436 MAMMALIA— PHYSIOLOGY 



Precipitin Reaction. — An interesting test for the detection of 

 human blood in murder cases is dependent on the solubility of blood 

 protein. It is said that soluble protein will give the test after a 

 period of fifty years. The serum of an animal (a rabbit is ordinarily 

 used), injected with the blood or blood serum of another animal, will, 

 when added to a homologous serum, precipitate the albumin in the 

 form of a light flocculent precipitate. (See p. 528, Evidence for 

 Evolution.) 



Carbohydrates consist of carbon, hydrogen and oxygen, and occur 

 in both the crystalline and colloidal state. Carbohydrates can be 

 converted into fats. In plants the starches are commonly dis- 

 tributed, but glycogen is the only example of animal starch. Gly- 

 cogen occurs in all the growing cells of the animal body, but is 

 found chiefly in the liver and muscles (see page 444). 



There are three chief groups of carbohydrates. The monosac- 

 charids with one sugar group include dextrose or grape sugar. The 

 disaccharids include cane sugar, milk sugar and maltose. Several 

 monosaccharid molecules become united into a polysaccharid. Corn 

 and potato starch and wheat flour are common examples of polysac- 

 charids. The monosaccharid, dextrose (C6H12O6), is formed in the 

 leaf of a green plant from carbon dioxide and water. Plants manu- 

 facture carbohydrates from carbon dioxide and water. Recently 

 Professor E. C. C. Baly of Liverpool University has produced sugar 

 synthetically in his laboratory by utilizing ultra-violet rays on quartz 

 vessels of water in which carbon dioxide was dissolved. With small 

 quantities of either iron or aluminium compounds as catalysts (see 

 page 440) he obtained sugar. In nature, sugar is made through the 

 action of the sun on chlorophyll-containing plant cells, and from this 

 sugar other foods are formed by the addition of different chemical 

 elements. Sugar, then, is the source of all food, and the process 

 just mentioned, " photosynthesis," is the most important chemical 

 reaction in nature. 



Fats contain carbon, hydrogen, and oxygen in different propor- 

 tions, containing much less oxygen in proportion to the carbon than 

 in the carbohydrates. Fats can readily be converted into carbohy- 

 drates. They are found in seeds and nuts, and in animals occur in 

 the connective tissue called adipose. Lipins contain carbon, oxy- 

 gen, hydrogen, and nitrogen, and may contain phosphorus also. 

 The lecithin of egg yolk is an example of a lipin. Fats of the body 

 are derived not only from the fatty substances consumed, but are 



