NATURE OF CHYMIFICATION. 343 



) 



dissolve in a dilute acid containing pepsin. M. Wasmann has remarked that 

 the pepsin of the stomach of the pig is entirely destitute of the power to coagu- 

 late milk, although the pepsin of the stomach of the calf possesses it in a 

 very high degree ; from which he is led to suppose, that the power of the 

 latter depends upon a particular modification of pepsin, or perhaps upon 

 another substance accompanying it, which ceases to be formed when the 

 young animal is no longer nourished by the milk of its mother."* It appears 

 from the recent inquiries of Liebig and others, that the solvent principle is a 

 protein-compound in a state of change; and that, like ferments in general, it 

 possesses the property of exciting change in other substances, with which it is 

 brought in contact. Hence we should regard the Digestive process, as result- 

 ing from the combined actions of Fermentation and Chemical solution; the 

 alimentary substances being first made to undergo an incipient fermentation, 

 by the agency of the pepsin, which so alters their condition, as to dispose them 

 to solution in hydrochloric and acetic acids, with which they form definite 

 chemical compounds. This view harmonizes completely with the fact just 

 stated, that a small quantity of pepsin will perform its part in the digestion 

 of an unlimited quantity of aliment, which is analogous to what we know of 

 the action of true ferments ; whilst only a definite quantity can be dissolved 

 in a limited amount of acid, which is the case with all acts of proper chemical 

 solution. 



457. Our knowledge of the nature of the Digestive process has lately- 

 received another important addition from the discovery, that the chief proxi- 

 mate principles of the animal tissues, and those which have been regarded as 

 most nutritious among vegetables, have almost identically the same chemical 

 composition. This conformity will appear from the following comparative 

 analysis, lately executed in the laboratory of Liebig. 



ALBUMEN. 



FIBRIN. { A ^ CASEIN. 



Of E<:s. Of Serum. 



Carbon 54-56 54-48 54-84 54-96 



Nitrogen 15-72 15-70 15-83 15-80 



Hydrogen 6-90 7-01 7-09 7-15 



Oxygen ""] 



Phosphorus j> 22-82 22-81 22-24 22-09 



Sulphur J 



100-00 100-00 100-00 100-00 



The proportion of carbon to nitrogen in all these substances, is that of 8 equi- 

 valents of the latter to one of the former. They differ slightly in the quantity 

 of phosphorus and sulphur which they contain ; but agree in many other im- 

 portant chemical properties. Thus, they all dissolve, with the aid of heat, in 

 concentrated muriatic acid ; and the solutions, kept for a time at a pretty high 

 temperature, first assume a beautiful lilac, and then a rich violet-blue colour. 

 At this stage of the decomposition, each of the three substances reacts in the 

 same way with carbonate of ammonia and other reagents. The parallel vege- 

 table principles are vegetable fibrin (a constituent of gluten first properly dis- 

 tinguished by Liebig), gluten itself, vegetable albumen, and legumin ; this last 

 is termed by Liebig vegetable casein, from its holding the same relation to 

 vegetable albumen that animal casein does to animal albumen. The following 

 is the elementary composition of these substances : 



* Graham, op. cit. [Am. Ed. p. 696 ] 



