THE FLUIDS OF THE BODY 43 



to the passage of blood through the liver ; but pleurisy, the 

 accumulation of lymph in the space between the lungs and the 

 chest wall, cannot be explained in the same way. There is 

 no undue pressure on the vessels in which the blood circulates 

 through the inflamed pleura (the investing membrane of the 

 lungs and lining membrane of the chest), yet the walls of the 

 capillaries fail to maintain a proper balance between blood 

 and lymph. 



Hitherto we have spoken of the lymphatic system as a 

 labyrinth of communicating spaces containing stagnant fluid, 

 which is kept in a fitting state by egress and ingress out of 

 and into blood. Such a mental picture is substantially correct. 

 But the system is complicated by the presence of lymphatic 

 vessels. Cells of the connective-tissue sponge-work arrange 

 themselves side by side. They flatten into endothelial scales. 

 The borders of the scales close up. They form lymphatic 

 channels, wider than blood-capillaries, but strictly comparable 

 in every other respect. The lymph capillaries unite into 

 larger vessels. The larger vessels are connected by cross- 

 branches ; they form plexuses. Their walls are strengthened 

 with fibrous tissue. Like the veins, they are abundantly 

 provided with valves, which check any tendency to a backward 

 flow on the part of the fluid which they contain. Lymphatic 

 plexuses surround and accompany the larger bloodvessels. 

 They are disposed on the surface of muscles and glandular 

 tissues. They are abundant beneath the skin. Nearly three 

 centuries ago the lymphatic vessels of the mesentery, which 

 collect products of digestion, especially fat, from the walls of 

 the alimentary canal, were recognized owing to the milkiness 

 of their contents after a meal. They were, on this account, 

 termed "lacteals." Other lymphatic vessels, owing to their 

 transparent walls and colourless contents, are not easily seen ; 

 but they are readily injected with mercury or other fluids 

 which render them conspicuous. In the upper part of the 

 thigh, in the armpit, or in the neck, they are about large 

 enough to admit a crow-quill. Those from the lower limbs, 

 from the viscera, and from the walls of the abdomen converge 

 to a receptacle which lies in front of the spinal column. The 

 receptaculum chyli is continued upwards as the thoracic 

 duct, which pours the lymph into the great veins of the 



