668 PHYSIOLOGY 



carries the vaso-constrictor 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 stimu- 

 lation 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 condi- 

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

 to secrete for many hours 'afterwards, shows that there must be a continual 



renewal of the fluid which is turned out 

 space - n the secretion The gource of thig 



fluid must be the blood which is circu- 

 -Secreting lating through the gland. If we refer to 

 cells. the diagrammatic representation of the 

 elements which make up a secreting lobule 

 and which may be involved in the act of 

 secretion (Fig. 325), we see that between 

 the lumen of the duct and the blood, 

 Basement which must be regarded as the source of 



membrane the fluid, the following layers of cells 



FIG. 325. Diagram to show relation of , /1X ,-, j ,r r r ,-1 



the secreting 8 cells of a gland to the intervene: (1) the endothehum of the 



blood and lymph supply. blood capillaries ; (2) the basement mem- 



brane ; (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 dilatation 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 capillaries 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 that the force with which the secretion is turned out into the ducts 

 of the gland is greater than that represented by the blood pressure. The 

 blood pressure in the capillaries must be considerably lower even with full 

 vascular dilatation than the pressure in the carotid artery. If two mano- 

 meters be connected, one with the carotid artery and the other with the 

 duct, it will be found that on stimulating the chorda tympani nerve the 

 secretion will be excited, and the mercury will be driven up by the pressure 



