950 THE EXTERNAL SECRETIONS 



we might regard this liquid as a true secretory product of the glands of Lieberkiihn, 

 it should not be forgotten that a considerable portion of it may be produced by 

 transudation following the relaxation of the intestinal blood-vessels. In general, 

 this pnenomenon may be compared to the paralytic secretion of saliva. 



The intestinal juice contains several ferments, two of which are proteolytic in 

 their action. Of these enterokinase has already been mentioned in connection with 

 the activities of Brunner's glands. The other, which is known as erepsin, is present 

 in this juice as well as in almost all tissues of the body. Among the ferments 

 affecting the carbohydrates, may be mentioned invertase which transforms sugar 

 into glucose and levulose or fructose, and maltase which changes maltose into 

 glucose. * Excepting enterokinase, these ferments have also been regarded as intra- 

 cellular agents and not as constituents of the juice itself. In this form, they should 

 exert their action upon the different foodstuffs while the latter traverse the epithe- 

 lial cells on their way to the channels of absorption. This contention is founded 

 upon the fact that the liquid obtained by extracting the intestinal mucosa forms 

 a more powerful digestive medium than the intestinal juice itself. In all previous 

 instances, we have observed that a simple extract of the mucosa is inactive, but may 

 be activated very readily by giving to it the reaction which it necessitates. Entero- 

 kinase, on the other hand, is not contained as such in the epithelial lining cells but 

 only in the form of a precursor which assumes its activity immediately after its 

 discharge into the general juice of the intestine. 



The regulation of this secretion is effected by a nervous as well 

 as a chemical factor. The former is mediated by the peripheral 

 expanse of the autonomic system of this particular region of the body, 

 which presents itself in the form of the plexuses of Meissner and Auer- 

 bach. These networks of sympathetic fibers are situated beneath the 

 submucosa. The fact that reflexes play a part in the secretion of 

 intestinal juice may be gathered from the close dependency of this 

 process upon extraneous stimuli. 2 Thus, a dog which had not been 

 fed for a period of about 24 hours showed a flow within 15 minutes 

 after the ingestion of food; moreover, this flow reached its maximum 

 in about 3 hours, i.e., at a time when the pancreatic juice was produced 

 most copiously. But since the intestinal secretion does not cease 

 after the intestine has been completely isolated from the central 

 nervous system by the division of the vagi and sympathetic nerves, 

 some other agent must be at work, presumably in the form of a se- 

 cretogogue. Although the nature and place of origin of this hormone 

 have not been made out with any degree of definiteness, Delezenne 

 and Frouin 3 have proved that the injection of secretin into the blood 

 stream of animals provided with an intestinal fistula gives rise to 

 a copious flow of this juice. It seems, therefore, that this chemical 

 messenger acts simultaneously upon three organs, namely, upon the 

 pancreas, liver and glands of Lieberkiihn, insuring thereby a concerted 

 action of these secretions upon the acid gastric chyme. But certain 

 evidence is also at hand to show that some other chemical agent is 

 liberated in the lower part of the small intestine, synchronously with 

 the intestinal juice. The nature of this hormone is not known. 



1 Weinland, Zeitschr. fur biol. Chemie, xlvii, 1905, 279. 



2 Bayliss and Starling, Ergebn. der Physiol., 1906. 

 8 Proc. Soc. Biol., Ivi, 1906, 319. 



