TEANSUDATION. 505 



vital action offer this in explanation of the important influ- 

 ence of the nervous system on this function. Precisely how 

 the nervous system affects absorption, in all instances, it is 

 impossible in the present state of our knowledge to deter- 

 mine ; but modifications are frequently effected through the 

 sympathetic system. These nerves, as is well known, are 

 capable of producing important local changes in the circula- 

 tion, and can even temporarily arrest the capillary circula- 

 tion in some parts ; and it is in this way that many of the 

 variations in absorption may be produced. 



Transudation. Although the endosmotic is the princi- 

 pal current in the functions which have thus far been consid- 

 ered, this is always accompanied by a certain amount of 

 exosmosis. All the soft and vascular tissues, and all the cav- 

 ities and tubes, contain a certain amount of exhaled or transu- 

 ded fluid. This may be small in quantity and hardly more than 

 vaporous, as it is over the pleura or peritoneum, or it may be 

 liquid and in a decidedly appreciable quantity, like the inter- 

 muscular fluid, etc.; but all these vascular tissues are moist- 

 ened by liquid which has transuded the permeable walls of the 

 blood-vessels. The subject of transudation, however, is so 

 closely connected with secretion, that its full consideration 

 will come properly under that head, in another volume. At 

 present it will be sufficient to notice only a few facts which 

 relate to this subject. There is probably a feeble exosmot- 

 ic current in the alimentary canal ; which will account for 

 the presence of certain salts, etc., in the faeces, which are 

 evidently derived from the blood. In the subcutaneous 

 areolar tissue and in the substance of and between the mus- 

 cles, there is a certain quantity of a liquid containing a very 

 minute proportion of albumen and some inorganic salts. 

 This is transuded from the blood-vessels by so simple a pro- 

 cess that it hardly merits the name of secretion. 



Transudation of these liquids is influenced chiefly by the 

 pressure of blood in the vessels and by the constitution of 



