33o CHEMISTRY OF THE DIGESTIVE PROCESSES. 
to the observations of van d. Velden x there is no free hydrochloric acid 
found in the stomach until, on an average, three-quarters of an hour 
after a carbohydrate meal. During- this time the diastatic action of the 
saliva must continue, and probably during most of the interval more 
intensely than it would with its natural reaction. In this stage gastric 
juice removed by the pump possesses a diastatic action on starch, but 
later, when free acid is present, even when saliva is added to it, has no 
such power. After all the proteid present in solution in the stomach 
has been combined with the acid first secreted in the gastric juice, 
and still more acid is secreted which remains free, the ptyalin not only 
becomes inert, but is rapidly destroyed, and does not come into action 
again after the acid of the gastric juice is neutralised in the small 
intestine. 2 
Free organic acids also act destructively on ptyalin : the concentration 
of acid required is greater than in the case of hydrochloric acid, and 
varies with the particular acid as well as with the concentration of the 
ferment in the solution. Different neutral metallic salts possess different 
actions ; some diminish the activity, such as mercuric chloride, which 
even in a concentration of - 005 per cent, is sufficient to stop all action ; 
others increase it when present in small quantity, such as magnesium 
sulphate up to 0'025 per cent., but have an opposite effect in greater 
concentration. 3 Carbolic acid does not produce much effect, digestion 
with 5 per cent, solution for some hours being required to destroy the 
ferment. 4 
Pepsin. — Pepsin is very widely distributed in the animal kingdom; 
it is found in the gastric juice of all vertebrates, with the possible excep- 
tion of some fishes. 5 In the frog it is found chiefly in the oesophagus. 6 
In the crayfish a yellowish-brown fluid is found in the mouth, of strong 
acid reaction, which digests fibrin readily." And in many insects an acid 
proteolytic secretion has been observed. Similar acid proteolytic secre- 
tions are also known in the vegetable kingdom, such as those which may 
be obtained by stimulating the leaves of insectivorous plants. s Whether 
these acid proteolytic ferments of the invertebrates and plants are 
identical with pepsin is not known with certainty, but they are very 
similar in their action. 
Pepsin is found in the stomach of the herbivora at birth, and in 
some other animals, including man: in others, it first appears two or 
three weeks after birth, as in the dog and cat, 9 
The different regions of the stomach do not, on extraction, yield 
1 Ztschr. f. physiol. Chem., Strassburg, 1S79, Bd. iii. S. 205. 
2 See Langley, .!< urn. Physiol., Cambridge and London, 1882, vol. iii. p. 246; Tvylen, 
Juhrcsb.il. d. Fortschr. d. Thier-Ckem., Wiesbaden, 1882, Bd. xii. S. 241; and other 
authorities quoted above. Opposite results were obtained by Cobnheim, Virchow's Arehiv, 
1863, Bd. xxviii. S. 24S : Scbitf, " Lecons sur la digestion," tome i. p. 162 ; and Dufresne, 
Com-pt. rend. Acad. d. sc, Paris, 1S79J tome lxxxix. p. 1070. 
a Nasse, Arch./, d. ges. Physiol., Bonn, 1S75, Bd. xi. S. 13S ; Chittenden and Painter, 
Stud. Lab. Physiol. Chem., Xew Haven, 1885, vol. i. p. 52. 
4 Plugge, Arch.f. d. ges. Physiol., Bonn, 1S72, Bd. v. S. 550. 
5 Hammarsten, " Lehrbuch der Physiol. Chem.," Wiesbaden, 1895, Aufl. 3, S. 234. 
,; Swiecicki, Jahresb. ii. d. Fortschr. d. Thier-Ohem., Wiesbaden, 1876, Bd. vi. S. 172. 
7 Hoppe-Seyler, ibid., S. 170. 
8 Darwin. "Insectivorous Plants" ; Goebeb and Loew, Chem. Centr.-BL, Leipzig, 1893, 
Bd. ii. S. 1065. 
9 Moriggia, Untcrsvch. : Naiwrl. d. Mensch. w. d. Thiere, 1876, Bd. xi. S. 455; 
Hammarsten, Beitr. z. Anat. u. Physiol, als Festgabe C. Ludwig, Leipzig, 1874, S. 116; 
Zweifel, "Leber d. Yerdauungsapparat der Xeugeborenen," Berlin, 1S74. 
