CHAPTER XIII 

 THE PHYSIOLOGICAL ROLE OE SALIVA 



The main functions of saliva are to aid in the digestion of food and 

 in the protection of the entrance to the alimentary tract. In addi- 

 tion, various other functions can be attributed to saliva, some of 

 which only belong to certain species. The present chapter deals 

 with the role of saliva in mammals, and, particularly, in humans ; 

 the comparative physiology of salivary secretion is reviewed in 

 Chapter XIV. 



Although saliva has important functions, it is generally not essen- 

 tial to life. Death occurs in sheep in which saliva is drained off 

 through fistulae of the parotid ducts, but seems to be due to severe 

 loss of electrolytes rather than to loss of a juice with a vital func- 

 tion. Plagge (1938) removed the submaxillary and sublingual 

 glands in new-born rats or ligated the ducts of these glands. The 

 animals died from inanition within five days if not forcibly fed with 

 cow's milk. When the glands were removed after ten days of age 

 the rats could survive but growth was retarded; when the opera- 

 tion was made after twenty days or more, development was normal. 

 Lack of saliva is, however, not without effects in full grown in- 

 dividuals. Many investigations indicate that the incidence of dental 

 caries is increased in "desalivated" animals (for references see 

 Schwartz and Shaw, 1955), illustrating the importance of the pro- 

 tective functions of saliva. The effect of loss of a digestive action 

 of saliva may be exemplified by an oft-quoted observation of 

 Claude Bernard ; a horse, supplied with fistulae of both the parotid 

 ducts, had great difficulty in chewing and swallowing its dry food 

 and lost its appetite. 



In humans xerostomia may exist because of congenital hypo- 

 plasia or absence of the salivary glands or may be due to loss of 

 gland substance after surgical operation or x-ray treatment. Cases 

 have also been described in which the glands are present but for 

 unknown reasons unable to secrete reflexly or in response to in- 

 jected drugs (Hadden, 1889). Such a patient has been observed to 

 secrete reflexly during pregnancy only (Waldenstrom, personal 

 communication). In cases of aptyalism, mastication and swallowing 



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