CHAPTER XXIX. 357 



LIPOIDS. This term was used originally to denote substances 

 having solubilities similar to those of the true fats, i.e., substances 

 which may be present, together with the true fats in alcohol, ether 

 and chloroform extracts of tissues. This definition is, however, 

 too crude, and the term as now used is restricted to certain chemi- 

 cally well-defined groups of substances, which in fact constitute the 

 bulk of such extracts, namely, cholesterol and its esters, phospha- 

 tides, cerebrosides and phosphorised cerebrosides. The phos- 

 phorised cerebrosides are ' compound lipoids." Their molecule 

 is very large and consists of a chemical combination of several groups 

 of simple lipoids, e.g., cerebrosides and a phosphatide in protagon. 

 The existence of such compound lipoids is denied by some authors, 

 who look upon them merely as mixtures of cerebrosides and phos- 

 phatides. These groups of substances, although differing in their 

 chemical constitution from each other and from true fats, frequently 

 occur together in the tissues, and may, therefore, be presumed to 

 have a similar physiological significance. They also resemble each 

 other and true fats in their general staining reactions, but some of 

 them exhibit characteristic differences in this respect. Some 

 authors use the term " lipoid " as including the true fats. Others, 

 morphologists in particular, use the term ; fat ' so as to include 

 lipoids. It seems advisable, however, to separate the true fats 

 from lipoids, as is done here, since the two groups of substances 

 fulfil different physiological functions. The term ; LIPIN ' has 

 been proposed by some authors to denote certain groups of lipoids. 

 The advantage of this nomenclature is not obvious, and since 

 almost every author who has used this term has given it a new 

 definition it will not be used here. 



For a detailed account of the chemistry of these substances the 

 reader is referred to the larger text-books and monographs, such 

 as HAMMARSTEN'S Text-book of Physiological Chemistry, ABDER- 

 HALDEN'S Biochemisches Handlexicon, Vol. iii, MACLEAN'S monograph 

 on Lecithin and the Allied Substances. The table on p. 358 gives 

 only a few elementary data concerning the chemical constitution 

 of these substances and their solubilities, which are of importance 

 in connection with their staining reactions. 



In the following, the term " true fats " will always be applied to 

 mixtures of unsaturated and saturated fats, since in the tissues 

 these substances always occur together. The table shows that all 

 fatty substances occurring in the tissues contain somewhere in their 

 molecule a double linkage (marked thus ii in the table overpage), 

 and are therefore unsaturated compounds. This fact is of importance, 



