326 THE TISSUES AND MECHANISMS OF DIGESTION. 



the chyle during and after the meal, so long as it remains milky, showing 

 that fat is being absorbed, we can ascertain the quantity of absorbed fat, 

 which would, but for the operation, have passed into the venous system. 

 When this has been done, a very remarkable deficit, amounting it may be to 

 40 or 50 per cent., has been observed ; that is to say, of every 100 parts of 

 fat which disappear from the alimentary canal only about 60 'parts find their 

 way through the thoracic duct into the venous system. 



Are we then to conclude that the missing quantity finds its way into the 

 portal system? Now the portal blood does, during digestion, contain a cer- 

 tain quantity of fat; indeed, the serum is said at times to appear milky from 

 the presence of fat. But the whole circulating blood during the digestion 

 of a fatty meal contains, for a while, the fat poured into it by the thoracic 

 duct ; and it has been ascertained in the dog that the blood of the portal 

 vein during digestion contains not more but less fat than the blood of the 

 carotid artery, so that the fat which appears in the portal blood during 

 digestion is, for the most part at least, not fat absorbed by the capillaries of 

 the alimentary canal, but fat absorbed by the lacteals. Moreover, when the 

 chyle of the thoracic duct is diverted through a canula, and not allowed to 

 flow into the blood, the quantity of fat in the portal blood, as in the blood 

 at large, is very small indeed. Lastly, when a villus of an intestine in full 

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

 microscope within the capillaries or other bloodvessels, 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 information. 



260. Water and salts. If, in an animal, the rate of flow of lymph or 

 chyle through a canula 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 canula 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 under some cir- 

 cumstances pass into the lacteals. 



With regard to the course taken by ordinary saline matters we possess 

 no detailed information. When special salts, such as potassium iodide and 

 others, easily recognized by appropriate tests, are introduced into the intes- 

 tine, 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. 



261. Sugar. Both blood and chyle contain, normally, a certain small 

 amount of sugar ; and careful inquiries show 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 con- 



