SECRETION 363 



source of the energy required. Certain possibilities are indicated in the text as to 

 the way in which these osmotic forces are available. But, in the end, the 

 production of osmotically active material must be ascribed to what, in our present 

 ignorance, we call " protoplasmic " activity, by means of which the chemical energy 

 derived from oxidation of food is converted into the various other forms of 

 energy required. 



In most cases, the process of secretion is found to be accompanied by the 



disappearance of certain granules, " zymogen," from the cells of the gland. These 



granules appear to be a stage in the formation of the constituents of the 

 secreted fluids. 



The production of osmotically active substances, together with changes in the 

 permeability of the cell membrane, appear to be the chief factors in the actual 

 process of secretory activity. 



The first stage in the formation of urine is the filtration in the glomerulus of 

 a liquid which is identical with blood-plasma minus its colloids ; so that the rate 

 of secretion under a given pressure is inversely proportional to the osmotic 

 pressure of these colloids in the blood, and if the blood pressure is lower than this 

 osmotic pressure, no filtration takes place. 



The work done in secretion is, in the main, of two kinds, although the ultimate 

 source of both lies in chemical energy utilised in cell processes. The work 

 done in producing a secretion of higher osmotic pressure than the blood can be 

 calculated by the method given in the text. That done in the various chemical 

 reactions can only be estimated approximately by the amount of oxygen consumed. 

 The work in glomerular filtration is not derived from the kidney itself, but from 

 the contractions of the heart muscle. 



The measurements of oxygen consumption give some indications as to the 

 nature of the cell process. The increase is found to take place, not only during 

 the secretory process, but for some time afterwards. This obviously means that 

 energy from some reaction is being stored up during rest and in a form available 

 for the next period of activity. There does not appear to be any storage of 

 " intramolecular" oxygen, since the secretory activity is greatly dependent on the 

 supply of oxygen in the blood at the time of secretion itself. 



Glands are set into activity either by means of chemical substances circulating 

 in the blood, such as drugs or the natural " hormones " such as secretin, or by 

 the agency of nerves supplying the gland cells. This statement does not exclude 

 the possibility that the final link in the chain of excitation processes may be the 

 same chemical substance in all cases. 



Evidence, taken as a whole, indicates that there are two kinds of nerve fibres 

 to glands ; one kind, the " secretory " of Heidenhain, presides over the secretion of 

 water, together with diffusible substances present in blood, and must, therefore, 

 affect both permeability of cell membrane and the osmotic pressure of cell contents. 

 The other set, "trophic" of Heidenhain, are concerned with the production of the 

 specific solid constituents and have little or nothing to do with the phenomena 

 connected with the production of a flow of water. 



The combination of various facts indicates that during rest gland cells 

 form, by means of reactions which are reversible, certain substances which are 

 preliminary stages of the constituents of the actual secretion formed on stimula- 

 tion. When the gland is excited to activity, a current of water is set flowing 

 through the cell by a combination of increased permeability of the outer end of 

 the cell with the splitting up of some cell constituent into smaller molecules and 

 thus raising the osmotic pressure. This current of water washes into the duct 

 various substances stored in the cell, sometimes after these have been changed by 

 the excitation process, before being given off. As these substances are removed, 

 further formation takes place by the cell reactions in order to re-establish 

 equilibrium. 



The results of experiments on artificial perfusion of salivary glands show that 



