FATS AND LI poms 23 



acid radical they contain. The assumed structural formulaof one lecithin, stearyl- 

 olcyl lecitliin, is as follows: 



CH2 — O — C18 — HssO 



I 

 CH— O— Ci,— H,30 



I 

 CH2— 0— PO— OH 



I 

 0— CHo— CH2— N = (CHa),. 



OH 



It differs from ordinary fats, therefore, in having two special groups, one the phos- 

 phoric acid, the other the choline radical, w^hich last may be of some importance 

 in pathological processes. In its phj^sical properties it is quite similar to the 

 ordinary fats, although it forms even finer emiilsions in water, which are practically 

 colloidal solutions (W. Koch). 



Cephalin diiTers in having for the base amino-ethyl alcohol (XH0CH2CH2OH) 

 instead of choline, and is probably as widely spread in the tissues as lecithin. 1^ 

 It has been held by some that there are many phospholipins, which may be speci- 

 fic for different cells, tissues and species, but it seems more probable that these 

 supposed specific lipoidal substances are merely mixtures of lecithin, cephalin and 

 their derivatives in varying proportions (Levene).^^ 



Cholesterol, which is another lipoid, is nearly as universally present as leci- 

 thin, it exists both free and in combination ^\•ith fatt}' acids, for cholesterol is 

 an alcohol and not at all similar to the fats chemically, although very similar 

 physically. The empirical formula is C27H4SOH or C27H46OH, and it is related 

 to the terpenes. It seems to be relatively inert chemically, and therefore is 

 probably important only because of its effect on the phj'sical properties of the ceUs. 

 By some it is considered to be a decomposition or cleavage product of the proteins, 

 which is in accordance 'ndth its abundance in masses of old necrotic tissue, e. g., 

 atheromatous masses, old infarcts, and old exudates. 



Doubly Refractive Lipoids and Myelins. '^ — In practically aU normal tissues 

 there are present droplets of lipoid nature which are characterized bj' sho-ndng 

 prominent crosses when examined with crossed Nicol prisms (anisotropic), the 

 adrenal and corpus luteum containing them most abundantly. Chemically they 

 seem to be mixtures of various lipoids in inconstant proportions, but probably the 

 anisotropic character is most usuallj' dependent upon the presence of cholesterol 

 esters. The term myelin was first appUed by Virchow to pecuHar fatty substances 

 found in various normal and pathological tissues, because they showed physical 

 characters similar to those of the myeUn substance of nerves, but as many of these 

 substances are doubly refractive, or can be easily made so, some authors use the 

 term myelin as if it were synonymous with doubly refractive Hpoids. There are, 

 however, myelins which are not always doubly refractive, and also doubly 

 refractive hpoids w^hich do not swell up in water to form myeUn figures, etc., as is 

 characteristic of true myeUns. Chemically, however, the mj^ehns and doubly 

 refractive substances are probably related, consisting of mixtures of cholesterol, 

 cholesterol esters, phospholipins and perhaps soaps, in varying proportion. They 

 will be considered further in discussing Fatty Metamorphosis, Chap. XVI. 



Carbohydrates 



The third great class of food-stuffs, the carbohydrates, is represented in the ceU by 

 pentoses and hexoses combined with proteins and with lipoids, and also by glycogen, 

 which exists free. Glycogen is a difficult substance to isolate in minute quantities 

 and, therefore, although it is not found in all cells by our present methods, yet it 

 may well be that it is a constant constituent of the protoplasm. There is no e\a- 



" Koch and Woods, Jour. Biol. Chem., 1905 (1), 203. 



12 Jour. Biol. Chem., 1919 (39), S3. 



"See Adami, Join:. Amer. Med. Assoc, 1907 (48), 463; Karwdcka, Ziegler's 

 Beitr., 1911 (50), 437; Schultze, Ergebnisse Pathol.,-1909 (13, pt. 2),»253; Bang, 

 Ergebnisse Physiol., 1907 (6), 131; 1909 (8), 463. 



