l8o BIOLOGICAL LECTURES. 



There is, however, here as elsewhere, a second mechanism — 

 that of muscle. This contractile tissue exists, like that of the 

 skin, sweat, lachrymal, and milk glands, between the cells and 

 the basement membrane, but it is present in much smaller 

 amounts. The sympathetic nerve innervates this muscle, and 

 the secretions following stimulation of this nerve, secretions 

 coincident with vaso-constriction, secretions caused by picro- 

 toxine, supra-renal extract, physostigmine, are caused by this 

 muscle. 



From this short survey, necessarily very incomplete, it may 

 clearly be seen that there is not one but many different 

 mechanisms of secretion; that secretion proper — that is, the 

 discharge from the cell — is generally constant ; its intermit- 

 tence, when it is intermittent, is caused first by contractile 

 tissue compressing the cells, second by the action of sphinc- 

 ters in the case of some unicellular glands, and third by the 

 vascular system. The intermittence of discharge of the secre- 

 tion from the gland's duct is due first to sphincters, second 

 to the muscular contractions of the gland or of its ducts 

 (bladder) ; or, third, to vasomotor changes in the gland. 

 Nerves which produce this intermittence act, therefore, upon 

 one of these three mechanisms. They do not act on the gland 

 cell. 



But besides this indirect control of secretion, do special 

 nerves exist which control secretion by action on the secreting 

 cells themselves .'* As every one knows, it is generally believed 

 by physiologists that such nerves do exist, and that they con- 

 trol secretion quite apart from any condition of the vascular 

 system, and quite apart from muscle action. It is impossible 

 to go into this matter here at any length, — something which I 

 have already done elsewhere, — but we can at least consider one 

 of the main evidences for the existence of such special secre- 

 tory nerves. The theory of their existence is, I believe, quite 

 erroneous. 



According to Ludwig, the originator of the theory, these 

 nerves did not affect any peculiar secretory power of the cell at 

 all. They caused only a decomposition of substances in the 

 cell, which thus affected its turgor and hence secretion. In the 



