CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 541 



digestion of fat is treated with osmic acid, fat cannot be recognized 

 by the microscope within the capillaries or other blood vessels, 

 though it abounds outside them in the substance of the villus 

 and in the lacteal radicle. 



We may probably therefore infer with safety that all or at 

 least very nearly all the fat absorbed from the intestine takes the 

 path of the lacteals. As to the deficit mentioned above, that is as 

 yet without explanation. It may be that in some way, on its 

 course, in the lymphatic glands, for instance, the fat is taken away 

 from the chyle, hidden so to speak somewhere away from both 

 chyle and blood; but on this point we have no exact infor- 

 mation. 



307. Water and Salts. If, in an animal, the rate of flow of 

 lymph or chyle through a cannula placed in the thoracic duct be 

 watched, and water or, to avoid the injurious effect of simple 

 water on the mucous membrane, normal saline solution be then 

 injected in not too great quantity into the intestine, no marked 

 increase in the flow of chyle through the cannula is observed. 

 From this we may infer that the water of the intestinal contents 

 is absorbed not into the lacteals but into the portal system. If 

 however a very large quantity of the normal saline solution be 

 injected so as to distend the intestine, then the flow of chyle is 

 increased to some extent. It would appear therefore that while 

 under normal conditions the water passes from the intestine mainly 

 into the portal blood, some of it may pass into the lacteals. 



With regard to the course taken by ordinary saline matters we 

 possess no detailed information. When special salts such as potas- 

 sium iodide and others, easily recognized by appropriate tests, are 

 introduced into the intestine, they may be speedily detected 

 both in the blood and in the contents of the thoracic duct ; 

 but whether, in such cases, these salts find their way into the 

 thoracic duct by the lacteal radicle of the villi, or pass into the 

 lymph stream at some later part of its course, we do not know. 

 Nor can we with regard to such a salt as sodium chloride, state 

 absolutely that it passes mainly with the water into the portal 

 blood, though we may fairly suppose this to be the case. 



308. Sugar. Both blood and chyle contain, normally, a 

 certain small amount of sugar ; and careful inquiries shew that 

 the percentage of sugar in chyle and in general blood is fairly 

 constant, neither being to any marked extent increased by even 

 amylaceous meals ; on the other hand, a meal containing sugar or 

 starch does temporarily increase the quantity of sugar in the 

 portal blood. From this we may infer that such portions of the 

 sugar of the intestinal contents as are absorbed as sugar pass 

 exclusively by the portal vein. We may however here call 

 attention to the difficulties attending an argument of this kind. 

 In the first place the quantitative determination of a small amount 

 of sugar in so complex a fluid as blood is attended with great 



