716 SPECIAL HISTOLOGY. 



fibrils, and in the place of the elastic membranes themselves, only layers 

 of more or less coalescent, fusiform cells. The internal longitudinal 

 fibrous membrane alone, in many vessels, is at this time demonstrable 

 as a homogeneous elastic tunic immediately under the epithelium, but in 

 the smaller vessels this is wanting, and is replaced by a layer of elon- 

 gated cells, out of which it appears to be developed. It is thought that 

 similar cells are occasionally to be seen also in the adult, in which the 

 elastic inner membrane is likewise merged. The muscular fibres of the 

 heart arise, as in other situations, from the union of cells, but I have 

 not yet seen how their anastomoses are formed, whether from a branch- 

 ing of certain formative cells, or by the lateral apposition of small rows 

 of cells probably in both ways. 



The mode in which the development of the capillaries is effected, dif- 

 fers in toto from that observed in the larger vessels. The former, as 

 Schwann and I have shown, proceed from the coalescence of single cells. 

 At the primary origin of these vessels, tubules of some size are 

 formed, at first by the successive apposition in a straight line and the 

 coalescence of rounded-angular cells, and the subsequent absorption 

 of the septa -and of the contents, but not of the nuclei, which remain 

 attached to the former cell-membrane, now become the capillary tunic. 

 Delicate pointed processes .then project from the walls of these little 

 vessels, which rapidly elongate, and meeting similar pointed processes 

 of stellate cells dispersed in the surrounding tissue, coalesce with them. 

 At the same time, the other processes of these cells join, so that there 

 is soon produced a network of stellate cells, continuous with the already 

 formed capillary tube or tubes. This net, however, is never spread, for 

 the prolongations, given off from already formed and pervious capil- 

 laries, and the neighboring cells connected with them are constantly and 

 rapidly transformed into fresh capillaries, by the continual increase in 

 size of the coalescing processes from their point of origin onwards, and 

 their becoming hollow. In this way are produced vessels which are at 

 first extremely fine, and admit only blood-plasma true vasa plasmatica 

 s. serosa; but they rapidly enlarge, until at last the blood-globules are 

 transmitted through them, and the capillaries are perfected. Owing to 

 the circumstances, that while these processes of the stellate formative 

 cells thus enlarge, the bodies of the cells do not expand in a correspond- 

 ing manner, but appear as simple nodular points in the vessels, all ves- 

 tige of the original cellular network is gradually lost, and subsequently 

 the situation of the bodies of the cells can only be determined by the posi- 

 tion of the persistent nuclei. When finer tubules have once been formed 

 from the previous larger capillaries, the facilities for the passage of the 

 blood are continually undergoing augmentation, inasmuch as new stel- 

 late cells are constantly enlarging into vessels, whilst, at the same time, 

 fresh vascular material is as constantly furnished by the apposition of 



