714 PHYSIOLOGY OF DIGESTION AND SECRETION. 



at the beginning of the swallow proves inefficient, that is to saj^, 

 if it succeeds only in forcing the bolus into the top of the esophagus, 

 it may be assumed that this secondary or accessory mechanism 

 comes into play and provides for the transportation of the bolus 

 to the stomach. In this series of secondary reflexes, as in the more 

 complicated primary reflex, the vagus nerve forms a part of the 

 path and the reflex center lies, therefore, in the meduUa. 



Cardiac Sphincter. — At the cardia or cardiac orifice the cir- 

 cular layer of muscle acts as a sphincter. When the stomach is 

 empty this sphincter is probably relaxed. When the stomach 

 contains food the sphincter is thrown into tone, and thus shuts off 

 the gastric cavity from the esophagus. The sphincter receives 

 nerve-fibers from the vagus and from the sympathetic system 

 by way of the celiac ganghon. In addition, it is supphed from 

 the intrinsic plexus, plexus of Auerbach, which, as elsewhere 

 in the alimentary canal, seems to be capable of regulating the 

 movements of the musculature independently of the extrinsic 

 nerves. The tonic contraction of the sphincter that occurs 

 when the stomach contains food is maintained, according to 

 Cannon,* by a reflex through this intrinsic plexus, the stimulus 

 initiating the reflex being due to the acid present in the gastric 

 secretion. The peristaltic wave sweeping down the esophagus is 

 preceded, in the lower part of the esophagus at least, by a wave of 

 inhibition or relaxation. This latter phase affects the sphincter 

 and the adjacent musculature of the stomach and clears the path 

 for the bolus, so that the following wave of contraction may squeeze 

 it through the relaxed orifice without serious resistance. After 

 the passage of the bolus into the stomach the sphincter again falls 

 into tone until another peristaltic wave approaches from the 

 esophageal side. 



The Anatomy of the Stomach. — The stomach in man belongs 

 to the simple type as distinguished from the compound stomachs 

 of some of the other mammaha — the ruminating animals, for 

 example. Physiological and histological investigations have shown, 

 however, that the so-called simple stomachs are divided into parts 

 that have different properties and functions. The shape and rela- 

 tions of the stomach in man vary somewhat with the posture and 

 the amount of food. According to the conventional description 

 which is derived from dissection of the cadaver the organ occupies 

 an obliquely transverse position in the abdomen (Fig. 283, A). 

 It may be divided into two main portions, the fundus and the 

 antrum pylori or pyloric part; the line of division being indi- 

 cated on the smaller curvature by a fissure, the incisura angularis. 

 The fundus forms the dilated rounded portion extending from the 

 * Cannon, "The Mechanical Factors of Digestion," 1911. 



