564 SPECIAL PHYSIOLOGY. 



tures at two different points, the gases of the blood, oxygen, carbonic 

 acid, and nitrogen, have been found to pass into the interior of the 

 intestine ; so that these gases may be both absorbed from, and ex- 

 creted into, the intestinal canal. In the large intestine, besides car- 

 bonic acid, as the principal gas, carburetted hydrogen may appear, 

 owing to the slow decomposition of its contents ; nitrogen abounds 

 after a flesh diet, and hydrogen after a milk diet ; lastly, though, it 

 would seem, but seldom, as a consequence of the decomposition of the 

 albuminoid substances containing sulphur, or of the taurin of the bile 

 so rich in that element, or possibly from the deoxidation of sulphates, 

 small quantities of sulphuretted hydrogen gas are evolved. These two 

 last-mentioned gases may also be absorbed into the blood ; indeed, it 

 has been shown experimentally, that animals may be quickly poisoned 

 by injecting sulphuretted hydrogen into the large intestine. 



The time taken by different articles of diet to descend through the 

 alimentary canal, varies. Laxative medicines may pass in four hours, 

 carbonate of iron in twelve hours, the coloring matter of spinach and 

 other vegetables in eighteen hours, and grape-pips and cherry-stones 

 in from three to four days. It has been shown that it is useless, and 

 perhaps imprudent, to administer purgatives immediately after the 

 accidental swallowing of buttons, coins, or stones ; it is better to ad- 

 minister thick tenacious food for a day or two, and then give a dose of 

 castor oil. 



Summary of the Chemistry of Digestion. 



We have seen that the digestible and absorbable parts of food con- 

 sist chiefly of the carbhydrates, or the amylaceous, gummy, and sac- 

 charine substances ; of hydrocarbons, or fats and oils ; of nitrogenous 

 gelatinoid and albuminoid substances, and extractive matters; of hy- 

 drocarbonaceous alcohol and organic acids ; of saline substances, and 

 of water. 



Part of the starch is converted, by the saliva in the mouth, into 

 glucose or grape sugar ; this change still goes on in the stomach, even 

 in the presence of the gastric juice ; it is completed, or, according to 

 some, chiefly accomplished, in the interior of the small intestine, by 

 the continued action of the saliva, and by the superadded agency of 

 the pancreatic and intestinal juices. Cooked starch is changed more 

 .rapidly than raw starch, the cells of which sometimes escape digestion; 

 the emptied envelopes of the starch grains commonly remain undi- 

 gested. Cane sugar (Bouchardat), and milk sugar (Lehmann), are 

 for the most part converted into grape sugar in the stomach, more 

 particularly in the intestine ; small quantities of cane sugar are said 

 to be absorbed without change (Bernard). The grape sugar, thus 

 formed from starch and other sugars, or that which may be contained 

 in the food, is principally absorbed as such by the bloodvessels; but 

 it appears partially to be changed, especially when abundantly taken, 

 within the alimentary canal, into lactic acid, and this again into buty- 



