PERITONEUM. 205 



laries are formed from these or have a separate origin and later 

 become connected with them. This subject was worked over 

 by Hendrickson. According to this author, the capillaries 

 cannot be demonstrated by the Golgi method in pigs' embryos 

 less than 5 cm. in length. In these only a few appear imme- 

 diately around the large branches of the portal vein. In 

 human embryos 5 cm. in length the network of capillaries is 

 considerably more extensive. In older embryos the main capil- 

 laries gain side branches, and those encircling different portal 

 branches finally meet. The meshes of the network in the places 

 where it first appears are smaller than where they are subse- 

 quently formed. This is due to the division of the older meshes 

 by side branches. In some of the older embryos the capillaries 

 are seen to be continuous with a larger vessel in the region of 

 the interlobular vein, which probably represents the interlobu- 



lar bile duct. 



H. PERITONEUM. 



The peritoneum lines the whole abdominal cavity and is 

 reflected over the organs contained therein. As it passes out to 

 the organs (e. g., the intestine) it forms a double layer, known 

 as the mesentery, and on the surface of the organs themselves 

 it is spoken of as the tunica serosa. 



The peritoneum is a thin membrane consisting of a connec- 

 tive-tissue layer and a single layer of flat endothelial cells. 

 The latter cover the free surface, and are usually polygonal in 

 outline. The cement lines between the cells can be made out 

 readily in specimens treated with nitrate of silver ; and by 

 special methods, especially that proposed by Kolossow, struct- 

 ures which generally are understood to be protoplasmic bridges 

 can be demonstrated. The outlines of the cells are often wavy 

 or quite irregular. 



The connective-tissue layer consists of interlacing connec- 

 tive-tissue bundles, containing numerous elastic fibres and con- 

 nective-tissue cells. The peritoneum is bound to the underlying 

 parts by means of a connective tissue rich in fat and elastic 

 fibres. The so-called subserous connective tissue is developed 

 more strongly in some places than in others. In the intestine and 



