732 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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

 through which the reflex is effected Hes 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. 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. 



