326 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



posed by the contraction of the longitudinal muscle-fibres of the oesophagus 

 and the oblique fibres of the muscular coat of the stomach. The stomach 

 contents are, therefore, forced violently out of the stomach through the oesoph- 

 agus, the negative pressure in the latter probably assisting in the act. The 

 passage through the oesophagus is effected mainly by the force of the contrac- 

 tion of the abdominal muscles; there is no evidence of antiperistaltic move- 

 ments on the part of the oesophagus itself. During the ejection of the contents 

 of the stomach the glottis is kept closed by the adductor 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 vomit- 

 ing, 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 abun- 

 dantly 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 equi- 

 librium 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, 

 artificial stimulation 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 products 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 fibres of the vagus. The efferent paths 

 of the reflex are found in the motor nerves innervating the muscles con- 

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

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

 ing centre 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 determined. It has been shown that the portion 

 of the nervous system through which the reflex is effected lies in the me- 

 dulla. But it has been pointed out that the muscles concerned in the act 

 are respiratory muscles. Vomiting in fact consists essentially in a simul- 

 taneous spasmodic contraction of expiratory (abdominal) muscles and inspi- 

 ratory muscles (diaphragm). It has therefore been suggested that the reflex 

 takes place through the respiratory centre, or some part of it. This view 

 seems to be opposed by the experiments of Thumas, 1 who has shown that 

 when the medulla is divided down the mid-line respiratory movements con- 

 tinue as usual, but vomiting can no longer be produced by the use of emetics. 

 Thumas claims to have located a vomiting centre in the medulla in the imme- 

 diate neighborhood of the calamus scriptorius. Further evidence, however, 

 1 Virchow's Archiv fiir pathologische Anatomic, etc., 1891, Bd. 123, S. 44. 



