156 Comparative Animal Physiology 



mammals the submaxillary and sublingual glands receive parasympathetic 

 stimulation by the chorda tympani nerve, and the parotid gland by the auricu- 

 lo-temporal nerve; in addition the salivary glands are innervated from the 

 cervical sympathetics. Both parasympathetic and sympathetic nerves stimulate 

 secretion, although the nature of the fluid secreted differs according to the 

 nerves stimulated and the glands concerned. Normally there is some contin- 

 uous secretion, but salivary secretion can be stimulated reflexly by the presence 

 of food or other substances in the mouth or by stimulation of other sense organs 

 which are associated with food by conditioning. Control of salivary secretion 

 is, therefore, entirely nervous. The stimulating action of parasympathetic 

 nerves is prevented by atropine, which probably blocks the action of acetyl- 

 choline liberated at the nerve endings. Pilocarpine stimulates the glands to 

 continuous secretion. 



Secretion of gastric juice is stimulated by the vagus nerve and is antagonized 

 by atropine. The sympathetic nerve to the stomach may antagonize the vagus 

 and may elicit a sKght mucous secretion. Reflex stimulation of gastric secretion 

 can be elicited by odor, taste, or sight of food; in addition, mechanical stimula- 

 tion of the gastric mucosa elicits secretion. More important is the fact that 

 certain foods, particularly meat, initiate the liberation into the blood of a sub- 

 stance which stimulates the gastric glands. This hormone, gastrin, has the 

 properties of and may be identical with histamine. Stimulation of the vagus 

 nerve causes limited secretion of pancreatic juice, particularly rich in enzymes. 

 When the acid chyme from the stomach enters the duodenum, a hormone is 

 liberated from the intestinal mucosa; this substance, secretin, carried in the 

 blood, strongly stimulates pancreatic secretion. Nervous influence upon in- 

 testinal gland secretion is slight; the sympathetic are said to be inhibitory, and 

 the vagus is said to cause secretion in the absence of influence of the sympa- 

 thetics. The intestinal glands respond readily to mechanical stimulation of 

 the mucosa. A hormone from the intestinal wall, enterocrinin, apparently 

 stimulates the intestinal glands to secrete. Secretion of bile can be elicited by 

 impulses in the vagus nerve; certain foods and secretin also stimulate secretion 

 of bile from the liver. In some mammals bile is retained and concentrated in 

 a gall bladder; liberation of bile from the gall bladder is elicited by a hormone, 

 cholecystokinin, which is produced by the duodenal mucosa under the influ- 

 ence of the acid food mass. The three hormones from the intestinal mucosa, 

 secretin, enterocrinin, and cholecystokinin, diff^er in their eff^ects, although 

 all three have many chemical properties in common (see Chapter 22). 

 A progressive series from nervous to hormonal control is evident. Salivary 

 secretion is entirely nervous; gastric and pancreatic secretion are nervous and 

 hormonal; and secretion of intestinal fluids and that of bile are predominantly 

 hormonal. 



The control of secretion in non-mammalian vertebrates is known only for 

 the stomach and pancreas. Secretion of gastric juice by the proventriculus of 

 birds, as by the mammalian stomach, is stimulated by the vagus nerve, also 

 by secretin. '^'^ In the frog, mechanical stimulation by food or by objects such 

 as beads in the stomach causes gastric secretion by a reflex via the sympathetic 

 nervous system. ^"^ Similar sympathetic stimulation occurs if various sensory 

 nerves are stimulated in frogs with the central nervous system intact. The 

 vagus and vagomimetic drugs are ineff^ective. There is uncertainty whether any 



