1 96 Theories of Secretion 



cells are much richer in mitochondria and succinic dehydrogenase 

 than the acinar cells. These latter features are both good indica- 

 tions that the respiratory activity of the duct cells is higher than 

 that of the acinar cell and suggest that duct cells may have an 

 important role to play in material transfer. It is interesting at this 

 point to compare the salivary gland with the aglomerular kidney 

 found in some fish which is a simple tubule capable of secreting a 

 complex mixture of water, electrolytes and organic substances ; in- 

 deed, the histological resemblance between the tubules of these 

 kidneys and the striated tubule of the salivary gland is very strik- 

 ing. Against these arguments there is no reasonable doubt that 

 degranulation of acinar cells occurs during stimulation and that 

 the liberated granules can be seen to enter the acinar saliva (see 

 for instance Holtzlohner and Niessing, 1936, and numerous other 

 authors) ; secondly, soluble blood group substances that are secreted 

 in the saliva apparently take origin only from the mucous cells 

 of the submaxillary gland and are not present at all in the duct cells 

 (Glynn and Holborow, 1959). At the very least, enough water must 

 be secreted by the acini to carry these products into the saliva. 

 This fundamental question of the proportion of the salivary water 

 secreted through different routes remains uncertain and a quanti- 

 tative evaluation must await the development of new techniques. 

 Theories of secretion based on the comparison of the histological 

 structure of various glands with the chemistry of their secretion 

 have been widely adopted but the evidence on which they have 

 been based is of the flimsiest. The suppositions made are that a 

 specific cell type carries out a single function and further that a 

 cell whose appearance differs from this cannot carry out this func- 

 tion. Against this it may be noted firstly that even the basic facts 

 on which these theories are based are shaky. For instance, it has 

 been suggested that the cat sublingual gland produces an isotonic 

 saliva because it lacks an organized striated duct epithelium whose 

 function is supposedly to reabsorb sodium and chloride (Lundberg, 

 1958); yet the human sublingual gland which apparently is just 

 as poor in striated duct cells can produce a saliva as hypotonic as 

 the parotid gland (Kostlin and Rauch, 1957). Cohen and Myant 

 (1959), in a study of the comparative physiology of iodide secretion 

 in the salivary glands, point out that they could find no correlation 

 between the histology of the glands in different species and their 

 ability to concentrate iodide. Unfortunately, cytological theories 



