i34 THE CHEMISTRY OF THE TISSUES AND ORGANS. 
acids contain equal quantities of caproic, caprylic, and capric acids, and 
the merest traces of butyric acid. The principal acids present, as is 
usual in animal fat, are palmitic, stearic, and oleic acids, and one or 
more acids of lower molecular weight, including myristic acid. The 
melting point of the mixture of acids is 37° to 39°, and of the fat itself 
30° to 31° C. 
The proteids of milk. — The proteids which occur in milk are 
three in number. The most abundant and most important of these is 
caseinogen. It is this proteid which is acted upon by rennet, and 
converted into casein or cheese. 1 The other two proteids are only 
present in small quantities ; they are called lactoglobulin and lact- 
albumin. Proteoses and peptone were described in milk by many of 
the older workers. This was due to the use of faulty methods of 
analysis (see p. 41). 2 
Coagulation of milk. — When milk is allowed to stand at the 
ordinary temperature exposed to the air, the chief change it undergoes 
is the lactic acid fermentation. The acid formed precipitates a part of 
the caseinogen, but this is a different thing from the conversion of 
caseinogen into casein. Sometimes, however, certain aerobic bacterial 
growths act like rennet in causing a true curd. Certain of the higher 
plants (Ficus, etc.) also curdle milk. 
The agency by which the clot is most readily formed is that of 
rennet. This is a ferment secreted by the stomach, and is usually obtained 
from the stomach of sucking animals, like the calf. The pancreatic 
juice also has a curdling action on milk (see p. 137), and extracts of 
many tissues (such as testis, liver, lung, muscle) have a feeble action of 
the same nature. 3 
Hammarsten 4 and, later, Friedberg 5 showed conclusively that the 
active principle of rennet is not pepsin ; that it requires for its efficient 
action the presence of calcium salts, of which the phosphate is the one 
which is mostly present hi the milk, and that it will act in a weakly 
acid, neutral, or alkaline solution. It acts most readily at 40° C, and 
is destroyed at 70° C. The ferment itself in the rennet extracts is 
termed chyinosin by Friedberg, and reimin by Foster. 6 
When rennet is added to cows' milk the result is a coherent clot or 
curd, which expresses a clear yellowish fluid, the whey. The curd 
contains the fat entangled with the casein ; the whey contains the other 
proteids, sugar, and salts of the milk. In human milk the curd is 
usually composed of smaller flocculi, and a similar flocculent coagulation 
can be produced in cows' milk by previously boiling it, or by diluting it. 
Lime water, soda water, or barley water are generally used as diluents 
for this purpose. 
The coagulation of milk is somewhat analogous to that of blood, 
and the analogy is accentuated by the fact that in both cases calcium 
1 The utility of this nomenclature is at once apparent when casein and caseinogen are 
contrasted with fibrin and fibrinogen, myosin and myosiuogen, even although the analogy 
is not complete in details. Hammarsten, however, prefers to call the proteid in milk, 
casein ; while the coagulated proteid he terms, after Schulze and Rose (Laiidic. 
Versuchs. Stat., Berlin, Bd. xxxi.), paracasein. 
2 For a critical article on the estimation of the various proteids in milk, see Schlossmann, 
Ztschr. f. physiol. Chem.. Strassburg, 1896, Bd. xxii. S. 197. 
3 Edmunds, Journ. Physiol., Cambridge and London, 1896, vol. xix. p. 465. 
4 Jahresb. ii. d. Fortschr. d. Thier-Chem. , Wiesbaden, 1874, S. 135, 
5 Journ. Am. Chem. Soc., N". Y., 18S8, p. 15. 
6 "Text-book," 5th edition, p. 519, 
