CHAP. I.] THE PROTEIDS. 5 



animal body, though the latter possesses the power of converting any 

 vegetable or animal proteid into the various proteids which are 

 characteristic of its solids and liquids. By the action of certain fer- 

 ments present in the alimentary juices, all proteids are capable of 

 being converted into closely allied bodies called peptones, which 

 after absorption are capable of reconversion into proteids. In the 

 organism the proteids thus introduced, after forming part of the 

 circulating blood, are partly employed in the reconstruction of slowly 

 wasting proteid tissues and organs ; for the most part, however, they 

 are subjected to a rapid series of decompositions, of which presumably 

 the most important take place in the liver, and which finally result in 

 the formation of carbonic acid, water and various imperfectly oxidized 

 organic bodies which contain all the nitrogen originally present in 

 the proteid; of those bodies the most abundant by far is carbamide 



or urea, C^ J NH * 



To the assemblage of chemical processes, or rather to the assemblage 

 of transformations, which a constituent of the organism, such as a proteid, 

 undergoes in its passage through the body, the term metabolism has been 

 applied, and we shall frequently employ it in this sense, the processes 

 themselves being designated when convenient metabolic processes. 



In the processes of metabolism to which the proteids are subjected 

 and which result in the formation of C0 2 , H 2 and urea, there are 

 formed intermediate bodies, such as glycogen and fats, which play an 

 important part in the economy of the body. 



It is further unquestionable that within the animal body certain 

 remarkable synthetic processes occur, by which proteids are built up 

 into bodies of a yet more complex structure, such for instance as the 

 blood colouring matter, Haemoglobin. 



Percentage The various Proteids differ somewhat in elemen- 



S^?wSi 0f tary com P sition > within the limits of the following 

 numbers 1 : 



C H N S O 



From 51-5 6'9 152 (V3 209 



to 54-5 to 7'3 to 17'0 to 2'0 to 23'5. 

 In addition to these essential constituents, the proteids, however 

 carefully they may have been purified, usually leave when ignited a 

 small quantity of ash, the composition of which varies in different 

 cases, chlorides and phosphates of the alkaline metals being the pre- 

 dominant constituents. 



Proteidsfor Certain of the proteids exist in a state of solution 



the most part in the liquids of the organism; others are present in 

 soluble. the same state in the tissues; all may be dissolved by 



certain reagents, though in some cases riot without suffering radical 

 changes. 



1 Hoppe-Seyler, Handbuck d. pkys.- und path.-chem. Analyse, 4te Aufl. p. 223. 



