14 BULLETIN OF THE BUREAU OF FISHERIES. 
hydrochloric acid in one liter. To each portion thymol was added to prevent the 
action of microbes. Into test tubes containing the alkaline and acid solutions, 
respectively, fibrin was placed. In no case was the fibrin digested, whether the 
tubes were kept at 18° C. or at 87° C. 
Therefore the esophagus produces neither trypsin nor pepsin. If the mucous 
membrane is scraped from the esophagus of a fish in full digestion, extracts of this 
mucus may have a slight digestive action on fibrin in acid solution. This digestive 
action, however, is due to some pepsin which has come from the stomach, for if the 
esophagus is well washed before scraping the esophageal mucus is found to have no 
action on fibrin. 
In like manner neither water extracts nor weakly alkaline extracts of the mucous 
membrane of the esophagus have any diastatic action on starch paste even after a 
lapse of ten hours. Yung (1899) in two cases found a diastatic ferment in the 
mucus of the esophagus. These cases were one Scylliwn canicula in full digestion 
and one Acanthias vulgaris. We concludes from his experiments, however, that as 
a rule the epithelial elements do not produce a diastatic ferment. 
Water extracts of the esophageal mucus have no action on olive oil. 
Conclusion: The esophagus has of itself no digestive action. 
MUCOUS MEMBRANE OF THE STOMACHIC SAC, 
The mucous membrane of the stomachic sac was scraped and triturated in equal 
parts of glycerin and 0.5 per cent hydrochloric acid. Neutralized extracts did not 
coagulate milk. Therefore the rennet enzyme and its zymogen are absent from the 
mucous membrane of elasmobranchs.“ The acid extracts digested uncooked white of 
egg and fibrin rapidly, but acted very slowly on cooked egg. Asa rule the products 
of digestion by the acid extracts were peptones. Alkaline extracts of the stomachic 
sac showed no digestive activity. 
The only proteolytic ferment in the stomachic sac of elasmobranchs is, accord- 
ingly, pepsin similar to that of higher vertebrates. There is one great difference, 
however, between the pepsin of mammals and that of fish: The pepsin of fish acts at 
a low temperature far better than does that of mammals. Moreover, Fick and 
Murisier (1873) and Hoppe-Seyler (1877, cited by Yung, 1899) claimed that the fish 
pepsin acts better at 10° C. or 15° C. than at 37° C. Luchhau (1878) and Yung 
(1899), on the other hand, observed that the peptonizing action of the stomach of 
fishes is greater at 40° C. than at 15° C. 
While I should admit that the pepsin of fish acts rapidly on fibrin at 15° C., I 
must conclude from my experiments that the pepsin of M/ustelis canis, Carcharias 
littoralis, and Galeocerdo tigrinus digested fibrin better at 37° C. than at 20°C. In 
this conclusion I am in exact agreement with Yung, who found in Seyllium canicula 
that the pseudopepsin of fish acted more rapidly at the higher temperatures. Before 
leaving the question as to the action of pepsin it must be said that the artificial 
digestion in no way approximates the natural digestion as carried on in the stomach, 
«Certain experiments have led me to believe that the rennin zymogen (pexinogen) may exist in the mucous of the 
stomachie sac of at least some of the elasmobranchs and may be extracted by appropriate methods as the active enzyme 
rennin (pexin). To this question I hope to return at another time. 
