THE SECRETORY TISSUES AND ORGANS. 



secretion (the water and mineral salts), and another, quite 

 different, which governs the chemical transformations of the 

 cells so as to make them produce mucin from mucigen pre- 

 viously stored in them, in a way comparable to the production 

 of trypsin from trypsinogen in the active pancreas. These 

 latter fibres may be called " trophic," since they directly con- 

 trol the cell metabolism: while the former may be called 

 " transudatory " fibres. Some of the evidence which leads to 

 thisconclusionis a little complex, but it is worth while to con- 

 sider it briefly. In the first place, on stimulation of the 

 chorda of an unexhausted gland (that is, a gland not over- 

 fatigued by previous work) the following points can be 

 noted : 



With increasing strength of the stimulus the quantity of 

 the secretion, that is of the water poured out in a unit of 

 time, increases ; at the same time the mineral salts also in- 

 crease, but more rapidly, so that their percentage in a rap- 

 idly formed secretion is greater than in a more slowly 

 formed, up to a certain limit. The percentage of organic 

 constituents of the secretion also increases up to a limit; but 

 soon ceases to rise, or even falls again, while the water and 

 salts still increase. This of course is readily intelligible; 

 since the water and salts can be derived continually from the 

 blood, while the specific elements, coming from the gland- 

 cells, may be soon exhausted ; and so far the experiment 

 gives no evidence of the existence of distinct nerve-fibres 

 for the salts and water, and for the specific elements: all 

 vary together with the strength of the stimulus applied to 

 the nerve. But under slightly different circumstances their 

 quantities do not run parallel. The proportion of specific 

 elements in the secretion is largely dependent on whether 

 the gland has been previously excited or not. Prior stimula- 

 tion, not carried on of course to exhaustion, largely increases 

 the percentage of organic matters in the secretion produced 

 by a subsequent stimulation; but has no effect whatever on 

 the quantities of water and salts. These are governed en- 

 tirely by the strength of the second stimulation. Here, 

 then, we find that under similar circumstances the transuda- 

 tory and specific elements of the secretion do not vary to- 

 gether; and are therefore probably dependent upon different 

 exciting causes. And the facts might lead us to suspect 

 that there are in the chorda, besides the vaso-dilator, two 



