180 THE FLUIDS. 



the contained fluid is set free, and is then recognized as the proper 

 secretion of the surface on which it is found. 



Transudation, on the other hand, is merely a physical phenome- 

 non, dependent upon the permeability of membranes, by certain 

 elements of the blood contained in their vessels ; and must not be 

 confounded with the vital process before defined. The word effu- 

 sion is sometimes used to express the same phenomenon. 



Exudation is also a vital process, as will appear. 



I. THE SEROUS SECRETIONS. 



Lehmann has included all the serous secretions under the head 

 of transudations. This is, however, incorrect, since it has been 

 proved that the normal serous secretions are separated from the 

 blood by the epithelial cells of the serous membranes. The serous 

 surfaces are, however, very liable to transudations also ; and in all 

 cases where an excessive amount of fluid is accumulated in a serous 

 cavity, a transudation (or an exudation, which consists of the blood- 

 plasma very nearly), directly from the blood, has occurred, and be- 

 come mixed with the proper secretion ; e. g. in ascites from pressure 

 of abdominal tumors, the accumulation is almost exclusively from 

 transudation, and very slightly from the natural secretion. 



The proper serous secretions are, therefore, the fluids normally 

 found upon the various serous membranes, viz., on the pleura, peri- 

 toneum, pericardium, the cerebral layer of the arachnoid, and the 

 membrane lining the ventricles of the brain. To these may also 

 be added the aqueous humor of the eye, the liquor Cotunnii, and 

 the endolymph of the internal ear. The liquor amnii is a serous 

 secretion ; but, apparently, to a still greater extent, also a transuda- 

 tion. The synovial are incorrectly termed serous membranes, and 

 their secretion is intermediate between the serous and the mucous 

 secretions. 



None of the serous secretions contain histological elements, ex- 

 cepting fragments of the epithelial cells (or still perfect ones) which 

 secreted them. Molecular granules or cytoid corpuscles are merely 

 accidental constituents, when present. 



In fact, the serous are the simplest of all secretions. They ap- 

 proximate more or less nearly to the serum of the blood ; while 

 other secretions contain elements peculiar to themselves, which they 

 have formed from the blood-plasma. It is, indeed, for this reason 

 that physicists would regard them as mere transudations. Still, 



