1012 DIGESTION 



traction of the.upper constrictor and the approximation of the pharyn- 

 geal wall and pillars of the fauces, the force of the ejection is sometimes 

 so great that this hindrance is overcome. 



The act of vomiting is controlled by a special center situated in the 

 medulla oblongata. On the efferent side, it is connected with the 

 different muscles mentioned previously and, on the afferent side, with 

 various local and general receptors. Thus, it is a well-known fact that 

 the sight and smell of offensive food or objects may serve as ade- 

 quate exciting causes, and that it may also be evoked by the mechanical 

 stimulation of the fauces and pharynx, as well as by irritations of the 

 gastric and intestinal mucosa. Even extragastric stimuli in the form 

 of abdominal tumors and the gradually enlarging uterus of pregnancy 

 may instigate it. Apomorphin produces its characteristic effect by 

 a direct stimulation of the vomiting center. 



The Innervation of the Gastric Musculature. — The stomach is 

 wholly under the control of the autonomic nervous system, the distal- 

 most fibers of which are expanded between its circular and longitudinal 

 layers of muscle tissue into the plexuses of Meissner and Auerbach. 

 This organ, therefore, is well equipped with a local reflex mechanism 

 which is destined to. regulate its various motor activities. Thus, it 

 has been ascertained that the excised stomach, if kept under proper 

 conditions of moisture and temperature, may be made to contract 

 upon local stimulation, and may even show a spontaneous activity. 

 Under normal conditions, this peripheral sympathetic mechanism 

 is connected with the central nervous system by way of the two vagi 

 and splanchnic nerves. The former terminate in the vicinity of the 

 cardia in two ramifications which are known as the ventral and dorsal 

 gastric plexuses. From here fibers pass over to the left suprarenal 

 plexus of the splanchnic system, as well as to the neighboring region 

 of the lesser curvature. At the present time, however, no evidence 

 is at hand to show that these plexuses also send fibers directly to the 

 greater curvature or to the region of the pylorus. The latter seem to 

 derive their innervation from the celiac ganglion of the solar plexus 

 by way of the celiac and splenic plexuses. 



It can no longer be doubted that the vagi nerves embrace musculo- 

 motor nerves for this organ. This is proved by the fact that their 

 stimulation above the diaphragm evokes well marked contractions 

 which involve chiefly its pyloric segment and possess all the charac- 

 teristics of regular peristaltic waves. While it is commonly stated 

 that the splanchnic nerves exert an inhibitory influence upon the 

 movements of this organ, it cannot be said that this view possesses a 

 satisfactory experimental basis. Inasmuch as these nerves contain 

 powerful vasoconstrictors, the relaxing effect sometimes observed 

 upon their stimulation, may in reality be caused by a diminution in 

 the gastric blood-supply.^ Since the musculo-motor function of the 

 vagi nerves has been well established, it may be said that their nuclei, 

 1 Burton-Opitz, Pfliiger's Archiv, xxxv, 1910, 205. 



