THE WORK OF MUSCLES AND GLANDS 43 



minute pits in the inner surface of the stomach or the in- 

 testine. The fundamental structure is the same in both 

 classes. The microscopic gland is a depression of a cel- 

 lular surface a pocket, one might say out of which 

 when it is active the secretion wells. The cells which 



Fig. 5. The principle of glandular structure. In the upper 

 figure a simple microscopic gland is supposed to be laid open by a 

 section along its vertical axis. The cells are seen to surround a recess 

 into which they discharge their secretion. Below, the same struc- 

 ture is shown in its entirety, and in addition the encircling blood- 

 vessels which contribute to make good the losses suffered by the 

 secreting cells. 



bound its cavity are the produ^^ and 



are in turn dependent for renewal upon the lymph which 

 underlies them and the blood which is flowing close by. 

 A superficial view would suggest that such a gland is a 

 filtering device adapted for straining off certain portions 

 of the blood. This is, however, an entirely inadequate con- 



