TRANSUDATIONS. 181 



they are not identical in composition on all the serous membranes, 

 but vary to suit the requirements of each ; e. g. the fluid of the 

 ventricles of the brain contains the least albumen of all (0.5 per 

 cent.), that of the peritoneum 1 per cent., and that of the pleura the 

 most of all (1.8 per cent.). Albumen is entirely wanting in the 

 aqueous humor, and in the liquor amnii during the last months of 

 pregnancy. (Lehmann.) But to the physiologist the idea that this 

 adaptation of the fluid to the requirements of the surface is dele- 

 gated to a mere physical force, is, d priori, in the highest degree 

 improbable, were there no facts to disprove it. 



It is impossible to ascertain the normal quantity of the serous 

 secretions, their amount is so small. Much of the fluid found after 

 death in the serous cavities is a mere transudation, doubtless, occur- 

 ring during the last moments of life. 



Origin. From the epithelial cells of the serous membranes, as 

 already described. 



Uses. The serous secretions are merely for the prevention of 

 friction of the opposed surfaces of the serous membranes. In the 

 eye and the ear they evidently conduce to the perfection of the 

 senses of sight and hearing. 



II. TRANSLATIONS. 



It has been already stated that mere transudations are very liable 

 to occur on serous surfaces, and these also are very similar in com- 

 position to blood-serum. Indeed, the true serous secretions being 

 normally in so small amount that enough for analysis can hardly 

 be obtained, the analyses of serous secretions (so called) are usually 

 those of transudations, mixed with a very small amount of the 

 former. In all cases when the secretion is abundant we may as- 

 sume that the major part is a transudation merely. And, except in 

 a few instances, transudations are to be regarded as pathological, 

 while the serous secretion is physiological. 



The transudations which may be regarded as physiological are 

 the serous fluid in the areolae of areolar tissue, the halitus from the 

 lungs, and a certain amount of fluid also given out on the skin, in- 

 dependently of the true perspiration. 



The pathological transudations include all cases of simple oedema, 

 and of dropsy (ascites, anasarca, hydrothorax, hydrocephalus, hy- 

 drocele, hy drops articuli, ovarian dropsy, &c.). Colliquative sweats, 

 diarrhoeas, and the discharges in cholera must be also referred to 



