388 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



stomach firmly contracted so as to drive the contents toward the dilated 

 cardiac portion. The act of vomiting is in fact a complex reflex movement 

 into which many muscles enter. The following events are described : The 

 vomiting is usually preceded by a sensation of nausea and a reflex How of 

 saliva into the mouth. These phenomena are succeeded or accompanied by 

 retching movements, which consist essentially in deep spasmodic inspirations 

 with a closed glottis. The effect of these movements is to compress the 

 stomach by the descent of the diaphragm, and at the same time to increase 

 decidedly the negative pressure in the thorax, and therefore in the thoracic 

 portion of the oesophagus. During one of these retching movements the 

 act of vomiting is effected by a convulsive contraction of the abdominal 

 wall that exert- a sudden additional strong pressure upon the stomach. 

 At the same time the cardiac orifice of the stomach is dilated, possibly 

 by an inhibition of the sphincter, aided it is supposed by the contrac- 

 tion 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 oesophagus, 

 the negative pressure in the latter probably assisting in the act. The pas- 

 sage 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 it-elf. During the ejection of the contents 

 of the stomach the glottis i- kepi 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 aud 

 be ejected partially through the nose. 



Nervous Mechanism <>f 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 [tarts 

 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 uerves of the gastric mucous membrane is the most common 

 cause of vomiting. This effecf may result from the product- 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 phrenic-, and the spinal nerves 

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

 in- centre in which the afferent impulses are received and through which 



