ABSORPTION OF SOLUBLE MATTERS INTO THE VEINS. 279 



filled with water, and have a stopcock attached at the other extre- 

 mity, and be then immersed in water acidulated with sulphuric or 

 hydrochloric acid, it will be some time before the acid will penetrate to 

 the interior of the tube, which is distended with water; but if the 

 stopcock be opened, and the water be allowed to discharge itself, the 

 presence of the acid will be immediately discovered (by tincture of 

 litmus) in the liquid which flows out. Thus th<e continuance of the Cir- 

 culation is obviously one of the most important of the conditions of 

 Absorption. It is whilst passing through the system of capillaries, 

 which forms a minute plexus immediately beneath the free surface of 

 the mucous membrane, that the blood thus receives an admixture of the 

 soluble matters contained within the digestive cavity ; and hence it is 

 that these substances are detectible in the tblood of the gastric and 

 mesenteric veins, sooner than in any part of tne arterial system. They 

 are very rapidly diffused, however, through tHe general circulation ; and 

 may even show themselves in the excretions within so short a period, 

 that it is obvious that they must have been absorbed immediately on 

 their introduction into the stomach. Thus Mr. Erichsen found that he 

 was able to detect the presence of ferrocyanide of potassium in the 

 urine, within one minute after it had been swallowed in solution. This, 

 however, was only when it was taken after a Ibng fast ; more commonly 

 the absorption is less rapid ; and if the substance' be introduced within an 

 hour or two after a full meal, it may be as much as half an hour before 

 its presence in the urine gives evidence of its having been received into 

 the circulating current. Although it is difficult to speak with certainty 

 on the point, yet there appears a strong probability, thafr, both in the 

 stomach and intestinal tube, the absorption of nutritive matters in a 

 state of perfect solution (such as gum, sugjar, pectine, gelatine, and 

 soluble albumen) is thus accomplished through the medium of the blood- 

 vessels ; which also take up the chief supply of 'water that is required by 

 the system. It is difficult else to see the purpose of the extraordinary 

 vascularity of the mucous membrane, and in particular of those filaments 



Fig. 82. ' 



Distribution of Capillaries in the Villi of the Intestine. 



or narrow folds, termed villi, which so thickly cover its surface. Each 

 of these villi is furnished with a plexus of minute blood-vessels, of 

 which the larger branches may even be seen with the naked eye, when 

 they are distended with blood or with a coloured injection. By these 

 villi, the vascular surface of the mucous membrane is enormously 

 extended. In Man, they are commonly cylindrical or nearly so, and 

 are from about a quarter of a line, to a line and a half in length ; but 



