ABSORPTION, AND EXCRETION 141 



The agents which come into operation in the external part 

 of secretion may be classified as follows : 



(1) The alteration in the supply of fluid or solvent and of dis- 

 solved and nutrient matter to the cell, such as variations of the 

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

 centration of the dissolved substances in the blood supply, for 

 example, alteration of percentage of glucose in the blood which 

 influences the glycogen-secreting power of the liver cells. 



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

 external causes, when the supply 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 

 arrangements in the different ducts, are retained ready and 

 already secreted for use at intermittent periods. Examples of 

 such external mechanisms 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 retention 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, &c., 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 



