38 



THE BODY AT WORK 



long as the endothelial cells are alive they keep their edges in 

 such close apposition as to constitute a continuous membrane 

 which shuts off blood from lymph. They are always close 

 enough together to prevent red blood-corpuscles from escaping 

 from -the capillary vessels ; but their resistance to the passage of 

 the" ^different^ constituents of plasma varies greatly. The 

 membrane which they compose is more complete and less 

 pervious, or less complete and more pervious, in accordance 



FIG. 3. A DUOTULE AND Two ACINI OF A Mucous GLAND OF THE MOUTH, WITH A MUSCLE- 

 FIBRE CUT LONGITUDINALLY ; CAPILLARY BLOODVESSELS AND CONNECTIVE TISSUE. 



Stellate connective-tissue cells form a labyrinth of intercommunicating lymph-spaces which 

 separate the gland-cells and the muscle-fibre from the walls of the capillary bloodvessels. 

 The capillaries contain circular red blood-corpuscles and nucleated leucocytes. Some of 

 the leucocytes are squeezing their way either out of a capillary into a lymph-space or vice 

 versa. A granular leucocyte is to be seen in a lymph-space at the bottom of the picture. 



with the nature of the tissues which surround it, and their 

 varying needs. The blood-passages of the liver may be 

 described as filters. The escape of red blood-corpuscles into 

 lymphatic vessels is prevented, but they offer practically no 

 resistance to the plasma. Plasma " lymph," as it is termed 

 as soon as it is outside bloodvessels passes through the walls 

 of the capillaries of the liver unchanged in constitution. Where 

 they traverse glands (other than the liver), muscles, skin, and 



