748 PHYSIOLOGY 



be remembered that the sympathetic nerve carries the vaso-con- 

 strictor fibres to most or all of the vessels of the head and neck, and 

 therefore in most cases we should expect, and we find, that stimulation 

 of this nerve causes vascular constriction in the gland affected. 

 Before attempting to decide this point we must study in somewhat 

 greater detail the changes which occur in the gland concomitantly 

 with the secretion. 



CHANGES IN THE GLAND ACCOMPANYING SECRETION 

 The fact that a sub-maxillary gland of the dog under favourable 

 conditions will secrete its own weight of saliva in five minutes, and will 



continue to secrete for many hours 

 spaces afterwards, shows that there must 

 be a continual renewal of the fluid 

 -Secretinq which is turned out in the secretion, 

 cells. The source of this fluid must be the 

 blood which is circulating through 



grammatic representation of the 

 elements which make up a secreting 

 i t Basement lobule and which may be involved 



membrane j n t ^ e act O f sec retion (Fig. 323), we 



FIG. 323. Diagram to show relation of gee that between the lumen of the 



the secreting cells 01 a gland to the 



blood and lymph supply. duct and the blood, which must be 



regarded as the source of the fluid, 



the following layers of cells intervene : (1) the endothelium of the 

 blood capillaries ; (2) the basement membrane ; (3) the epithelial 

 cells of the gland proper. We have, in the first place, to decide 

 to which of these elements can be ascribed the chief part in the act 

 of secretion. 



The secretory activity of the sub-maxillary gland, whether evoked 

 reflexly by the introduction of acids into the mouth or directly by the 

 injection of pilocarpine or by stimulation of the peripheral end of the 

 chorda tympani nerve, is always associated with a considerable dilata- 

 tion of the vessels of the gland, and a consequent large increase in the 

 blood flow through the gland which may amount to between three and 

 eight times the quantity passing during rest. Such an increase in the 

 supply of blood is necessary in order to afford a source for the large 

 quantity of fluid which is turned out in the saliva. Another effect of 

 the dilatation will be to raise the pressure in the capillaries of the 

 gland. We cannot, however, regard this rise of pressure in the capil- 

 laries as an essential factor in the act of secretion. If atropine be 

 administered excitation of the chorda causes the same vaso- dilatation 

 as before, though no secretion is produced. Moreover Ludwig showed 



