22 SECRETION. 



through the part in moderate quantity and undergoes the usu- 

 al change from arterial to venous, in which a great part of the 

 oxygen disappears and carbonic acid is formed ; but, when 

 secretion commences, the ordinary nutritive changes are not 

 sufficient to deoxidize the increased quantity of blood, and 

 the venous character of the blood coming from the part is 

 very much less marked. 1 



These facts enable us to form a pretty clear idea of the 

 mechanism of secretion; though the exact nature of the 

 forces which effect the changes of the organic princi- 

 ples of the blood into the characteristic elements of the 

 secretions is not understood. Experiments, however, have 

 shown that in the act of secretion there are two tolerably 

 distinct processes : 



1. It may be assumed that at all times the peculiar se- 

 creting cells of the glands are forming, more or less actively, 

 the elements of the secretions, which may be washed out of 

 the part or extracted by maceration ; but during the inter- 

 vals of secretion, the quantity of blood received by the 

 glands is relatively small. 



2. In obedience to the proper stimulus, when a gland 

 takes on secretion, the quantity of blood which it receives 

 is four or five times greater than it is during repose. At 

 that time, water, with certain of the salts of the blood in 

 solution, passes into the secreting structure, takes up the 

 characteristic elements of the secretion, and fluid is dis- 

 charged by the duct. 



In all the secretions proper, there are intervals, either of 

 complete repose, as is the case with the gastric juice or the 

 pancreatic juice, or periods when the activity of the secretion 

 is very greatly diminished, as in the saliva. These periods 

 of repose seem to be necessary to the proper performance of 

 the function of the secreting glands ; forming a marked con- 

 trast with the constant action of the organs of excretion. It 



1 This subject is more fully discussed in vol. i., Blood, p. 106, under the 

 head of " Color of the Blood." 



