172 MANUAL OF HISTOLOGY. 



plete tube in some of the narrow trabeculse, in which a sin- 

 gle cell may complete the circumference. Klein states that 

 when these openings take place, the connective-tissue cells pre- 

 viously situated between the connective-tissue bundles, come 

 to lie on the lateral surface which is now free. This, he 

 thinks, establishes the fact that the latter may be converted 

 into true endothelial cells. Delafield, in considering the ques- 

 tion of the re-formation of the endothelium on serous mem- 

 branes, after hydrothorax, remarks that it would seem to be 

 reproduced from the old endothelium, or from migrating white 



Fro. 77. From silver-stained omentum of cat: A, fenestra; B, intercellular lines of upper surface ; 

 C, nuclei of same ; D, intercellular lines of lower surface ; B, nuclei of same ; F, nuclei in wall of opening ; 

 corresponding cell-forms, part of upper and part of lower surface. 



blood-cells, or from sub-endothelial connective- tissue cells, 

 although he has not seen sufficient proof to establish any of 

 these theories. 



TJie nerves of the peritoneum have been studied by Cyon. 

 They enter the mesentery with the blood-vessels as fasciculi of 

 medullated nerve-fibres, and, dividing laterally, lose their med- 

 ullary sheath and form a plexus, the fibres of which show 

 projecting nuclei at various points. The walls of the arteries 

 receive a rich supply of these fibres. A lymph-space, surround- 

 ing the fibres, can sometimes be demonstrated. 



Intimate structure of lymphatic vessels. A lymphatic ves- 

 sel may be considered as a serous membrane with only one free 

 surface, rolled in the form of a tube, its endothelial layer form- 

 ing the intima, resting upon an elastic reticulum, and an ad- 

 ventitia or external envelope of connective-tissue fasciculi, as 

 in the serous membranes. In the finest capillaries only the en- 

 dothelial layer is independent, although they lie surrounded 



