ABSORPTION. 



313 



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

 like character, colloids. 



This distinction, however, between colloids and crystalloids which is 

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

 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 re- 

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

 and ordinary saline substances are examples of crystalloids. 



(b.) Filtration, or transudation. A distinction must be drawn be- 

 tween osmosis and filtration. The latter 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. 

 Collojds 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 in- 

 filtrated with serous fluid. The fluid passes out of the vein when the 

 intra-venous pressure passes a certain point, the fluid is, 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 

 an 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, moreover, 

 for six7 seven, or eight days. 



Some experiments on the absorption of various mineral and vegetable 

 poisons have brought to light the singular fact that, in some cases, ab- 

 sorption takes place more rapidly from the rectum than from the 

 stomach. Strychnia, for example/ when in solution, produces its poi- 

 sonous effects much more speedily when introduced into the rectum than 

 into the stomach. When introduced in the solid form, however, it is. 

 absorbed more rapidly from the stomach than from the rectum, doubt- 



