THE PHYSIOLOGY OF SECRETION. 171 



instances, however, the discharge from the cell is also inter- 

 mittent. 



Let us consider the simpler secretions first. The secretion 

 of carbonic anhydride is evidently controlled by its production, 

 since the gas is normally secreted as fast as it is produced. 

 The production is largely dependent on the food and external 

 conditions. In the case of muscle the secretion of carbonic 

 anhydride is largely dependent on the contraction of muscle, 

 large amounts of this gas being formed when muscle contracts. 

 In this case it is under the direct control of nerves. But let it 

 be remarked that these nerves do not control the secretion; 

 they only control the production. They have no action on any 

 hypothetical secretory function of the muscle cell at all. They 

 cannot, hence, be called secretory nerves in the proper sense. 

 They act on a very special substance in a very special cell. 

 These simpler secretions, then, so far as they are intermittent 

 at all, are rendered so by the intermittence of their production. 

 This in turn is largely affected by many different circumstances, 

 chief among which is the food supply of the cell. This is quite 

 beyond the cell's control, and any nerve affecting the secretion 

 of these substances, beyond the very special case of muscle, 

 affects it through controlling the food supply. This is, for 

 example, the case in the liver. Whenever the oxygen supply 

 of the liver is reduced either by vaso-constriction, insufficient 

 ventilation of the lungs, or in any other way, the production 

 of sugar by the liver cell is increased and its secretion increased. 

 Just as in the muscle, where nerves control the secretion of 

 carbonic anhydride by controlling the production, so in the 

 liver the secretion of sugar is controlled by its production. 

 And if we may speak of the muscle nerves as secretory nerves, 

 so must we equally well speak of nerves which indirectly con- 

 trol the secretion of sugar as secretory nerves. The vaso- 

 constrictor nerves going to the liver or to the blood vessels of 

 the intestinal area become, hence, secretory nerves. But they 

 do not directly affect any secretory power of the cell ; they only 

 affect the production of sugar. They do this, moreover, by 

 their action on blood vessels and not, as far as is known, by 

 their action on the liver cell. All vasomotor nerves, in fact, 



