THE MECHANICAL FACTORS OF DIGESTION 



415 



The movements of the stomach and its sphincters may be modified 

 by the action of the vagus nerve. Stimulation of the peripheral end 

 of the vagus nerve causes contraction of the cardiac sphincter, but 

 after the intravenous administration of atropine the action of the 

 same nerve on this sphincter becomes inhibitory. On vagal stimula- 

 tion, the tone of the stomach increases, and the peristaltic waves 

 augment. In some cases there may be a preliminary transitory 

 inhibitory effect. The effect on the pyloric sphincter is uncertain. 

 It is probably dependent upon the condition, at the moment of stimu- 

 lation, of the local nervous mechanism which controls that orifice. 

 Sometimes it opens, and sometimes it closes. The sympathetic nerve 

 is generally believed to be inhibitory to the stomach, but this action 

 is open to question. 



^ro 



o Umbilicus. 



FIG. 211. SEGMENTATION OF THE SMALL INTESTINE IN MAN. (Hurst.) 



Vomiting is controlled by a nervous reflex, and can be induced 

 either centrally or peripherally. Tickling the throat between the 

 pharynx and top of the oesophagus will often induce it. The effector 

 fibres run in the vagus nerve. The cardiac orifice is relaxed, the body 

 of the stomach rendered flaccid and dilated, and the tone of the pylorus 

 increased. A strong contraction occurs at the incisura angularis, 

 dividing the stomach into two separate portions. The stomach 

 contents are then voided by a simultaneous contraction of the dia- 

 phragm and the muscles of the belly wall. As vomiting proceeds, the 

 stomach wall contracts down on the remaining contents; otherwise 

 the stomach is essentially passive during the act of vomiting. 



The Movements of the Small Intestine. On observing by the 

 X-ray method a length of small intestine, it is noticed that the first 

 movement, after food enters it, is the sudden division of the length 

 into a number of small ovoid segments of almost equal size (Fig. 209). 

 A moment later, >nese small segments themselves divide into two, the 



