DIGESTION. 259 



diminished by the contraction of the diaphragm, and subsequently of 

 the abdominal muscles. The experiments and observations, however, 

 which are supposed to confirm this statement, only show that the con- 

 traction of the abdominal muscles alone is sufficient to expel matters 

 from an unresisting bag through the oesophagus; and that, under very 

 abnormal circumstances, the stomach, by itself, cannot expel its con- 

 tents. They by no means show that in ordinary vomiting the stomach 

 is passive ; and, on the other hand, there are good reasons for believing 

 the contrary. 



It is true that the facts are wanting to demonstrate with certainty 

 this action of the stomach in vomiting; but some of the cases of fistu- 

 lous opening into the organ appear to support the belief that it does take 

 place; and the analogy of the case of the stomach with that of the other 

 hollow viscera, as the rectum and bladder, may be also cited in confirm- 

 ation. 



The muscles concerned in the act of vomiting are chiefly and prima- 

 rily those of the abdomen; the diaphragm also acts, but usually not as 

 the muscles of the abdominal walls do. They contract and compress 

 the stomach more and more towards the diaphragm; and the diaphragm 

 (which is usually drawn down in the deep inspiration that precedes each 

 act of vomiting) is fixed, and presents an unyielding surface against 

 which the stomach may be pressed. The diaphragm is, therefore, as a 

 rule passive, during the actual expulsion of the contents of the stomach. 

 But there are grounds for believing that sometimes this muscle actively 

 contracts, so that the stomach is, so to speak, squeezed between the de- 

 scending diaphragm and the retracting abdominal walls. 



Some persons possess the power of vomiting at will, without applying 

 any undue irritation to the stomach, but simply by a voluntary effort. 

 It seems also, that this power may be acquired by those who do not natu- 

 rally possess it, and by continual practice may become a habit. There 

 are cases also of rare occurrence in which persons habitually swallow their 

 food hastily, and nearly unmasticated, and then at their leisure regurgi- 

 tate it, piece by piece, into their mouth, remasticate, and. again swallow 

 it, like members of the ruminant order of Mammalia. 



The various nerve-actions concerned in vomiting are governed by a 

 nerve-centre situate in the medulla oblongata. 



The sensory nerves are the fifth, glosso-pharyngeal and vagus prin- 

 cipally; but, as well, vomiting may occur from stimulation of sensory 

 nerves from many organs, e. g., kidney, testicle, etc. The centre may 

 also be stimulated by impressions from the cerebrum and cerebellum, 

 so called central vomiting occurring in disease of those parts. The 

 efferent impulses are carried by the phrenics and other spinal nerves. 



