aside the 

 general changes, in diameter, which result from circumstances affecting all the 

 capillaries of a part, it may be observed that a single capillary will sometimes 

 enlarge or contract by itself without any obvious cause. Thus, the stream 

 of blood will sometimes be seen to run into passages, which were not before 

 perceived ; and it has hence been supposed that they were new excavations, 

 formed by the retreating or removal of the solid tissue through which it passes. 

 But a more attentive examination shows, that such passages are real capilla- 

 ries, which did not, at the time of the first observation, admit the stream of 

 blood-corpuscles, in consequence of the contraction of their calibre, or of some 

 other local impediment ; and that they are brought into view by the simple 

 increase in their diameter. The compression of one of the small arteries 

 will generally occasion an oscillation of the corpuscles of blood in the small- 

 est capillaries, which will be followed by the disappearance of some of them ; 

 but when the obstruction is removed, the blood soon regains its former velocity 

 and force, and flows exactly into the same passages as before. 



220. The opinion was long entertained, that there are vessels adapted to sup- 

 ply the white or colourless tissues ; carrying from the arteries the Liquor 

 Sanguinis, or fluid portion of the blood ; and leaving the Corpuscles behind, 

 through inability to receive them. But such a supposition is altogether 

 groundless. Some of the white tissues, as Cartilage, are altogether destitute 

 of vessels ; and in others, the supply of blood is so scanty, as not to commu- 

 nicate to them any decided hue. It is evident from what has been already 

 stated, that the idea that Nutrition can only be carried on by means of Capil- 

 lary vessels, is entirely gratuitous. There is no essential difference, in fact, 

 between the nutrition of the non-vascular tissues, and that of the islets in the 

 midst of the network of capillary vessels, which traverses the most vascular. 



Fig. 91. 



Capillary vessels from the pia mater; a. calibre of the tube, partly occupied by oval nuclei, alter- 

 nately arranged lengthways, and epithelial in their character; 6, 6, 6, nuclei projecting on the exterior 

 of the tube ; c, c, walls, and d, calibre, of a large branch ; /,/, oval nuclei, arranged transversely. Mag- 

 nified 410 diameters. 



