82 INTRODUCTION TO GENERAL PHYSIOLOGY 



sense. Thus, not only is the taste of food effective, but also the 

 sight and smell. Hence, as we saw, the importance of the 

 possession of appetite. 



But in the case of the stomach we find that another mode of 

 stimulation begins to show itself. Glands can be excited by 

 chemical agents, as we know from the fact that there are drugs 

 which have the property of causing them to secrete. Such a drug 

 is pilocarpine. In the course of digestion, certain substances are 

 produced by the action of the contents of the alimentary canal on 

 the cells of its walls. These substances are absorbed into the 

 blood and arrive in this way at some gland which is sensitive 

 to them. The most obvious of these mechanisms is that of the 

 pancreas. When the acid contents of the stomach pass into the 

 small intestine, they cause the formation in the cells of a substance 

 which has been called " secretin," but whose chemical nature is still 

 unknown. This passes into the blood, and arriving at the pan- 

 creas, excites this organ to secrete. It is a remarkable fact that 

 the trypsin contained in the pancreatic juice is inactive until it has 

 been acted upon by another substance, " enterokinase," apparently 

 an enzyme, secreted by the walls of the intestine. 



So far as can be made out, what happens in the secreting cell 

 when stimulated is somewhat as follows. We have seen that the 

 cell at rest contains a store of material, which has been produced 

 by the expenditure of energy. Its molecules, or some of them, 

 are large ones, and onset of activity is associated with a splitting 

 up of these molecules into smaller ones. The consequence of this 

 is that the osmotic pressure of the cell contents rises, water is 

 absorbed from the blood and, supposing that the cell membrane is 

 impermeable to these solutes, the cell merely becomes distended. 

 But now, supposing that the end in relation with the duct becomes 

 permeable, then the pressure will cause a flow of liquid into the 

 duct, carrying with it in solution the constituents of the secretion. 

 Sometimes granules appear to escape bodily from the cells, be- 

 coming dissolved in the ducts later. This process will continue as 

 long as any osmotically active material is present in the cell and 

 the membrane at the duct end remains permeable. It can be 

 imitated in a model (E., p. 197). 



In some secreting mechanisms of plants it can be shown that 

 changes of permeability occur at the end of the cell at which the 

 liquid appears. A similar state of affairs is at the basis of the 

 root pressure. If the stem is cut through, a flow of liquid, under 

 fair pressure, occurs from the cut end, being absorbed from the 

 soil. 



In the animal cell, proofs of increased permeability in activity 

 are more indirect. The cells take up certain dyes more readily. 



