370 HANDBOOK OF PHYSIOLOGY. 



are supposed to confirm this statement, only show that the contraction 

 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 contents. 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 facts are wanting to demonstrate with certainty this 

 action of the stomach in vomiting; but some of the cases of fistulous 

 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 pri- 

 marily 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 toward 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 

 nrhich 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 nat- 

 urally 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 re- 

 gurgitate 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 situated 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. 



THE INTESTINES. 



The Intestinal canal is divided into two chief portions, named from 

 their differences in diameter, the small and large intestine (fig. 215) 



