928 THE EXTERNAL SECRETIONS 



essential for the maintenance of health and growth. While as yet 

 chemically unrecognized, their presence has been proved by physiolog- 

 ical means. They play the part of ^'building stones" in the syntheses 

 of the developing animal. As such, they act upon the secretory proc- 

 esses as well as upon the processes of assimilation and dissimilation. 

 Consequently, they must unfold their function as secretogogues shortly 

 after their entrance into the digestive tract, and niust also exert a 

 certain influence upon the activity of the gastric glands. This end 

 they accomplish (a) by stimulating the cells of the gastric glands 

 directly, (6) by entering the blood and influencing the secretory ac- 

 tivity of these glands in an indirect way, and (c) by causing a liberation 

 of gastric secretin which in turn affects the glandular elements. 



The Nervous Control of the Secretion of Gastric Juice. — ^Like 

 saliva, gastric juice is secreted continuously, but only in amounts suf- 

 ficient to lubricate the mucous surfaces. At various times, however, 

 a more copious flow is initiated which finds its origin in stimuli of an 

 occasional character. The latter embrace (a) chemical agents which 

 reach the gastric cells either directly from the cavity of the stomach or 

 indirectly by the blood, and (6) stimuli which act reflexly through 

 particular reflex circuits. It must be evident, however, that these 

 reflex arcs need not be situated in the stomach, at least not their 

 afferent path, or analyzer, because a secretion of gastric juice results 

 not only in consequence of the entrance of food into the stomach, but 

 also upon its being taken into the mouth; in fact, even the mere sight 

 and smell of nutritive substances may serve as an efficient stimulus. 

 The act of chewing, however, does not serve as a stimulant, nor does 

 the intake of an indifferent substance not related to food. Conse- 

 quently, we may say that the secretion of gastric juice is controlled 

 by nerve impulses of intragastric and extragastric origin produced by 

 palatable substances. 



This statement implies that the afferent side of the reflex circuit 

 concerned in gastric secretion is diversified, while the efferent side is 

 not. Relative to the latter, it has been proved that it is contained in 

 the vagus system, because the division of the ventral and dorsal vagi 

 below the origins of their pulmonary and cardiac branches abolishes 

 this reflex, while the stimulation of the distal end of either nerve 

 is followed by a secretion. In addition, it has been stated that these 

 nerves contain musculomotor fibers for the stomach.^ The splanchnic 

 nerves do not seem to be involved in this secretory reaction, because 

 their division and stimulation remains without effect. We know, 

 however, that these nerves embrace vasomotor fibers for the stomach 

 which traverse the ganglia of the solar plexus and reach this organ by 

 following the highways of the different gastric arteries. Consequently, 

 since their stimulation must lead to a constriction of the gastric blood- 

 vessels, they must limit the quantity of the gastric juice, but in an in- 

 direct way and not as secretory nerves. Now, since their action is 

 1 Burton-Opitz, Pfiiiger's Archiv, cxxxv, 1910, 205. 



