108 ELEMENTARY PHYSIOLOGY less. 



Pix>teins are, in the first place, extremely complex 

 substances, so much so that chemists have not as yet been 

 able to determine their constitution or assign any formula 

 to them. Some are soluble in water, others only soluble 

 in solutions of a neutral salt such as sodium chloride, 

 while others are insoluble in either of the preceding 

 solvents. When heated, those most frequently found in 

 the body are altered or coagulated, as in the well-known 

 change wliicli the white of an egg, itself a typical protein, 

 undergoes when boiled. 



In the next place proteins are composed of the four 

 elements, carbon, hydrogen, oxygen and nitrogen, with 

 frequently a small amount of sulphur ; of these the 

 nitrogen stands out as having a supreme importance. All 

 the tissues of the body contain nitrogen and are continu- 

 ally undergoing a nitrogenous waste, and the body is 

 quite unable to make use of nitrogen for the repair of 

 this waste except it is presented in the form of a protein. 

 The general percentage composition of proteins is, roughly 

 speaking, the same for all of them, and varies but slightly 

 on either side of the following numbers : — carbon 53 

 parts, oxygen 22, hydrogen 7, nitrogen 16, and sulphur 

 1-2. In the absence of any formula this percentage 

 composition becomes a most important characteristic. 



All proteins give the three following reactions, 

 (i) When boiled with nitric acid they turn yellow, and 

 this yellow turns to orange on tlie addition of ammonia, 

 (ii) Boiled with Millon's reagent (a mixture of the 

 nitrates of mercury) they give a pink colour, (iii) When 

 mixed with caustic .soda and a small amount of a solution 

 of sulphate of copper they give a violet colour. These 

 reactions suffice for the detection of any protein in 

 solution or as a solid. 



The solids in the plasma of blood are chiefly protein.s, 

 of which there are three. The first is known as 

 fibrinogen and is piecipitated by the addition to plasma 

 of 15 per cent, of sodium chloride (ordinai'y salt). This 



