436 CHEMISTRY OF THE DIGESTIVE PROCESSES. 
Ingestion of large quantities of solution of sugar leads to the appear- 
ance of sugar in the urine (alimentary glycosuria), due to the assimilation 
of the sugar not keeping pace with its absorption. 1 Absorption itself 
is also disturbed by the appearance of diarrhoea. But when carbohydrate 
is introduced into the alimentary canal, in the form of starch, immense 
quantities can be rapidly and completely absorbed without any glycosuria 
or other disturbance ensuing. 
Thus Riibner 2 found that a man consuming a daily ration of 508-070 
grins, of carbohydrate contained in wheaten bread, left unabsorbed only 
- 8-2 - 6 per cent. ; of the carbohydrate of peas (357-588 grms.) 3*6-7 "0 per 
cent, was unabsorbed; and of potatoes (718 grms.) 7*6 per cent. This 
complete absorption and utilisation of carbohydrate, when taken in the 
form of starch, is probably due to the rate of assimilation and storage as 
glycogen in the liver, being able to keep pace with that of absorption 
from the intestine. 
Conditions of absorption of proteids.— The power possessed by 
the intestinal cells of absorbing various forms of proteid affords one 
of the best illustrations that this process is not one of mere 
physical diffusion. The products found towards the end of a proteid 
digestion in vitro are distinguished from the proteids from which they 
originate by being slightly diffusible. To this fact great importance 
was at one time attributed, because it was thought that only proteids 
in a diffusible form were capable of absorption, and hence that peptonisa- 
tion was in all cases a necessary preliminary. It is now generally 
admitted that many forms of native proteid are capable of entering the 
epithelial cells without previous change by digestion or otherwise ; and 
in those cases in which a proteid is incapable of direct absorption a much 
less profound change than peptonisation is sufficient to render it so, 
namely, conversion into acid or alkali albumin. Such an absorption of 
soluble proteid, other than albumose or peptone, takes place not only in 
the small intestine, but in the large intestine, and even in the rectum. 
Voit and Bauer 3 cleared a loop of small intestine of its contents as 
completely as possible by stroking, and separated it from the rest of the 
gut by double ligatures at each end. Various forms of proteid, in solu- 
tions of known amount and strength, were then injected into this loop ; 
the intestine was replaced, and its contents examined on killing the 
animal (cat or dog) some hours later. It was found that variable amounts 
of these proteids had disappeared ; thus, in one to four hours, 16-33 per 
cent, of white of egg had disappeared, and of syntonin from ox muscle 
28-95 per cent. It might be supposed that the portion of proteid ab- 
sorbed had been peptonised by traces of proteolytic enzyme which might 
be present in the intestine ; but in the unabsorbed proteid remaining at 
the end of the experiment, no albumose or peptone was found. Voit and 
Bauer also injected solutions of white of egg and sodium chloride into the 
rectum of man and animals in a fasting condition, and found a marked 
increase (6 to 8 grms. in 24 hours) in the amount of nitrogen eliminated 
by the kidneys ; in fact, an equilibrium of nitrogenous metabolism may 
even be maintained in this way. It has been shown by Eichhorst, 4 who 
confirms these results, that no appreciable amount of peptonisation takes 
1 C. Voit, Ztschr.f. JJioL, Miinclien, 1S91, Bd. xxviii. S. 245. 
- Ibid., Bd. xix. S. 45. 
3 Ibid., Bd. v. S. 562. 
4 Arch./, d. yes. Physiol., Bonn, 1871, Bd. iv. S. 570. 
