CHAP. L] TISSUES AND MECHANISMS OF DIGESTION. 453 



material occurs to a much less extent or not at all ; neither leucin 

 nor tyrosin can be considered as natural products of the action of 

 pepsin. We may here call attention to an interesting point. We 

 said ( 207) that probably more kinds of peptone than one exist. 

 We have indeed reasons for thinking that in gastric digestion 

 (and indeed in other decompositions, as by acids and high tempera- 

 tures) the proteid molecule is split into two peptone molecules, 

 which differ from each other in this important respect, that while 

 the one peptone molecule is readily converted by the further action 

 of trypsin into leucin and tyrosin, the other resists this action and 

 remains a peptone. The former has been called hemipeptone, the 

 latter antipeptone. This result indicates an action on the part of 

 pepsin preparatory to that of trypsin ; but since when a proteid is 

 acted upon by trypsin alone, only a part, even after the most 

 prolonged action, is converted into leucin and tyrosin, some peptone 

 always remaining as an end product, it would seem that tryptic 

 digestion also gives rise to antipeptone. And we may perhaps 

 infer that the proteid molecule, by its very nature, consists of what 

 we may call a ' hemi '-moiety and an ' anti '-moiety. 



On the gelatiniferous elements of the tissues as they actually 

 exist in the tissue previous to any treatment pancreatic juice 

 appears to have no solvent action. The fibrillse and bundles of 

 fibrillse of ordinary untouched connective tissue are not digested 

 by pancreatic juice, which in this respect affords a striking 

 contrast to gastric juice. But when they have been previously 

 treated with acid or boiled so as to become converted into actual 

 gelatin, trypsin is able to dissolve them, apparently changing 

 them much in the same way as does pepsin. Trypsin will also 

 dissolve mucin. Like pepsin, it is inert towards nuclein, horny 

 tissues, and the so-called amyloid matter. 



On fats pancreatic juice has a twofold action. In the first 

 place it emulsifies fats. If hog's lard be gently heated until it 

 melts and be then mixed with pancreatic juice before it solidifies 

 on cooling, a creamy emulsion, lasting for almost an indefinite time, 

 is formed. So also when olive oil is shaken up with pancreatic 

 juice, the separation of the two fluids takes place very slowly, 

 and a drop of the mixture under the microscope shews that the 

 division of the fat is very minute. An alkaline aqueous infusion of 

 the gland has similar emulsifying powers. In the second place pan- 

 creatic juice splits up neutral fats into their respective acids and 

 glycerin. Thus palmitin (or tripalmitin) (C 16 H 81 . CO . O) 3 . C 3 H 5 

 is with the assumption of 3H 2 O split up into three molecules of 

 palmitic acid 3(C J8 H 81 .CO . OH) and one of glycerin C 3 H 5 (OH) 3 ; 

 and so with the other neutral fats. If perfectly neutral fat be 

 treated with pancreatic juice, especially at the body-temperature, 

 the emulsion which is formed speedily takes on an acid reaction, 

 and by appropriate means not only the corresponding fatty acids 

 but glycerin may be obtained from the mixture. When alkali 



