PEPTIC DIGESTION OF PROTEIDS. 401 



Ultimate chemical analysis shows that the composition of the peptones 

 and albumoses is practically the same as that of the proteids from which they 

 are formed ; in some cases the proteids show a somewhat higher percentage of 

 carbon and lower of hydrogen and oxygen than the proteids of their digestion, 

 in others the reverse, but in no cases any very considerable variation. So 

 that, if the process of peptonisation is one of hydrolysis, the peptone molecule 

 must be out of all proportion greater than that of the molecule of water. 

 Nevertheless the hydrolytic theory is the one most generally received, and 

 against this unfavourable argument from analytical results must be set the 

 other experiments already quoted. 



The main differences between the proteids and peptones of physio- 

 logical importance are the physical ones, that the latter are much more 

 soluble, and are diffusible, though with difficulty, through membranes. 

 It is indeed purely by physical means that we at present differentiate 

 proteids and peptones and the intermediate products between them, 

 and not by any well-marked chemical differences shown by them. The 

 different proteids, albumoses, and peptones are classified and marked 

 off from one another almost entirely by the behaviour of their solutions 

 towards solutions of neutral salts of different strength, according to 

 whether these dissolve or precipitate them. It is questionable whether 

 it is justifiable on such a slender basis to assume, as is commonly done 

 that these precipitates correspond to pure compounds. It ought to be 

 remembered that these names at present only apply to certain precipi- 

 tates, and that it is not at all known whether these represent distinct 

 chemical substances, nor indeed what they do represent. Still less right 

 have we to assume from mere proteid .analyses that the products of 

 digestion of different proteids yield substances distinctive of them and 

 worthy of distinctive names. 



The two proteolytic enzymes, pepsin and trypsin, closely resemble 

 each other in their action on proteids, a series of very similar products 

 being in each case evolved, which, generally speaking, become more 

 soluble and probably simpler in constitution as the end of the process is 

 approached. Still there is sufficient difference to warrant a separate 

 consideration of the two processes. 



PEPTIC DIGESTION OF PROTEIDS. 



The stomach was recognised even by the ancients as a digestive 

 organ, and its action attributed in many cases to the " animal heat " 

 assisted by mechanical force. Digestion seems to have been first con- 

 sidered as a similar process to fermentation by van Helmont, and this 

 view was also maintained by Sylvius. 



Reaumur 1 seems to have been the first to experiment on gastric 

 digestion. He carried out his successful experiments on a tame buzzard, 

 which, like some other birds of prey, regurgitates after a time the more 

 indigestible portions of its food. He administered various kinds of 

 food, enclosed in small metallic tubes closed at one end and covered by 

 muslin at the other, so as to prevent any mechanical action of the 

 gizzard and yet allow the gastric juice to act ; he found that meat was 

 digested in the course of some hours, and in a shorter period was digested 

 partially on the outside while the interior still remained untouched. 

 Reaumur also obtained gastric juice by enclosing pieces of sponge in 



1 "Hist. Acad. roy. d. sc. de Paris," 1752, pp. 266, 461. 

 VOL. I. 26 



