8 ESSENTIALS OF PHYSIOLOGY. 



occurring especially in t^e nervous tissues, bile, and the plasma and 

 corpuscles of the blood. 



Lecithin is a complex fat, and when boiled with baryta water yields 

 two fatty acids, glycerophosphoric acid, and choline, which belongs to 

 the group of amines. Lecithin, in virtue of its nitrogen content, has 

 also affinities with proteins. It belongs to the group of phospholipines, 

 which also includes cephaline and sphingo-myeline. Some other lipoid 

 substances found in the brain resemble lecithin in containing nitrogen, 

 but contain no phosphorus. 



Cholesterol, C 27 H 45 OH, is a complex monatomic alcohol, and is 

 included in the group of lipoids simply on account of its solubilities. 

 It forms colourless, square, flat crystals, often notched at one corner, 

 and gives a red colour with strong sulphuric acid. 



Lipoid substances enter into the composition of cell protoplasm, 

 occurring especially in the superficial layer, or "plasma skin." This 

 surface layer is less permeable to salts and other substances in watery 

 solution than to water. Substances such as alcohol and alkaloids, 

 which are soluble in oily media, can penetrate the cell. 



THE NITROGENOUS SUBSTANCES. 



The nitrogenous substances contained in the body are (1) proteins, 

 which form the greater proportion of the solid constituents of the cells, 

 tissues, and body fluids, and (2) derivatives of proteins. 



The Proteins. 



The physical properties of proteins are those of colloid substances 

 (p. 16). Some proteins, however, may be obtained in the crystalline 

 form, for example egg-albumin, and haemoglobin, the pigment of blood. 

 Others, for example peptone (which is not, however, a constituent of 

 the body), are capable of diffusing through an animal membrane. 



The molecule of protein is very large ; it contains the elements 

 C, H, N, 0, and S in the following proportions : 



C 50-6 - 54-5 per cent. 



H 6-5- 7-3 



N 15-0-17-6 



21-5-23-5 



S 0-3- 2-2 



Proteins have the power of combining either with acids or alkalies, and 

 this property is of service in maintaining the reaction of the cells and 

 fluids of the body at its normal* level. 



When a solution of a protein, such as egg-albumin, is warmed 

 with dilute acid or alkali, it is converted into metaprotein and shows 



