CHEMICAL BASIS OF THE ANIMAL BODY. 1225 



blood-vessels it appears unaltered in the urine. It is therefore 

 presumably changed in the alimentary canal into some form of 

 sugar which is assimilable, it may be into dextrose and galac- 

 tose. It does not appear that any such conversion can be 

 markedly observed, if at all, under the action of any of the 

 secretions of the alimentary canal, hence the change may more 

 probably take place, as in the case of maltose, rather during 

 than before the passage of the sugar through the intestinal 

 walls. 



FATTY ACIDS AND FATS, THEIR DERIVATIVES 

 AND ALLIES. 



I. ACIDS OF THE ACETIC SERIES. 



General formula C n H 2n+1 .COOH (monobasic). 



The free acids are found only in small and very variable 

 quantities in various parts of the body ; their derivatives on 

 the other hand form most important constituents of the human 

 frame, and will be considered further on. 



Some of the lower acids of the series have been obtained by 

 treating proteids with molten caustic potash. They also occur 

 among the products of the putrefaction of proteids, as for instance 

 in old cheese. 



Of the primary alcohols from which this series of acids is 

 derived only two have as yet been obtained from animal tis- 

 sues or secretions, viz. ethyl-alcohol, C 2 H 5 . OH, and cetyl-alco- 

 hol, C 16 H 33 .OH. The former from muscle, brain and liver, 

 the latter in union with palmitic acid in spermaceti and the 

 secretion of the caudal glands of birds. 



Formic acid. H . COOH. 



When pure is a strongly corrosive, fuming fluid, with power- 

 ful irritating odour, solidifying at C., boiling at 100 C., and 

 capable of being mixed in all proportions with either water or 

 alcohol. It has been obtained from various parts of the body, 

 such as the spleen, thymus, pancreas, muscles, brain, and blood ; 

 in the latter its presence may be due to the action of acids on 

 the haemoglobin. It also occurs in minute traces in urine. It 

 is excreted by some ants (Formica rufa) in a fairly concen- 

 trated form and may be present to the surprisingly large extent 

 of 40 p.c. in the secretion of certain caterpillars. The separa- 

 tion of so acid a fluid from the alkaline cell-substance is remark- 

 able and of considerable interest. When heated with strong 

 sulphuric acid it is decomposed into carbon monoxide and water. 

 It is further characterized by readily effecting the reduction 

 of metallic salts, as of mercury or silver, when heated with their 

 solutions. 



