4 CHEMISTRY OF THE PROTEIDS 



2. As so-called ' protoplasm ' in the cells and tissues of plants and 



animals. This plasm is a mixture of albuminous substances 

 and other organic and inorganic compounds ; it possesses a 

 definite structural arrangement and occupies a middle position 

 between the solid and the fluid states. The animal supporting- 

 structures built up of the so-called albuminoids are in a 

 somewhat firmer state than is the ordinary cell-plasm. It is 

 possible to extract these albuminous substances from the 

 organs in which they occur either by the simple process of 

 dissolving them out or by employing stronger measures. 



3. As reserve material in the form of firm or even crystalline 



structures, which act as storehouses for the developing em- 

 bryos of plants and animals. 



' Proteids ' or albuminous bodies form a well-defined group of 

 organic compounds with definite physical and chemical properties. 

 They are for the greater part built up of a-amino-acids linked to one 

 another as acid-amines. Their general characters agree to such an 

 extent that a doubt hardly ever arises in our minds as to whether a 

 given substance is, or is not, a proteid, and already amino-acids have 

 been combined into aggregates giving the chemical tests of such pro- 

 teids as are found normally in animals and in plants. 



For purposes of classification ' proteids ' may be divided into three 

 groups : 



1. Albumins which occur in nature as 'native albumins.' They 



include the ' albuminoid ' substances which form the support- 

 ing or connective tissues of the animal body. 



2. Proteids proper, which are combinations of the native 



albumins with such other organic compounds as sugars or 

 radicals containing phosphorus or iron. 



3. Derivatives of the natural albumins and proteids, which retain 



in their chemical configuration the characteristics of albumin- 

 ous substances, and are represented by the albumoses, peptones, 

 peptids, and other compounds. These bodies are met with in 

 nature as products of digestion and metabolism, but they may 

 also be obtained artificially by hydrolysis of the more com- 

 plex albuminous substances. 



It is customary in England to use the term ' proteid J for all 

 albuminous substances, while in France the term * substances albumin- 

 oides' is used similarly. Of late there has been a tendency in 

 Germany to use the English phraseology and to speak of ' Protein- 

 substanzen.' The author has thought it best to follow Cohnheim in 

 restricting the terms albumin and proteid. 



