PRODUCTION OF THE TRUE SECRETIONS. 345 



discharged from the vein pulsatim to the distance of several inches. The cause of this 

 difference in color is very easily understood. During the intervals of secretion, the blood 

 is sent to the gland for the purposes of nutrition and the manufacture of the elements of 

 the secretion. It then passes through the part in moderate quantity and undergoes the 

 usual 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 charatcer 

 of the blood coming from the part is very much less marked. These facts enable us to 

 form a pretty clear idea of the mechanism of secretion ; although the exact nature of 

 the forces which effect the changes of the organic principles of the blood into the charac- 

 teristic 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 secreting 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 intervals of secretion, the quan- 

 tity 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 discharged 

 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 contrast with the constant action of the organs of excretion. It is well 

 known, for example, that the function of digestion is seriously disturbed when the act is 

 too prolonged from the habitual ingestion of an excessive quantity of food. 



From the considerations already mentioned, it is evident that the secretions, as a 

 rule, are formed by the epithelial structures of the glands. There has been a great deal 

 of speculation with regard to the mechanism of this action of the cells. As we before 

 remarked, this question cannot be considered as settled. It does not seem probable that 

 the cells are ruptured during secretion and discharge their contents into the ducts, for, 

 under these circumstances, we should expect to find some of their structure in the 

 secreted fluid ; whereas, aside from accidental constituents, the secretions are homogene- 

 ous and do not contain any formed anatomical elements. There is no good reason for 

 supposing that this action takes place and that more or less of the glandular epithelium 

 is destroyed whenever secretion occurs ; and, in the present state of our knowledge, we 

 can only assume that the secreting cells induce certain transformations in the organic 

 elements of the blood and modify transudation, without pretending to understand the 

 exact nature of this process. 



The theory, that the discharge of the secretions is due simply to mechanical causes and 

 is attributable solely to the increase in the pressure of blood, cannot be sustained. Press- 

 ure undoubtedly has considerable influence upon the activity of secretion ; but the flow 

 will not always take place in obedience to simple pressure, and secretion may be induced 

 for a limited time without any increase in the quantity of blood circulating in the gland. 



The glands possess a peculiar irritability, which is manifested by their action in 

 response to proper stimulation. During secretion, they generally receive an increased 

 quantity of blood; but this is not indispensable, and secretion may be excited without 

 any modification of the circulation. This irritability will disappear when the artery sup- 

 plying the part with blood is ligated for a number of hours; and secretion cannot tlu-n 

 be excited, even when the blood is again allowed to circulate. If the gland be not 

 deprived of blood for too long a period, the irritability is soon restored; but it may be 



