CELLS AND THEIR ASSOCIATION 47 



From this they draw nutriment and oxygen; to it they 

 discharge carbon clioxid and other products of their 

 activity. The lymph adjacent to any one cell is a very 

 limited quantity and if there were no provision for its 

 renewal its usefulness would soon be at an end. But 

 the blood is flowing close by in capillaries whose thin 

 walls scarcely impede the passage of dissolved gases and 

 other materials between the blood and the lymph. 

 By a continuous exchange between the two fluids the 

 lymph is relieved of cell waste and held to a standard 

 composition as regards oxygen and food. 



FIG. 4. To suggest the confinement of the blood within definite 

 vessels (bb). The lymph occupies the dotted region between these 

 blood-vessels and the tissue-cells. 



One of the striking facts we note when we compare 

 free-living and associated cells is expressed by the term 

 " division of labor" which is used with reference to the 

 latter. The cells are evidently of different orders and 

 specially adapted to particular functions. Those of 

 muscle are eminently contractile. Those of the nervous 

 tissues have most highly developed the property of 

 conduction. Some, as in the skin, have it their chief 

 duty to provide hosts of descendants whose dead remains 

 may form a protective covering. The specific service of 

 the cells in the connective tissues is to elaborate inter- 

 cellular deposits. 



