THE DIGESTIVE SYSTEM 



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surface distinct lobes which are subdivided into smaller portions 



called lobules. A duct to convey away the product of the gland 



is usually present. The duct connects internally with the gland 



alveoli, the receptacles into which the 



secretory cells discharge their fluid. 



Blood-vessels encircle the alveoli and 



supply lymph to make good the loss 



caused by the activity of the cells (Fig. 



28). Special secretory nerves are provided 



to influence functional activity. 



Secretory products are of various 

 kinds and for many different purposes. 

 They are fluid in character and act in 

 a chemical manner. Among the most 

 familiar secretions are the saliva and 

 the gastric juice. 



The^ process of secretion is inter- 

 mittent, periods of rest occurring in the 

 work of glands as in all other organs. 

 During the rest period the protoplasm 

 of gland-cells becomes loaded with small 

 granules that are the stored elements, 

 from which the organic constituents of 

 the secretory product are manufactured. 

 As secretion goes on these granules 

 gradually disappear. If it is continued 

 for some hours, they disappear com- 

 pletely. 



If the process of secretion were a mere 

 filtration from the blood through cells 

 ,into gland alveoli, as was at one time 

 believed, we should expect to find the 

 same amount of soluble, diffusible salts 

 in the secretion as in the blood. Take 

 the saliva as a typical secretion to illus- 

 trate the point. It contains only half the 

 amount of diffusible salts as the blood, which makes it clear that 

 the gland-cells must prevent diffusion of the balance. Further- 

 more, by hindering the outflow of saliva from a duct, the pres- 

 sure will rise above that of the blood in the blood-vessels which 



Fig. 28.— The principle 

 of glandular structure. In 

 the upper figure a simple 

 microscopic gland has been 

 laid open by cutting through 

 its vertical axis. The cells 

 are seen to surround a recess, 

 or alveolus, into which they 

 discharge their secretion. 

 Below, the same structure 

 is shown in its entirety, and, 

 in addition, the encircling 

 blood-vessels, which help to 

 make good the loss due to 

 the activity of the secreting 

 cells. (Stiles, Nutritional 

 Physiology.) 



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