116 THE BODY AT WORK 



Pancreas. In structure the pancreas presents a marked 

 resemblance to the salivary glands. Probably this resem- 

 blance is merely superficial. Minute examination reveals 

 points, apparently of great morphological importance, in which 

 they differ. In the gland-tubes of the salivary glands, and, 

 indeed, in all glands with the exception of the pancreas, 

 secreting cells project into the lumen. The secreting cells 

 of the pancreas are invested internally by a layer of 

 flattened scales . (intra-acinar cells). They lie, therefore, 

 between the basement membrane which invests them exter- 

 nally and this second layer of flattened cells which separates 

 them from the lumen of the tube. At a very early date in em- 

 bryonic life the gland-cells of the pancreas are filled with highly 

 refracting granules. As this occurs long before any digestive 

 action is called for, it may be taken as indicating that the 

 pancreas has functions which other glands the salivary, for 

 example do not possess. These granules do not, however, 

 appear in all parts of the tubes. Certain portions of the tubes 

 remain undeveloped fail, that is to say, to acquire a secreting 

 function even in adult life. Such patches of cells, not dis- 

 posed in gland-tubes, are known as islands of Langerhans. 

 When the pancreas is over-stimulated by artificial means, 

 leading to its extreme exhaustion, large portions of its 

 glandular substance return to this primitive condition. The 

 gland-cells not only discharge their stores of granules, but 

 they lose the greater part of their cell-protoplasm. It would 

 seem that, in their effort to meet the demand for ferments, 

 they use up their own cell-substance in their manufacture. 

 Having exhausted their coal, they stoke the furnace with 

 the looms and furniture of the mill. It may be that other 

 glands would do the same if it were possible to stimulate them 

 as strongly as the pancreas can be stimulated. The result is 

 probably due to the extreme susceptibility of the pancreas 

 to the action of secretin, a substance made in the intestine. 

 Secretin can be isolated and injected into the blood. We 

 shall refer again to this chemical stimulation of the pancreas 

 when tracing the progress of food through the alimentary canal. 



The secretion of the pancreas is a clear, colourless, alkaline 

 liquid of syrupy consistence. The quantity of juice secreted 

 is relatively small, but the organic substances which it contains 



