362 HANDBOOK OF PHYSIOLOGY. 



be changed into peptone by prolonged digestion ; it does not occur in 

 physiological gastric digestion. The commonest proteose is the one 

 formed from albumin and is known as albumose : the class name, how- 

 ever, is proteose, and this name is used in the subsequent descriptions of 

 the digestive processes. 



Characteristics of Peptones. Peptones have a certain characteristic 

 which distinguishes them from other proteids. They are diffusible, i.e., 

 they possess the property of passing through animal membranes. 



In their diffusibility peptones differ remarkably from egg-albumin, and 

 on this diffusibility depends one of their chief uses. Egg-albumin as 

 such, even in a state of solution, would be of little service as food, inas- 

 much as its in diffusibility would effectually prevent its passing by absorp- 

 tion into the blood-vessels of the stomach and intestinal canal. When 

 completely changed by the action of the gastric juice into peptones, albu- 

 minous matters diffuse readily, and are thus quickly absorbed. 



After entering the blood the peptones are very soon again modified, 

 so as to reassume the chemical characters- of albumin, a change as neces- 

 sary for preventing their diffusing out of the blood-vessels, as the pre- 

 vious change was for enabling 'them to pass in. This is effected, prob- 

 ably, in great part by their passage through the vascular walls. 



Products of Gastric Digestion. The proteid is first changed into syn- 

 tonin, or acid proteid, by the combined action of the pepsin and acid. 

 Though the acid alone is capable of accomplishing this, the fact that it 

 does not do so physiologically is proven by the great length of time re- 

 quired, in laboratory experiments, for the change. The next change is 

 the conversion of the syntoniii into proteoses which, according to Neu- 

 meister, occurs in two successive stages. The first of these stages is the 

 conversion of syntonin into the primary proteoses, i.e., proto-proteose and 

 hetero-proteose ; the second is the conversion of both proto-proteose and 

 hetero-proteose into the secondary proteoses, i.e., deutero-proteose. 

 The last change is the conversion of the deutero-proteose into peptone ; 

 this change does not occur to any great extent physiologically and the 

 proteoses always predominate. Schematically the changes in the proteids 

 may be represented as follows : 



Proteid. 

 Syntonin (acid proteid). 



Proto-proteose. Hetero-proteose. 



Deutero-proteose. Deutero-proteose. 



Peptone. Peptone. 



