CHEMICAL BASIS OF THE ANIMAL BODY. 71 



into the surrounding medium ; when killed however, as by means of alcohol, they 

 yield it readily to a suitable extractive. This holds good also in the case of invertin, 

 which is not found in the nitrate from yeast while it may readily be extracted from 

 the cells when killed by ether or alcohol 1 . Similarly it appears that putrefactive 

 bacteria may excrete or yield an enzyme whose action is closely analogous to that 

 of trypsin 2 . 



The most prolific source of the urea enzyme is in all cases the 

 mucous urine passed in inflammatory conditions of the bladder. In 

 this case the enzyme appears to be closely associated with the mucin 

 and is presumably a secretory product of the mucous membrane, for it 

 is frequently obtained when there has been no operative use of surgical 

 instruments which could account for the introduction of micro-organisms 

 from the exterior. 



In concluding this account of the more important enzymes of the 

 animal body it may not be out of place to say a few words on the 

 probable mode of action of the ferments and enzymes. 



The term fermentation was applied originally to the changes, ac- 

 companied by characteristic frothing, foaming and evolution of gases, 

 which saccharine solutions such as the expressed juice of fruits or 

 infusions of grain undergo on exposure to the air. The chemical 

 changes and products of the fermentation were studied from the 

 earliest times, and in 1680 Leuwenhcek described, with the aid of 

 the newly-invented microscope, the small, spherical particles which 

 are now known, as yeast-cells, to be the exciting cause of the whole 

 process. He did not however ascribe any organisation to these particles, 

 and it was not until 1835 and 1837 that Cagniard de Latour and 

 Schwann respectively but independently took up the investigation 

 where Leuwenhcek had left it and established firmly and finally the 

 organised and plant-like nature of the yeast-cell and the absolute 

 dependence of fermentation upon its presence in the fermenting fluid 3 . 

 The yeast-cell having thus been definitely recognized as the cause of 



1 Hoppe-Seyler, Ber. d. deutsch. chem. Gesell 1871, S. 810. Confirmed by Lea. 

 For chemistry of invertin see Donath, Ber. d. deutsch. chem. Gesell. 1875, S. 795 ; 

 1878, S. 1089, Earth, Ibid. 1878, S. 474. Kjeldahl (Danish). See Abst. in Maly's 

 Pericht. 1881, S. 448. Mayer, Zt. f. Spirit-Indust. 1881, Nos. 16, 22. Low, Pnuger's 

 Arch. Bd. xxvn. (1882), S. 203. 



2 Hiifner, Jn. f. prakt. Chem. (N.F.) Bd. v. (1872), S. 372. Herrmann, Zt. f. 

 physiol. Chem. Bd. xi. (1887), S. 523. E. Salkowski, Zt.f. Biol. Bd. xxv. (1889), 

 S. 92. 



3 Erxleben in 1818 had described and spoken of yeast as a vegetative organism, 

 as also in 1825 had Desmazieres, who ascribed to it an animal rather than vegetable 

 nature. 



