32 HUMAN PHYSIOLOGY 



and from these the three fats 



Palmitin C 3 H 5 (0,C 18 H 3I 0) 3 - C 51 H 98 6 



Stearin 



Olein 



are produced. 



It will be observed that the molecules of these fats are 

 very rich in carbon and hydrogen, and very poor in oxygen 

 i.e. they contain a large amount of material capable of 

 being oxidised, and thus capable of affording energy in the 

 process of combustion. 



The fats resemble one another in being insoluble in water, 

 but soluble in ether and in hot alcohol. As the alcohol 

 cools, they separate out as crystals. They differ from one 

 another in their melting point, palmitin melting at the 

 highest and olein at the lowest temperature. Fat which is 

 rich in palmitin and stearin, like ox fat, is thus hard and 

 solid at the ordinary temperature of the air, while fats rich 

 in olein, like dogs' fats, are semi-fluid at the same tempera- 

 ture. The olein acts as a solvent for the fats of a higher 

 melting point. (For tests, see Chemical Physiology, p. 12.) 



The functions of adipose tissue are twofold : 



1st, Mechanical. The mass of adipose tissue under the 

 skin is of importance in protecting the deeper structures 

 from injury. It is a cushion on which external violence 

 expends itself. Further, this layer of subcutaneous fat pre- 

 vents the loss of heat from the body, being, in fact, an extra 

 garment. 



2nd, Chemical. Fat, on account of its great quantity of 

 unoxidised carbon and hydrogen, is the great storehouse of 

 energy in the body (p. 379). 



(C) Pigment Cells. In various parts of the eye the 

 connective tissue and other cells contain a black pigment 

 Melanin. The precise chemistry and mode of origin of this 

 pigment is not known. It contains carbon, hydrogen, nitro- 

 gen, oxygen, and it may also contain iron. It may be formed 

 directly in the cells, or it may be produced by the cells from 

 the pigment of the blood. Its function in the eye is to 

 prevent the passage of light through the tissues in which it 

 is contained. 



