330 CHEMISTRY OF THE DIGESTIVE PROCESSES. 



to the observations of van d. Yelden 1 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 0'005 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. 7 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. 8 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, 1879, Bd. iii. S. 205. 



2 See Langley, Journ. Physiol., Cambridge and London, 1882, vol. iii. p. 246 ; Nyle"n, 

 Jahresb. u. d. Fortschr. d. Thier-Chem., Wiesbaden, 1882, Bd. xii. S. 241 ; and other 

 authorities quoted above. Opposite results were obtained by Cohnheim, Virchow's Archiv, 

 1863, Bd. xxviii. S. 248 ; Schiff, " Lecons sur la digestion," tome i. p. 162 ; and Dufresne, 

 Compt. rend. Acad. d. sc., Paris, 1879, tome Ixxxix. p. 1070. 



3 Nasse, Arch. f. d. ges. Physiol., Bonn, 1875, Bd. xi. S. 138 ; Chittenden and Painter, 

 Stud. Lab. Physiol. Chem., New Haven, 1885, vol. i. p. 52. 



4 Plugge, Arch.f. d. ges. Physiol., Bonn, 1872, Bd. v. S. 550. 



5 Hamniarsten, " Lehrbuch der Physiol. Chem.," Wiesbaden, 1895, Ann. 3, S. 234. 



6 Swiecicki, Jahresb. u. d. Fortschr. d. Thier-Chem., 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, Untersuch. z. Naturl. d. Mensch. u. d. Thiere, 1876, Bd. xi. S. 455 ; 

 Hamniarsten, Beitr. z. Anat. u. Physiol. als Fcstgabe C. Ludwig, Leipzig, 1874, S. 110 ; 

 Zweifel, "Ueber d. Verdauungsapparat der Neugeborenen," Berlin, 1874. 



