82 LOCOMOTION. [CHAP. in. 



is one of those situations where the capillary vessels can be most un- 

 equivocally proved to possess distinct membranous parietes. 



Fat. Fat is a white or yellow unctuous substance, unorganized, 

 and secreted into the interior of the adipose vesicles. Chemists 

 have distinguished in it two solid proximate principles, stearine and 

 margarine, combined with a fluid one, or oil, elaine ; on the relative 

 proportions of which the principal of the numerous modifications of 

 its external qualities would seem to depend. These principles may 

 be obtained by different means. Boiling alcohol dissolves both, but 

 on cooling deposits the stearine in snow-white flakes ; and the elaine 

 may be set free by the addition of water, for which the alcohol has 

 a superior affinity. Or, the elaine may be separated by pressure. 

 Stearine preserves its solidity at a temperature of 167 Fahr., and 

 elaine remains fluid at 63 or 65 F. Margarine exists along with 

 stearine in most fats, and may be separated from it by ether, which 

 dissolves margarine, but not stearine ; it is said to exist alone in 

 human fat, which is therefore destitute of stearine. These proxi- 

 mate elements of fat are regarded by modern chemists as natural 

 compounds of certain organic acids with an organic base, to which 

 the name of glycerine has been given, from its sweet taste. The 

 acids are, the stearic, margaric, and elaic ; and the proximate prin- 

 ciples are, respectively, a stearate, a margarate, and an elaate of 

 glycerine. By boiling oil or fat with a solution of caustic alkali, 

 the acids unite with the potash, forming soap, and the glycerine re- 

 mains dissolved in the liquid. By evaporating this liquid (in which 

 any excess of alkali had been previously neutralized by tartaric acid) 

 to a thick syrup, the glycerine may be obtained from it in solution 

 by strong alcohol. 



We may often detect a spontaneous separation of these two proxi- 

 mate principles within the fat vesicle of the human subject. The 

 solid portion collects in a spot on the inner surface of the cell-mem- 

 brane, and looks like a small star (fig. 11, b. b. b). The elaine 

 Fitj , , occupies the remainder of the vesicle, except 



when the quantity of fat in the cell is smaller 

 than usual; in which case we may often 

 discern a little aqueous fluid between the 

 elaine and the cell-membrane on the side 

 farthest from the star (fig. 11, . .) ; a con- 

 dition, by the way, which is very favourabl 

 for the observation of this membrane itself. 

 The softer kinds of fat were denominat 

 by the older anatomists pinguedo, lard ; an< 



