THE VISCERAL AND GUSTATORY APPARATUS 241 



The ordinary processes of digestion are carried on partly by 

 automatic activities of the organs without nervous control 

 (see p. 224), and partly by the intrinsic sympathetic nervous 

 system of the digestive organs. Throughout the length of the 

 digestive tract there are two sympathetic ganglionated plexuses. 

 One of these is located between the muscular coats of the stom- 

 ach and intestine, known as the myenteric or Auerbach's plexus; 

 the other lies immediately under the lining mucous membrane 

 and is known as the submucous or Meissner's plexus. It has 

 been shown physiologically that the local reflexes concerned in 

 the typical peristaltic contractions of the digestive tube are 

 effected chiefly by the myenteric plexus. Accordingly, this 

 reflex is called by Cannon the myenteric reflex. 



The entire digestive mechanism (like most of the other visceral 

 systems) may also be influenced indirectly by nervous impulses 

 arising in the cerebral cortex, though these organs are not under 

 direct voluntary control. It is well known that the digestive 

 processes are especially sensitive to emotional states, pleasurable 

 experiences promoting digestion and painful or disagreeable emo- 

 tions inhibiting it. These facts can be studied on laboratory ani- 

 mals under experimental conditions (Cannon). A large amount 

 of information regarding the physiology of digestion has recently 

 been gathered by Carlson from the study of a man with an arti- 

 ficial opening into the stomach (gastric fistula), permitting 

 direct observation of the stomach at all times. 



The salivary glands are excited to secretion from two nuclei 

 of the medulla oblongata, the superior salivatory nucleus (Figs. 

 71, 114), whose preganglionic fibers go out with the VII cranial 

 nerve for the sublingual and submaxillary salivary glands, and 

 the inferior salivatory nucleus (Figs. 71, 73, 114), whose fibers 

 go out with the IX nerve for the parotid gland. The secretion 

 of saliva may be produced either as a simple reflex from the 

 presence of food in the mouth through the gustatory nerves and 

 fasciculus solitarius, or as so-called psychic secretion excited by 

 the sight or thought of food. All of the digestive secretions 

 are susceptible to this sort of indirect excitation, as, indeed, are 

 most other processes which are under the control of the cerebro- 

 spinal visceral nervous system. These visceral reactions, in 

 their turn, are reported back to the central nervous system and 



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