410 HANDBOOK OP PHYSIOLOGY. 



chloric acid in the contents of the stomach appears to determine the 

 direction of the osmosis, or at any rate to diminish or prevent exosmosis. 



The conditions for osmosis exist not only in the alimentary mucous 

 membrane, but also in the serous cavities and the tissues elsewhere. 



Various substances have been classified according to the degree in 

 which they possess the property of passing, when in a state of solution 

 in water, through membrane; those which pass freely, inasmuch as they 

 are usually capable of crystallization, being termed crystalloids, and those 

 which pass with difficulty, on account of their physically glue-like char- 

 acter, colloids. 



This distinction, however, between colloids and crystalloids which 

 is made the basis of their classification, is by no means the only differ- 

 ence between them. The colloids, besides the absence of power to assume 

 a crystalline form, are characterized by their inertness as acids or bases, 

 and feebleness in all ordinary chemical relations. Examples of them 

 are found in albumin, gelatin, starch, hydrated alumina, hydrated silicic 

 acid, etc. ; while the crystalloids are characterized by qualities the reverse 

 of those just mentioned as belonging to colloids. Alcohol, sugar, and 

 ordinary saline substances are examples of crystalloids. 



Filtration, or transudation, means the passage of fluids through the 

 pores of a membrane under, pressure. The greater the pressure the 

 greater the amount which passes through the membrane. Colloids will 

 filter, although less easily than crystalloids. The nature of the substance 

 to be filtered and the nature of the membrane which acts as the filter 

 materially affect the activity of the process. No doubt both osmosis 

 and filtration go on together in the process of absorption. An excellent 

 example of filtration or transudation occurs in the pathological condition 

 known as dropsy, in which the connective tissues become infiltrated 

 with serous fluid. The fluid passes out of the vein when the intra-ven- 

 ous pressure passes a certain point, the fluid being, as it were, squeezed 

 through the walls of the vessels by this excess of pressure. 



Rapidity of Absorption. The rapidity with which matters may be 

 absorbed from the stomach, probably by the blood-vessels chiefly, and 

 diffused through the textures of the body, has been found by experiment. 

 It appears that lithium chloride may be diffused into all the vascular 

 textures of the body, and into some of the non-vascular, as the cartilage 

 of the hip- joint, as well as into the aqueous humor of the eye, in a quar- 

 ter of an hour after being given on an empty stomach. Into the outer 

 part of the crystalline lens it may pass after a time, varying from half 

 un hour to an hour and a half. Lithium carbonate, when taken in five 

 or ten-grain doses on an empty stomach, may be detected in the urine 

 in 5 or 10 minutes; or, if the stomach be full at the time of taking the 

 dose, in 20 minutes. It may sometimes be detected in the urine, more- 

 over, for six, seven, or eight days. 



