POOD DIGESTION AND RESPIRATION 81 



a powerful stimulus to the secretion of the gastric juice, which is, 

 therefore, already present in the stomach when the food arrives 

 there. 



Secretion 



The digestive juices, as pointed out, are formed by the cells of 

 organs called "glands," specially devoted to this purpose. Like 

 all the other cells of the organism, these cells derive from the blood 

 the material from which they manufacture their special products. 

 These products are not present in the blood, whose composition 

 we shall have to discuss in other places. While all cells form their 

 own structure, some of them prepare also things for the use of 

 other parts of the body, giving them off by means of a current of 

 water, which washes them out, as it were. The production of this 

 current of water is one of the problems concerning secretion, to 

 which we shall have to give attention. 



As will have been evident from what has been said about the 

 process of digestion, glands are not always in action. If we 

 examine under the microscope the condition of secreting cells after 

 a period of rest, we notice that they are rilled with a number of 

 granules, differing in kind according to the nature of the gland 

 (E., p. 196). These granules are clearly the material out of which 

 the constituents of the secretion, or some of them, are produced 

 when the gland is excited to activity, because they disappear to a 

 greater or less extent during activity. When the stimulus to 

 secretion ceases, new granules are formed by the cells automatically 

 and, as it appears, by a chemical process of the nature of a balanced 

 reaction, because, when the cell has stored a certain quantity, the 

 formation ceases, the gland being then ready for renewed activity. 

 This stage of preparation requires the expenditure of energy, as 

 shown by the fact that there is an increased consumption of oxygen 

 for some minutes after secretion has ceased. During continued 

 activity, the using up and production of new material keep pace 

 with one another, unless very great demand is made on the gland. 

 In the latter case, the granules are used up faster than they are 

 formed, and they disappear first of all from that part of the cell 

 furthest from the side in connection with the duct. 



Before we pass on to discuss what happens in the cell when it 

 enters into activity, we may spend a little time on the ways in 

 which a gland is excited to secrete. 



Some glands are supplied with newes, and when these nerves 

 are stimulated, changes are produced in the cells leading to a flow 

 of liquid. This mode of setting into activity is particularly evident 

 in the salivary glands, and is also present in the gastric glands. 

 The nerves are stimulated by reflex action from various nerves of 

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