226 Theories of Secretion 



zero. They found not infrequently that the specific activity of 

 sodium and urea in the saliva was lower than that in the venous 

 blood. This finding argues in favour of a countercurrent flow but 

 because of the anatomical irregularity of the gland vasculature, 

 this argument cannot be regarded as completely proven. For in- 

 stance, if the terminal part of the duct were to receive an inde- 

 pendent arterial supply which might be a small fraction of the total 

 gland blood flow, this could serve to lower the specific activity of 

 the saliva below that in the venous blood. How do these ideas 

 conform with the anatomy of the gland vasculature as demonstrated 

 by injection procedures? A full account will be found in Chapter II. 

 It suffices here to say that injection procedures fully confirm the 

 presence of a rich vascularization of the ducts. It also confirms 

 that, in general, the flow of blood in the periductal capillaries is 

 indeed largely countercurrent. Anatomical evidence for a portal 

 system is very meagre. It should also be noted that a considerable 

 proportion of the blood water (10-20 per cent) is extracted by the 

 gland and if the bulk of the saliva water were secreted by the acini 

 and if, as some of the anatomical evidence suggests, the acinar 

 receives only a meagre blood supply, the haemoconcentration in 

 the acinar capillaries might be very considerable. As discussed 

 elsewhere there is also considerable doubt as to the proportion of 

 the saliva water that is contributed by the acinar and the proportion 

 contributed by the ducts. 



SUMMARY 



A number of new methods have been developed to attempt to 

 uncover the nature of secretory processes in the salivary glands. 

 The presently available evidence shows that the secretory process 

 is highly complex. This is particularly true of the salivary duct 

 system which seems to play a more important role in the formation 

 of this secretion than had previously been realized. By various 

 techniques, segregated sites and distinct processes in the ducts have 

 been distinguished. At least three types of cells can be distinguished 

 on electrophysiological grounds and some progress has been made 

 in correlating electrical events in salivary gland cells with the 

 ionic movements taking place in secretion. 



