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A MANUAL OF VETERINARY PHYSIOLOGY 



other secretion of the body — more, even, than venous blood. 

 Side by side with the above destructive changes construction 

 must also be going on, though there are no known nerve fibres 

 responsible for the process. As to the manner in which the 

 secretory nerves act very little is known ; the blood loses more 

 water than can be accounted for by the amount in the saliva, 

 the difference being supposed to be represented by an increase 

 in the lymph flow from the fluid ; but in what way nerve impulses 

 can cause the cells to secrete is at present inexplicable. 



The method by which secretion in the parotid gland is carried 

 out differs in no essential respect from that of the submaxillary. 

 The nerves supplying the parotid are the glossopharyngeal (the 

 action of which corresponds to the chorda of the submaxillary) 

 and the sympathetic. In the glosso-pharyngeal are dilator 

 fibres, and in the sympathetic constrictor fibres for the blood- 

 vessels, while both trunks contain secretory nerves. 



The influence of atropine and pilocarpine on the gland-cells has 

 previously been mentioned ; it remains to notice the effect of 





Fig. 60. — Changes in the Cells of the Living Parotid (Serous Gland) 

 during Secretion (Foster, after Langley). 



A, At rest ; B, in the first stage of secretion ; C, after prolonged secretion. 



nicotine, which prevents the secretion of saliva- — not, however, 

 by its action on the gland-cells, but by paralysing the connections 

 of the nerve fibres in such ganglia as the submaxillary, Langley's, 

 and the superior cervical. This action of nicotine, which essen- 

 tially consists in paralysing the nerve-cells in the ganglia, and 

 not the fibres, was discovered by Langley, and has largely helped 

 in building up our knowledge of the sympathetic system. 



The changes occurring in the cells of the salivary glands 

 during secretion depend upon the type of gland. We select 

 Langley's observations, since he examined the living gland, and 

 not one simply hardened and stained. During the stage of rest 

 in a living serous gland, such as the parotid, the cells are found 

 to be filled with a quantity of granular material, and the outline 

 of each individual cell is indistinct ; the lumen of the gland is also 

 occluded, and no nucleus can be observed in the cells ; in other 

 words, the gland is charged with its secretory products (Fig. 60, A). 



