DRUGS ACTING ON THE DIGESTIVE ORGANS 17 



when rubbed on a dog's, skin, and a small dose of tartar emetic cause 

 violent vomiting in a cow. Some animals are very susceptible to counter- 

 irritants. Well-bred animals are commonly more responsive to drugs 

 than others. 



GENERAL ACTION OF DRUGS. 

 DRUGS ACTING ON THE DIGESTIVE ORGANS. 



Sialagogues are agents increasing the secretion of saliva. Antisiala- 

 gogues are agents diminishing salivary secretion. Among the sources of 

 saliva — the parotid, sublingual and submaxillary glands— the latter have 

 received most study. The chorda tympani, with its centre in the medulla, 

 is one of the two nerves supplying the submaxillary gland. It contains 

 two sets of fibres, the secretory and vasodilator. Hence stimulation of 

 this nerve, or its centre, whether immediately or reflexly, leads, by means 

 of its vasodilator fibres, to dilatation of the blood vessels and enhanced 

 vascularity of the gland, and so indirectly to greater secretion; while, 

 through excitation of the secretory fibres, the protoplasm of the glandular 

 cells is influenced and secretion directly increased. 



Reflexly the gland is stimulated by drugs exciting the peripheral 

 terminations of the gustatory (lingual branch of the fifth nerve) and 

 glossopharyngeal nerves in the mouth ; the vagus endings in the stomach ; 

 by agencies sending pleasurable impressions to the brain through the 

 medium of the eyes or nose; or by stimulation of other sensory nerves. 

 The submaxillary gland is also supplied by a branch of the cervical 

 sympathetic accompanying the submaxillary arteries. Stimulation of this 

 nerve, or its centre, causes vascular constriction in the gland and inhibi- 

 tion of secretion. 



Sialagogues are often classed under three heads. 1st, Specific Siala- 

 gogues, acting directly on the mechanism concerned with secretion, i.e., 

 the gland cells, or nervous apparatus. Pilocarpine is the best example of 

 the specific class. It stimulates the gland cell or peripheral nerve end- 

 ings. 2nd, Reflex sialagogues ; exciting sensory nerve terminations and 

 indirectly or reflexly stimulating the nervous mechanism controlling secre- 

 tion. As examples of this class may be mentioned alkalies, acids, emetics, 

 and other agents stimulating the mucous membrane of the mouth and 

 stomach. 3rd, Mixed sialagogues, acting both specifically and reflexly. 

 Physostigmine, nicotine or tobacco and mercury preparations may be in- 

 cluded in this category. 



Antisialagogues. — These drugs may act in various ways to lessen 

 salivary secretion, but atropine is most notable in this regard. It acts by 

 depressing the peripheral endings of the secretory nerves. 



Uses. — Sialagogues are not of much therapeutic value. Some are 

 added to the drinking water given to animals suffering from fever, to 

 relieve dryness of the mouth and thirst. They are then called refrig- 

 erants; as, for example, potassium nitrate, diluted phosphoric and other 

 acids. The sialagogues are sometimes employed to stimulate the mucous 



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