PHYSIOLOGY OF THE DIGESTIVE GLANDS 423 



the process by which this glandular activity is controlled. Two mechan- 

 isms of control are known: (1) by the nervous system, and (2) by means 

 of hormones. 



Nervous Control. Control through the nervous system is most marked 

 indeed it may be the only means of control in glands which have to 

 produce their secretion promptly, whereas hormone control pre- 

 dominates in those in which prompt changes in secretory activity are not 

 required. Thus, nervous control alone is present in the salivary glands, 

 whereas hormone control is predominant in the pancreas, intestinal 

 glands and liver. The gastric glands are partly under nervous control, 

 and partly under hormone control. It should be pointed out here that 

 the glands of the body other than the digestive glands are also subject to 

 nervous or hormone control according to the promptness with which they 

 are required to secrete. The lachrymal and sweat glands, and the venom 

 glands of reptiles, for example, are practically entirely under nervous 

 control, whereas most of the ductless glands, with the exception of the 

 adrenals, are mainly under the influence of hormones. 



The exact nature of the nervous control of glandular function has, 

 therefore, been most extensively studied in the salivary glands, and that 

 of the hormonic in the pancreas. With regard to the salivary glands, 

 the following points are of importance: Their nerve supply comes from 

 two sources: the cerebral autonomic, and the sympathetic autonomic 

 (see page 877). These two nerve supplies have usually an opposite influ- 

 ence on the secretory activity of the glands, and very frequently also on 

 the vascular changes that accompany secretory activity. 



On account of its ready accessibility, the submaxillary gland in the 

 dog and cat has been most thoroughly investigated. The cerebral auto- 

 nomic nerve in this case is represented by the chorda tympani, and the 

 sympathetic autonomic by postganglionic fibers that run from the 

 superior cervical ganglion to the gland along its blood vessels (Fig. 145). 

 After tying a cannula into the duct of the gland, it will be found in the 

 dog that stimulation of the chorda tympani produces an immediate and 

 abundant secretion of thin watery saliva accompanied by a marked 

 dilatation of the blood vessels of the gland. 



That this secretion is not dependent on the vasodilatation is easily 

 shown by repeating the experiment after administering a sufficient dose 

 of atropine to paralyze the secreting cells. Stimulation of the nerve then 

 produces a vasodilatation but no secretion. The same conclusion is 

 arrived at by an experiment of an entirely different nature ; namely, by 

 observing the pressure produced in the duct when the chorda tympani is 

 stimulated. This pressure rises considerably above that in the arteries, 

 so that no such physical process as mere diffusion can be held accountable 



