PEPTIC DIGESTION OF PR O TEWS. 40 1 
Ultimate chemical analysis shows that the composition of the peptones 
and albumoses is practically the same as thai of the proteids from which they 
are formed ; in some cases the proteids show a somewhat bigher 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 peptonization 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 sel 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 
di Hi 'rent 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 cpiestionahle 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 Proteins. 
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 l 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 :t Hist. Acad. roy. d. sc. de Paris," 1752, pp. 266, 461. 
VOL. I. — 26 
