44 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



absorbed. Though it will replace in diet a certain quantity of such 

 proteins and thus acts as a ' protein-sparing ' food, it cannot 

 altogether take their place as a food. Animals whose sole nitro- 

 genous food is gelatin waste rapidly. The reason for this is that 

 gelatin contains neither the tyrosine nor the tryptophane radicals, and 

 so it gives neither Millon's nor the Adamkiewicz reaction. Animals 

 which receive in their food gelatin to which tyrosine and tryptophane 

 are added thrive well.; 



4. Chondrin. This is the name given to the mixture of gelatin 

 and mucoid which is obtained by boiling cartilage. 



5. Elastin. This is the substance of which the yellow or elastic 

 fibres of connective tissue are composed. It is a very insoluble 

 material. The sarcolemma of muscular fibres and certain basement 

 membranes are very similar. 



6. Keratin, or horny material, is the substance found in the 

 surface layers of the epidermis, in hairs, nails, hoofs, and horns. It 

 is very insoluble, and chiefly differs from most proteins in its high 

 percentage of sulphur. A similar substance, called neurokeratin, 

 is found in neuroglia and nerve fibres. In this connection it is 

 interesting to note that the epidermis and the nervous system 

 are both formed from the same layer of the embryo the epiblast. 



6, The Phospho-proteins 



Vitellin (from egg-yolk), caseinogen, the principal protein of milk 

 and casein, the result of the action of the rennet-ferment on 

 caseinogen (see Milk), are the principal members of this group. Among 

 their decomposition products is a considerable quantity of phosphoric 

 acid. They have been frequently confused with the nucleo-proteins 

 we shall be studying immediately, and the prefix nucleo- so often 

 applied to them is entirely misleading, since they do not yield the 

 products (purine bases) which are characteristic of nucleo-compounds. 



7. The Conjugated Proteins 



These very complex substances are compounds in which the 

 protein molecule is united to other organic materials, which are, as 

 a rule, also of complex nature. This second constituent of the 

 compound is usually termed a prosthetic group. They may be 

 divided into the following sub-classes. 



i. Chromo-proteins. These are compounds of proteins with a 

 pigment, which usually contains iron. They are typified by 

 haemoglobin and its allies, which will be fully considered under Blood 



