326 SECRETION. 



has found 1 that ferrocyanide of potassium, when injected into the 

 jugular vein, though it appears with great facility in the urine, 

 does not pass out with the saliva ; and even that a solution of 

 the same salt, injected into the duct of the parotid gland, is ab- 

 sorbed, taken up by the blood, and discharged with the urine ; but 

 does not appear in the saliva, even of the gland into which it has 

 been injected. The force with which the secreted fluids accumulate 

 in the salivary ducts has also been shown by Ludwig's experi- 

 ments 2 to be sometimes greater than the pressure in the bloodves- 

 sels. This author found, by applying mercurial gauges at the same 

 time to the duct of Steno and to the artery of the parotid gland, that 

 the pressure in the duct from the secreted saliva was considerably 

 greater than that in the artery from the circulating blood ; so that 

 the passage of the secreted fluids had really taken place in a direc- 

 tion contrary to that which would have been caused by the simple 

 influence of pressure. 



The process of secretion, therefore, is one which depends upon 

 the peculiar anatomical and chemical constitution of the glandular 

 tissue and its secreting cells. These cells have the property of 

 absorbing and transmitting from the blood certain inorganic and 

 saline substances, and of producing, by chemical metamorphosis, 

 certain peculiar animal matters from their own tissue. These sub- 

 stances are then mingled together, dissolved in the watery fluids 

 of the secretion, and discharged simultaneously by the excretory 

 duct. 



All the secreting organs vary in activity at different periods. 

 Sometimes they are nearly at rest ; while at certain periods they 

 become excited, under the influence of an occasional or periodical 

 stimulus, and then pour out their secretion with great rapidity and in 

 large quantity. The perspiration, for example, is usually so slowly 

 secreted that it evaporates as rapidly as it is poured out, and the 

 surface of the skin remains dry ; but under the influence of unusual 

 bodily exercise or mental excitement it is secreted much faster 

 than it can evaporate, and the whole integument becomes covered 

 with moisture. The gastric juice, again, in the intervals of digestion, 

 is either not secreted at all, or is produced in a nearly inappreciable 

 quantity ; but on the introduction of food into the stomach, it is 

 immediately poured out in such abundance, that between two and 

 three ounces may be collected in a quarter of an hour. 



1 Legons de Physiologie Experiinentale. Paris, 1856, tome ii. p. 96 et seq. 



2 Ibid., p. 106. 



