B UILDING- UP AND BREAKING-DO WN BODYSTUFFS. 893 
It is produced when these two substances are allowed to act upon one 
another at a high temperature, and under pressure, as when they are 
heated together for some hours in a glass tube to a temperature of 160° 
C, or more simply hy heating monochloracetic acid with benzamide :■ — 
C 6 H 6 CO.NH 2 +CH 2 CLCOOH=(C 6 H 6 CO)NH.CH 2 .COOH+HCl 
(benzaniide) (monochloracetic acid) (hippuric acid) 
This synthesis of hippuric acid in vitro was speedily followed by that of 
urea (Wohler, 1828). 
The synthesis of hippuric acid was proved by Bunge and Schmiede- 
berg to occur in dogs exclusively in the kidney, and may be produced 
even at the temperature of the room, by passing oxygenated blood 
containing benzoic acid, or a benzoate, and glycine through the blood 
vessels of the organ, or even by allowing such blood to stand for a 
while in contact with the minced kidney of a fresh-killed animal. 
When, however, the kidney cells are destroyed, as by being pounded with 
sand in a mortar, no hippuric acid is produced. ' If benzoic acid be 
given by the mouth, hippuric acid appears in the urine ; the glycine for 
the synthesis is furnished by the tissues. If the kidneys are previously 
extirpated, no hippuric acid is found in any of the organs after the 
exhibition of benzoic acid ; but if the ureters are merely ligatured, 
hippuric acid is found in abundance. 
In frogs and rabbits the synthesis of hippuric acid is not confined to the 
kidneys, but is found to occur after the extirpation of these organs. 1 
Other syntheses besides that of hippuric acid, which are known to 
occur in the animal body, are that of urea in the liver, from ammonium 
carbonate and ammonium carbamate ; that of uric acid in the bird's liver, 
also from ammonia compounds ; that of glycogen, from glucose in the 
liver, and also in muscles and in many other tissues ; that of proteids, 
from peptones in the mucous membrane of the alimentary canal ; that of 
fats, from fatty acids and glycerin in the intestinal mucous membrane ; 
that of fats from carbohydrates, or from the elements of the broken-down 
carbohydrate molecule ; and also, in all probability, that of fats from the 
non-nitrogenous moiety of the broken-down proteid molecule. It is clear 
from these instances that the importance of syntheses in the animal 
economy cannot be overrated, and although the most striking feature in 
animal metabolism is the breaking down of complex substances into 
others of more simple form, yet even in the case of these broken-down 
products there is frequently a subsequent synthesis before they are got 
rid of from the body. Instances of this occur in the case of several urinary 
products, such as hippuric acid, urea, and uric acid. 2 
As Bunge 3 remarks : " There are two reasons why these synthetic 
processes in the animal body have excited the interest of physio- 
logists and chemists. In the first place, they were in contradiction 
to the long dominant doctrine of Liebig, as to the contrast be- 
tween the metabolic processes in plants and animals ; 4 and, in the 
1 Bunge and Schruiedeberg, Arch. f. exper. Path. u. Pharmakol., Leipzig, 1S76, Bd. vi. 
S. 233 ; Hoffman, ibid., 1877, Bd. vii. S. 239 ; Kochs, Arch. f. d. ges. Physiol., Bonn. 1879, 
Bd. xx. S. 64 ; Salomon., Ztschr. f. physiol. Cliem., Strassburg, Bd. iii. S. 365. 
2 On the importance of synthetic processes in animal metabolism, see Pfluger, Arch./, d. 
ges. Physiol., Bonn, 18S8, Bd. xlii. S. 144. 3 "Lehrbuch," 1894, S. 288. 
4 Nevertheless, the main distinction propounded by Liebig, that most plants are able 
to obtain their nitrogen, and to build it up into proteid from inorganic materials, 'whereas 
animals do not possess this power, still holds good. 
