THE CIRCULATORY SYSTEM 169 



THE LYMPH -VESSEL SYSTEM 



The larger lymph vessels are similar in structure to veins. Their 

 walls are, however, thinner than those of veins of the same calibre 

 and they contain more valves. These are folds of the intima, and 

 always occur in pairs. They are arranged with considerable regu- 

 larity and in small vessels where the intima forms the entire wall 

 there is a distinct bulging just above the valve which gives the vessel 

 a quite characteristic appearance. They are capable of great 

 distention, and when empty collapse so that their thin walls are 

 in apposition. 



The largest of the lymph vessels, the thoracic duct, has three well- 

 defined coats: an intima consisting of the usual lining endothelium 

 resting upon a subendothelial layer of delicate fibro-elastic tissue, the 

 outermost elastic fibres having a longitudinal arrangement; a fairly 

 thick media of circularly disposed smooth muscle cells which occur in 

 groups separated by connective tissue containing a few elastic fibres 

 and an adventitia of longitudinally running connective tissue contain- 

 ing elastic fibres and strengthened by bundles of longitudinal smooth 

 muscle. 



Lymph capillaries resemble blood capillaries in that their walls 

 are composed of a single layer of endothelial cells. The cells are 

 rather larger and more irregular than in blood capillaries, the capil- 

 laries themselves are larger, and, instead of being of uniform diameter 

 throughout, vary greatly in caHbre within short distances. In cer- 

 tain tissues dense networks of these lymph capillaries are found. 

 Cleft-like lymph spaces — perivascular lymph spaces — partially sur- 

 round the walls of the smaller blood-vessels. 



Lymph spaces without endothelial or other apparent lining also 

 occur. Examples of these are the pericellular lymph spaces found in 

 various tissues and the canaliculi of the cornea and of bone (pages 

 87 and 105). 



Similar in character to lymph spaces are the body cavities, peri- 

 toneal, pleural, and pericardial, with their finings of serous membranes. 

 These cavities first appear in the embryo as a cleft in the mesoderm 

 — the ccelom, body cavity, or pleuro peritoneal cleft. This cleft is lined 

 with mesothelium beneath W'hich the stroma is formed. These mem- 

 branes not only line the cavities, but are reflected over most of the 

 viscera of the abdomen and thorax. Thev consist of a stroma of 

 mixed fibrous and elastic tissue, covered on its inner side by a layer 



