FILTEATION AND CELL ACTION. 191 



nothing extraordinary that, under the pressure of circumstances, one of 

 the special structures should, in an imperfect way, resume the action 

 which it once enjoyed, while it was yet a part of the common structure ; 

 but, however this may be, the cases of vicarious action are too numer- 

 ous and too well authenticated to admit of any doubt. 



Though these vicarious actions may be in a certain degree imperfect, 

 they are of the highest importance physiologically, since they indicate the 

 true nature of the function, and place the influence of structure in its 

 proper attitude. 



The separation of material from the blood may, however, for the pres- 

 ent, be considered as conducted in two different ways ; 1st, by filtration ; 

 2d, by cell action. 



Secretion by filtration is, of course, a purely physical act. The trans- 

 udation of water charged with saline substances, or with more Separation of 

 or less of albumen, seems to imply nothing but the escape of ^f^oo^by 

 pre-existing bodies through pervious or porous membranes, filtration. 

 Such a result is presented in the case of the lachrymal gland, the duty 

 of which is to accomplish a definite mechanical operation for the eye in 

 keeping the cornea clear and transparent. This mechanical function is 

 again observed in the case of the serous membranes, and particularly the 

 synovial ones, in which the relief of friction of movable parts seems to be 

 the object aimed at. 



As long as the material secreted clearly pre-exists in the blood, it is 

 needless to refer secretion to any other principle than the simple one of 

 transudation or filtration. It would be unphilosophical to suppose that 

 the lachrymal gland exercises any property for the formation or produc- 

 tion of water when by mere transudation copious supplies of that sub- 

 stance can be obtained from the blood. 



But secretion is, moreover, perhaps connected with cell life. On the 

 upper part of the intestine of the young chick, a few cells secretion by 

 make their appearance about the fourth day of incubation, cell action. 

 They are eventually recognized as bile-containing cells from the color of 

 their contents. As the process goes on, the spot they occupy buds off, 

 as it were, so as to produce a blind pouch. This offshoot, with its ex- 

 terior cells, is eventually, when perfect development is reached, the liver. 

 Secreting organs of this glandular class, and also membranes, possess a 

 general analogy: they consist of a structureless basement membrane, with 

 cells upon its surface, and a supply of blood-vessels. The cells are not 

 persistent, but lead a very transitory life, apparently elaborating the ma- 

 terial with which they are charged, and then undergoing rupture or deli- 

 quescence. 



Our conclusion respecting the mode of action of secreting cells turns 

 altogether upon the evidence of the power they possess of preparing ma- 



