en. xxv.] THE FATS 393 



greater difficulty. The various digestive ferments have little or no 

 action on cellulose ; hence the necessity of boiling starch before it is 

 taken as food. Boiling bursts the cellulose envelopes of the starch 

 grains, and so allows the digestive juices to get at the starch 

 proper. 



Cellulose is found in a few animals, as in the test or outer invest- 

 ment of the Tunicates. 



[Inosite, or muscle sugar '(GoH.^0^, is found in muscle, kidney, 

 liver, and other parts of the body in small quantities. It is also 

 largely found in the vegetable kingdom. It is crystallisable, and 

 has the same formula as the glucoses. It is, however, not a sugar, 

 and careful analysis has shown that it really belongs to the aromatic 

 series.] 



The Pats. 



Fat is found in small quantities in many animal tissues. It is, 

 however, found in large quantities in three situations, viz., marrow, 

 adipose tissue, and milk. 



The contents of the fat cells of adipose tissue are fluid during life, 

 the normal temperature of the body (37 C., or 99 F.) being con- 

 siderably above the melting-point (25 C.) of the mixture of the fats 

 found there. These fats are three in number, and are called palmitin, 

 stearin, and olein. They differ from one another in chemical com- 

 position and in certain physical characters, such as melting-point and 

 solubilities. Olein melts at 5 C., palmitin at 45 C., and stearin 

 at 53-66 C. It is thus olein which holds the other two dissolved at 

 the body temperature. Fats are all soluble in hot alcohol, ether, and 

 chloroform, but insoluble in water. 



Chemical Constitution of the Pats. The fats are compounds of 

 fatty acids with glycerin, and may be termed glycerides or glyceric 

 ethers. The term hydrocarbon, applied to them by some authors, is 

 wholly incorrect. 



The fatty acids form a series of acids derived from the monatomic 

 alcohols by oxidation. Thus, to take ordinary ethyl alcohol, C 2 H 6 0, 

 the first stage in oxidation is the removal of two atoms of hydrogen 

 to form aldehyde, C 2 H 4 ; on further oxidation an atom of oxygen is 

 added to form acetic acid, C 2 H 4 2 . 



A similar acid can be obtained from all the other alcohols, 

 thus : 



From methyl alcohol CH 3 .HO, formic acid H.COOH is obtained. 



ethyl 



propyl 



butyl 



amyl 



hexyl 



C 2 H 5 .HO, acetic CHo.COOH 

 C 3 H 7 .HO, propionic CoHj.COOH 

 C 4 H 9 .HO, butyric C;H 7 .COOH 



4 9., 



C 5 H n .HO, valeric 

 C 6 H, 3 .HO, caproic C 5 H n .COOH 



and so on. 



