PHYSIOLOGY OF THE CIRCULATION. 87 



the blood contained in the small vessels ; and the 

 anatomical arrangement of the blood-vessels of these 



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glands furnishing the impediment necessary for the 

 compression of the blood circulating through them ; 

 these two considerations do appear to me sufficient 

 to invest the old doctrine with a certain degree of 

 probability. Physiologists are too unsettled in their 

 views of the process of secretion to render necessary 

 many additional remarks in applying the general law 

 of effusion to this particular function. For whether 

 the secreted matters be derived directly from the 

 blood by a process of filtration, or are the results of 

 various chemical changes effected in that effused 

 fluid by the peculiar physical conditions to which it 

 is subjected previous to its final discharge, or are 

 generated according to vital laws in cells developed 

 by vital powers similar to those concerned in the 

 process of growth, or, as is perhaps most probable, 

 by the conjoint operation of all these agencies, it is 

 in each case equally evident that a constant supply 

 of material will be required ; and this supply can 

 only be obtained from the blood-vessels of the 

 secreting structure. It is unnecessary to recapi- 

 tulate any of the former arguments for the purpose 

 of proving that this effusion is due to the expelling 

 force constituted by the lateral pressure of the blood 

 contained in these vessels. But in closing this part 

 of my subject, I may remark 1st, That the free 

 surface on which secretion invariably occurs is 

 another provision for facilitating the process of 

 effusion, by substituting a less resisting for a denser 

 medium : so that the vessels are less firmly sup- 



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