CHAP, i.] DIGESTION. 307 



or by less diffusible forms of proteids arising as subsidiary products 

 of proteolytic digestion or by carbohydrate products, deserves 

 attention. 



It cannot be a matter of indifference which course is taken by 

 the particular digestive products. For if they pass by the portal 

 vein they fall into the general blood-current after having undergone 

 only such changes as they may experience in the lymphatic system ; 

 while if they pass into the portal vein they are subjected to the 

 powerful influences of the liver before they find their way to the 

 right side of the heart. What those influences are we shall study 

 in a future chapter. 



Fats. As we have seen, a special mechanism is provided for 

 the passage of fats into the lacteals. On the other hand, it is 

 difficult to suppose that solid particles of fat can pass into the 

 interior of the blood capillaries. So that we are led a priori to 

 the view that the whole of the fat takes the course of the lacteals. 

 But we cannot say that this is definitely proved. On the contrary, 

 a large deficit is observed when the quantity of fat disappearing 

 after a meal from the alimentary canal is compared with that 

 flowing out through a cannula placed in the end of the thoracic 

 duct; and if it be true, as is stated, that the blood of the portal 

 vein contains during digestion more fat than the general venous 

 blood, some of this deficit may be explained by the fat passing 

 into the blood capillaries, difficult as that passage may appear. 

 The portal blood, moreover, during digestion contains a small but 

 appreciable quantity of soaps. It may be however that the deficit 

 observed is due to some of the fat disappearing in some way, in 

 the glands for instance, from the interior of the- vessels in its 

 transit. 



The fat thus entering the blood either directly or indirectly is 

 rapidly got rid of in some way or other, for from experiments on 

 dogs it would appear that the percentage of fat in the blood 

 after a meal rich in fat, does not, after the lapse of 20 hours 

 from the swallowing of the food, differ materially whether the 

 fat has been during the whole time shut off from the blood by 

 being allowed to flow out of a cannula placed in the thoracic duct, 

 or has been allowed to pass into the venous system in the usual 

 way. 



Proteids. The question as to the course taken by the digested 

 proteids is complicated by the insufficiency of our knowledge con- 

 cerning the exact stages to which the digestion of proteids is 

 naturally carried in the alimentary canal. If we take it for 

 granted that the proteids taken as food are reduced to the 

 condition of soluble and diffusible peptone, it seems easy to 

 suppose that the proteids of food pass by diffusion as peptone into 

 the blood capillaries which as is well known are placed in the villus 

 between the epithelium and the lacteal chamber; though even 



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