DIGESTION IN THE MOUTH 719 



solids may preponderate, altogether apart from changes in the circulation simultaneously 

 evoked. 



THE ENERGY INVOLVED IN THE ACT OF SECRETION 



The source of the energy must be sought in the processes of oxidation 

 occurring in the cells of the gland, and Barcroft has attempted to determine 

 the total amount of energy put out by the gland in the act of secretion 

 by measuring its respiratory exchanges under the conditions of rest and 

 activity. He found that the resting submaxillary gland in a small dog took 

 up 0-25 c.c. of oxygen per minute and put out 0-17 c.c. C0 2 , while during 

 active secretion it absorbed 0*86 c.c. 2 and gave off 0-39 c.c. C0 2 . Assum- 

 ing that the total oxygen taken up is employed in the oxidation of a food 

 substance, such as glucose, and that the whole of the energy of the chemical 

 changes is set free in the form of heat, we find that a resting gland weighing 

 about six grammes produces about 1-1 calories per minute. We know 

 however that a certain amount of external work is performed in the secretion 

 of a saliva containing less salts than the original blood, and also, when there 

 is any resistance to the flow of saliva through the duct, in raising the hydro- 

 static pressure of the saliva in the duct to a height greater than that in the 

 blood capillaries. 



Can we from all these data form a conception of the total changes 

 occurring in the gland and involved in the formation of the secretion ? Even 

 during rest, changes are going on in the gland cells, changes which involve 

 the taking up of food material and its assimilation under the influence of 

 the nucleus, perhaps into the nucleus itself, and certainly into the undifferen- 

 tiated cytoplasm. In this cytoplasm a further change occurs, leading to its 

 transformation into granules. When activity is excited by the stimulation of. 

 secretory nerves, the primary change appears to involve simply the granules. 

 These structures must absorb water, apparently against osmotic pressure. 

 Those nearest the lumen swell up, become converted into spheres containing 

 water and salts in smaller proportion than exists in the lymph bathing 

 the cells (and presumably in the protoplasm surrounding the granules), and 

 in this swollen form are discharged or ruptured on the periphery of the cell 

 into the lumen, so giving rise to secretion. This discharge of a fluid with 

 a smaller molecular concentration than the cell or surrounding blood plasma 

 must lead to an increased concentration in the remaining parts of the cell. 

 The increased concentration would naturally induce a flow of water from 

 lymph into cell, and the consequent concentration of the lymph would in -the 

 same way cause a flow of water from blood to lymph. This pull of water by 

 the cell from the blood is still further increased in another way. The act 

 of secretion, involving as it does the expenditure of energy, can be carried out 

 only at the expense of chemical changes in the cell. These chemical changes, 

 as in all other metabolic processes of the body, will result in the formation of 

 a number of small molecules from the great colloid molecules of the proto- 

 plasm. The products of metabolism, or metabolites, will therefore accumulate 

 in the cell, pass into the lymph, and increase the concentration of the latter. 

 The increased concentration will call forth an increased transudation of 



