FORMATION OF CAPILLARY BLOOD-VESSELS. 101 







In both cases, the nutrient materials conveyed by the blood are absorbed by 

 the cells or other elementary parts of the tissue immediately adjoining the 

 vessels, and are imparted by them to others which are further removed ; and 

 the only variation which exists, is in the amount of the portion of tissue that 

 has to be thus traversed. There is great variety in this respect, as we 

 have seen, among the vascular tissues ; and we are only required to extend 

 our ideas, from the largest of the islets which we find in these, to the still 

 more isolated structures, of which the non-vascular tissues are composed. 

 The distribution of Capillaries through the vascular tissues, and the character 

 of the reticulation which they form, vary so greatly in the different parts of 

 the fabric, that it is possible to state with tolerable certainty the nature of the 

 part from which any specimen has been detached, whether a portion of 

 skin, mucous membrane, serous membrane, muscle, nerve, fat, areolar tissue, 

 gland, &c. But the arrangement of vessels peculiar to each evidently has 

 reference only to the convenience of the distribution of blood among the 

 elementary parts of the tissue, and varies with their form. It is not possible 

 to imagine that it has any other relation than this to their function ; since, as 

 already shown, the function of each separate element 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 furnishing the material, 

 on which these are to be exercised. 



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

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

 the straight and anastomosing Ducts of Plants. Bodies having the appear- 

 ance 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. Similar nuclei may be brought into view in the capillaries of adult 

 animals, by treating them with acetic acid ; and they are particularly well 

 seen in the Pia Mater, which consists almost entirely of a congeries of blood- 

 vessels (Fig. 91). The accompanying figure shows the contrast between the 

 long oval nuclei b, b, imbedded at intervals in the walls of the true capillaries, 

 and rather projecting on their exterior; and the nuclei of the epithelium-cells, 



f, /', lining the interior of a larger branch, which last are more numerous and 

 of less regular form, and are sometimes placed transversely to the direction of 

 the tube. 



222. The first formation of the Capillary blood-vessels in the vascular 

 area in the Bird's egg, is described by Schwann as being 



affected entirely by the coalescence of cells, which send -p itr go 



off prolongations in various directions, in the manner of 

 stellate pigment-cells, such as those seen at c, c, Fig. 90. 

 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 be- 

 tween these, at the points where their prolongations coa- 

 lesced, they are much contracted. The calibre of the ves- 

 sels, however, gradually becomes equalized (Fig. 92); and 

 the network becomes connected with the larger trunks, and 

 bears a part in the general circulation. Appearances indica- ,-,. . 



f> . . . 1 l 



live ot a similar process, have been seen in the tail ot very blood-vessels in the 



young Tadpoles; so that it may probably be regarded as the vascular layer of the 



general method, by which new capillaries are formed in germinal membrane of 



the natural process of growth. Observations upon the a FOM>* at the 36th hour 



history of the operations, which are performed for the ofl " cul 



