148 MANUAL OF HISTOLOGY. 



we can readily account for the phenomena in question. The 

 capillary-wall is elastic, extremely thin, and permeable. By 

 virtue of these qualities, it may allow the passage of a leu- 

 cocyte or colored globule through its substance without suf- 

 fering a permanent breach of continuity. 



The writer's views on endothelial desquamation as a normal process of physi- 

 ological import may strike the reader as insufficiently substantiated by known 

 facts. But when we remember that similar processes have been actually ob- 

 served taking place under the microscope, all doubts as to the probability of 

 this endothelial desquamation should vanish. The author refers to the recent 

 observations of Altmann (Arch./, mikros. Anat., Vol. XVI., p. 111). This his- 

 tologist investigated the changes which take place in the serous epithelium 

 (i.e., endothelium) of the exposed frog's mesentery. Multiple swellings of the 

 endothelia were seen to occur ; then portions of these cells would become de- 

 tached. Such detached bits were found to resemble in their appearance ordi- 

 nary leucocytes. But, in spite of this apparent breaking up of the endo- 

 thelia into these nucleated corpuscles, they often retained their individuality 

 unaltered. The production of bodies resembling leucocytes from endothelia 

 has, therefore, been actually observed in connection with serous membranes, 

 and vascular desquamation is essentially the same process. 



The capillary blood-vessels occupy the interstitial connec- 

 tive tissue of organs, without entering their parenchyma proper. 

 Cartilage, the teeth, the hairs and nails, the cornea, and cer- 

 tain structures of the nervous system and organs of special 

 sense are devoid of capillary supply. 



Most of the larger tubes are invested by a 'delicate, exter- 

 nal, sheath-like structure, called the capillary adventitia or 

 vascular perUhelium. It is composed of a rather close net- 

 work of delicate connective- tissue librils. Prolongations of pe- 

 culiar stellate cells, which clasp the capillary-tube, may some- 

 times be seen to join these fibrils. (Fig. 62.) Such branching 

 cells are also encountered at some distance from the capillaries. 

 They show delicate processes, which may anastomose with the 

 offshoots of the adv.entitial corpuscles. In other places we 

 only find external plates of connective-tissue cells (Krause's ino- 

 blasts), which have become more or less fused with the capil- 

 lary-wall. In many instances the perithelium is inseparable 

 from the connective-tissue stroma surrounding the vessel. 



In reference to the manner of anastomosis, the forms and 

 modes of ramification of different networks vary with the dif- 

 ferent tissues and organs of the body. Hence, a simple in- 



