ABSORPTION AND EXCRETION 247 



blood-supply to the secreting cells; or alterations in the concentra- 

 tion of the dissolved substances in the blood-supply for example, 

 alteration of percentage of glucose in the blood which influences 

 the glycogen- secret ing power of the liver cells. 



(2) Alterations in the secretory activity of the cells due to 

 external causes, when the supplies of solvent and nutrient remain 

 constant or do not change proportionately to the change in secretory 

 activity, such as the stimulation of the special secretory nervous 

 mechanism of the secretory cells, or the effects upon secretion of 

 chemical substances for example, secretin upon the pancreatic 

 cells, or gastrin upon the gastric cells or drugs such as pilocarpin 

 or atropin upon secretory cells in general. 



(3) Most external in character of all are the mechanisms by 

 which the secretions in certain cases are carried away from the 

 secreting cells, diverted into different channels so as to be carried 

 away to different parts of the body, or by certain muscular arrange- 

 ments in the different ducts, are retained ready and already secreted 

 for use at intermittent periods. Examples of such external mechan- 

 isms of secretion are found in the ducts of secreting glands, often, 

 when of an appreciable length, supplied with muscular walls which 

 by means of peristaltic contractions pass on the secretion, or by 

 sphincters at definite parts along their course, provide for its reten- 

 tion until a reflex stimulus causes it to be discharged when there is 

 physiological occasion for its use. Storage sacs for the secretion 

 are found in the gall-bladder, in the organs of generation, in the 

 dilated ducts of the mammary gland, and in the poison glands, etc., 

 of many animals. 



Exactly the same division into an intrinsic indispensable cellular 

 or internal part is seen in the processes of excretion and absorption, 

 together with a more or less expanded and varying accessory or 

 external part. 



Thus, in absorption, we have in the intestinal columnar cells 

 an active cellular absorption, with accompanying chemical change, 

 and work done against osmotic pressure, and the external part 

 of carriage of the products to the tissues, where again cellular pro- 

 cesses of absorption occur modified by the operation of the nervous 

 system, and new products are given out which are carried by an 

 extra- cellular process to other cells. Similarly in excretion, we 

 have cellular activity in which the excretory products are formed 

 in the various cells; external processes by which these are carried 



