674 PHYSIOLOGY 



contents as that which follows the introduction of acid into the mouth. 



Babkin has shown that the same differences are found after complete section 



pi the sympathetic and that there is the same acceleration of the circulation 



c through/the gland whether the secretion is aroused by introduction of meat 



powder or of acid. It is impossible therefore to explain the difference in the 



^composition of the saliva obtained under these two circumstances as due to 



'differences in the blood-supply to the gland, and we must conclude either that 



the chorda tympani contains different kinds of fibres which are excited to 



varying extent according to the nature of the reflex stimulations, or that one 



and the same nerve fibre can convey specifically different impulses. The latter 



explanation would not be in accord with the generally accepted Miiller'slaw 



^of specific irritability but is the explanation to which Babkin himself inclines. 



: :At any rate it is certain that according to the nature of the reflex stimulus 



either the secretion of water and salts or the secretion of organic 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 -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 . Assuming 

 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, rhe 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 sa]ts 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 



