CHAPTER X 

 THEORIES OF SECRETION 



GENERAL CONSIDERATIONS 



In the enumeration of the more abundant saliva constituents (in 

 Chapters VIII and IX) it was repeatedly brought to the attention 

 of the reader that the amounts of these substances in the saliva 

 were dependent on the nerve stimulated (whether sympathetic or 

 parasympathetic), the frequency of stimulation, the previous secre- 

 tory history of the gland, the plasma composition, as well as other 

 variables. In addition, the secretory products of the submaxillary, 

 parotid and other glands differ amongst themselves. Saliva is not a 

 simple fluid, the salivary glands are morphologically complex, and 

 we must expect that the secretory process will not be simple either. 

 None of the present theories of salivary secretion are adequate to 

 account for all the phenomena encountered, and the object of this 

 chapter will be to examine critically their merits and defects. 



The most deeply entrenched prejudice about salivary secretion 

 is that the acini are the main or sole begetters of the secretion. 

 Later in this chapter abundant evidence will be presented that the 

 salivary duct system is not a passive tube but plays a complex role 

 in salivary secretion. We may, for the moment, however, assume 

 the role of the devil's advocate and ask what evidence there is that 

 the acini have any important part to play in secretion. In both rats 

 and dogs at birth, the acini are primitive or altogether absent 

 (Leeson and Jacoby, 1959; Wechsler, i960), while the ducts, al- 

 though differing from the adult in some particulars, show a con- 

 siderable degree of elaboration. Secretion of saliva in the new-born 

 animal is as abundant as it is in the mature one. It seems, therefore, 

 very probable that most of the water of the saliva is secreted by the 

 ducts in the immature animal. What of the mature animal? Does 

 this capacity to secrete water disappear with the onset of maturity 

 or is it a persistent process in the adult gland? There are other 

 features of the gland which suggest a less important role for the 

 acini than the ducts; the ducts have a very rich blood supply, 

 probably richer than that of the acini (see page 15) and the duct 



i95 



