674 PHYSIOLOGY OF DIGESTION AND SECRETION. 



muscles, and usually the nasal chamber is likewise shut off from the 

 pharynx by the contraction of the posterior pillars of the fauces on 

 the palate and uvula. In violent vomiting, however, the vomited 

 material may break through this latter barrier and be ejected 

 partially through the nose. 



Nervous Mechanism of Vomiting. That vomiting is a reflex act 

 is abundantly shown by the frequency with which it is produced in 

 consequence of the stimulation of sensory nerves or as the result 

 of injuries to various parts of the central nervous system. After 

 lesions or injuries of the brain vomiting often results. Disagreeable 

 emotions and disturbances of the sense of equilibrium may produce 

 the same result. Irritation of the mucous membrane of various 

 parts of the alimentary canal (as, for example, tickling the back 

 of the pharynx with the finger); disturbances of the urogenital 

 apparatus, the liver, and other visceral organs; artificial stimula- 

 tion of the trunk of the vagus and of other sensory nerves, may all 

 cause vomiting. Under ordinary conditions, however, irritation of 

 the sensory nerves of the gastric mucous membrane is the most 

 common cause of vomiting. This effect may result from the prod- 

 ucts of fermentation in the stomach in cases of indigestion, or may 

 be produced intentionally by local emetics, such as mustard, taken 

 into the stomach. The afferent path in this case is through the 

 sensory fibers of the vagus. The efferent paths of the reflex are 

 found in the motor nerves innervating the muscles concerned in the 

 vomiting, namely, the vagus, the phrenics, and the spinal nerves 

 supplying the abdominal muscles. Whether or not there is a defi- 

 nite vomiting center in which the afferent impulses are received 

 and through which a co-ordinated series of efferent impulses is 

 sent out to the various muscles has not been satisfactorily deter- 

 mined. It has been shown that the portion of the nervous system 

 through which the reflex is effected lies in the medulla, and it may 

 be observed that the muscles concerned in the act, outside those 

 of the stomach, are respiratory muscles. Vomiting, in fact, consists 

 essentially in a simultaneous spasmodic contraction of expiratory 

 (abdominal) muscles and inspiratory muscles (diaphragm). It has 

 therefore been suggested that the reflex involves the stimulation of 

 the respiratory center or some part of it. Thumas claims to have 

 located a vomiting center in the medulla in the immediate neighbor- 

 hood of the calamus scriptorius. Further evidence, however, is 

 required upon this point. The act of vomiting may be produced 

 not only as a reflex from various sensory nerves, but may also be 

 caused by direct action upon the medullary centers. The action 

 of apomorphin is most easily explained by supposing that it acts 

 directly on the nerve centers. 



