

COLLOIDS. 297 



molecules, do not pass, or pass only with difficulty, through the pores of a membrane impreg- 

 nated with gelatinous bodies, which diffuse slowly. These substances are not actually in a true 

 state of solution, but exist in a very dilute condition of imbibition. Such substances are the 

 fluid proteids, starches, dextrin, gum, and gelatin. These diffuse when no septum is present, 

 but diffuse with difficulty or not at all through a porous septum. Graham called these sub- 

 stances colloids, because, when concentrated, they present a glue-like or gelatinous appearance ; 

 further, they do not crystallise, while those substances which diffuse readily are crystalline, and 

 are called crystalloids. Crystallisable substances may be separated from non-crystallisable by 

 this process, which Graham called dialysis. Mineral salts favour the passage of colloids through 

 membranes. 



That endosmosis takes place in the intestinal tract, through the mucous 

 membrane and the delicate membranes of the blood and lymph-capillaries, cannot 

 be denied. On the one side of the membrane, within the intestine, are relatively 

 concentrated solutions of highly diffusible salts, peptones, sugar, and soaps, and 

 within the blood-vessels are the colloids which are scarcely diffusible, e.g., the pro- 

 teids of blood and lymph. 



II. Filtration is the passage of fluids through the coarse intermolecular pores of a membrane 

 owing to pressure. The greater the pressure, and the larger and more numerous the pores, the 

 more rapidly does the fluid pass through the membrane ; increase of temperature also accelerates 

 it. Those substances which are imbibed by the membrane filter most rapidly, so that the same 

 substance filters through different membranes with varying rapidity. The filtration is usually 

 slower, the greater the concentration of the fluid. The filter has the property of retaining some 

 of the substances from the solution passing through it, e.g., colloid substances or water (in 

 dilute solutions of nitre). In the former case, the filtrate is more dilute, in the latter more 

 concentrated, than before filtration. Other substances filter without undergoing any change of 

 concentration. Many membranes behave differently, according to which surface is placed next 

 the fluid ; thus the shell-membrane of an egg permits filtration only from without inwards ; 

 [and the same is true to a much less extent with filter-paper ; the smooth side of the filter- 

 paper ought always to be placed next the fluid to be filtered. The intact skin of the grape pre- 

 vents the entrance of fungi into the fruit]. There is a similar difference with the gastric and 

 intestinal mucous membrane. 



[By using numerous layers of filter-paper, many colloids and crystalloids are retained in the 

 filter, e.g., haemoglobin, albumin, and many colouring matters, especially aniline colours, the last 

 being arrested by glass-wool (Krysinski). ] 



[Filtration of Albumin. Runeberg finds that the amount of albumin in pathological transu- 

 dations varies with (1) the capillary area,, being least in oedema of the subcutaneous tissue. 

 (2) The presence or absence of inflammatory processes in the vascular wall, non-inflammatory 

 pleuritic effusion containing 2 per cent., and inflammatory 6 per cent, of albumin. (3) The con- 

 dition and amount of albumin in the blood. The amount of albumin in the transudate never 

 reaches, although it sometimes approaches, that in blood. In ascites in general dropsy the 

 amount is '03 to "04 per cent. (4) The duration of the transudation. (5) Perhaps the blood- 

 pressure and the condition of the circulation.] 



Filtration of the soluble substance may take place from the canal of the 

 digestive tract when : (1) The intestine contracts and thus exerts pressure upon its 

 contents. This is possible when the tube is narrowed at two points, and the mus- 

 culature between these two points contracts upon the fluid-contents. (2) Filtration, 

 under negative pressure, may be caused by the villi (Briicke). When the villi con- 

 tract energetically, they empty their contents towards the blood- and lymph-vessels. 

 The lacteal remains empty, as the chyle is prevented from passing backwards into the 

 origin of the lacteal within the villi, owing to the presence of numerous valves in 

 the lymphatics. When the villi relax, they are refilled with fluids from the intes- 

 tine. 



192. ABSORPTION BY THE INTESTINAL WALL. I. True solutions 



undoubtedly pass by endosmosis into the blood-vessels and lymphatics of the 

 intestinal walls, but numerous facts indicate that the protoplasm of the cells takes 

 an active part in the process of absorption. The forces concerned have not as yet 

 been proved to be purely physical and chemical in their nature. 



(1) Inorganic Substances. Water and the soluble salts necessary for nutri- 

 tion are easily absorbed, the latter especially by the blood- and lymph-vessels. 

 When saline solutions pass by endosmosis into the vessels, water must pass from 



