112 HANDBOOK OF PHYSIOLOGY. 



proteids, but it is not certain whether or not it is an integral part of the 

 molecule. Chittenden defines a proteid as a substance which contains 

 carbon, hydrogen, oxygen, nitrogen, and sulphur, the nitrogen being in a 

 form which serves the physiological needs of the body ; and yield?, on 

 decomposition, a row of crystalline amido-acids and crystalline nitrogen- 

 ous bases; nearly all contain 52 per cent of carbon and 16 per cent of ni- 

 trogen. 



Properties of Proteids. Proteids are for the most part amorphous and 

 non-cry stallizable. Certain of the vegetable proteids have, it is said, 

 been crystallized, and according to Hofmeister, egg albumin is also capa- 

 ble of crystallization. They possess as a rulejio power (or scarcely any) 

 of passing through animal membranes. They are soluble, but undergo 

 alteration in composition in strong acids and alkalies ; some are soluble 

 in water, others in neutral saline solutions, some in dilute acids and al- 

 kalies, none in alcohol or ether. Their solutions exercise a left-handed 

 action on polarized light. 



The hope that it may be possible in the immediate future to synthe- 

 size proteids is rendered all the weaker because of the extraordinary va- 

 riety of compounds obtained by the decomposition of proteids by various 

 chemical methods, the compounds differing according to the method em- 

 ployed. In the body it seems clear that living proteid is built up by the 

 food supplied to it, which necessarily contains proteid derived either from 

 a vegetable or an animal source ; how this process takes place we are yet 

 unable to say. In the course of later chapters in this book we shall en- 

 deavor to trace the steps of the breaking up of proteid in the body, but 

 we may anticipate by mentioning that it is now generally believed that 

 the ultimate products of this decomposition are urea, a body the formula 

 of which is CO(NH 2 ) 2 , carbon dioxide and water, while the intermediate 

 substances or by products are probably ammonia compounds (ammonium 

 carbonate). When proteid material is decomposed by putrefaction, by 

 the action of chemical reagents, e.g., acids, alkalies, or by heat, various 

 bodies are produced, of which amido-acids (acids in which one or more of 

 the hydrogen atoms of the radical of the acid are replaced by amidogen, 

 NHJ and bodies belonging to the aromatic or benzene series predominate. 

 Hence it comes that various theories of the way in which proteids are 

 built up have arisen. The one which has appeared to have received the 

 greatest support is that of Latham. This observer has suggested that 

 proteid may be considered as made up of a series of cyan- alcohols (bodies 

 obtained by the union of any aldehyde with hydrocyanic acid) with a 

 benzene nucleus. Taking ordinary ethyl alcohol, CH s CH 2 OH, as the 

 type, the aldehyde of which is CH 3 CHO, the corresponding cyan-alcohol 

 would be CH 3 CHCNOH. 



Proteids give certain general chemical reactions. They are a little 



