THE MECHANISTIC CONCEPTION 31 



and distributors of materials which yield energy, 

 parts which get rid of waste materials, and still other 

 parts which have the function of ridding the organ- 

 ism of injurious invaders. Each of these functions 

 has its counterpart in a variety of labor done by a 

 citizen. Now it is clear that division of labor is 

 inseparable from interdependence, alike of the types 

 of workers that make a state and of the cells that 

 compose a living body. As the various kinds of 

 factories make different things necessary to the 

 healthful life of a community, so do various special 

 kinds of cells make substances required by other 

 parts. The cells of the adrenal glands, of the 

 pituitary gland, of the thyroid gland, of the sexual 

 glands, of the duodenal mucous membrane, and of 

 the pancreas, each make substances essential to 

 life or needed for the normal maintenance of the 

 body. In return, these cells get not merely their 

 food from the blood, but in some cases also substances 

 of a very special character needed for the performance 

 of their special duties. The waterways, the roads, 

 and the railroads may be roughly likened to the 

 lymphatics and blood vessels. They are the neces- 

 sary channels for the exchange of commodities, on 

 the one hand, and of nutritive and waste substances, 

 on the other. Just as the prolonged obstruction of 

 the roads and railroads of a district may give rise 

 to famine and possibly depopulation, so the obstruc- 

 tion of an artery (where there is no compensatory 

 flow of blood through other arteries) gives rise to a 



