VOMITING. 517 



large intestine. Each segment contains circular muscular fibres, areo- 

 lar tissue, and two layers of mucous membrane, continuous with each 

 other at the free edge of the segment. The mucous membrane of the 

 surface turned towards the ileum is covered with villi; whilst that 

 turned towards the large intestine is destitute of those processes. 



Notwithstanding the active absorption which takes place along the 

 whole length of the small intestine, its contents retain a pulpy consis- 

 tence. By the peristaltic action of the circular muscular fibres, they 

 are pressed through the slit-like opening between the segments of the 

 ileo-caecal valve, having passed which, they are received into the pouch 

 of the caecum, which now supports their weight, whilst the lateral 

 position of the valve relieves it from pressure. Once having passed 

 the valve, no force exerted upon the intestinal contents can ever re- 

 turn them into the small intestine, the valve-segments, owing to the 

 elasticity and muscularity of all the parts, meeting closely together 

 under every change of dimensions. Even after death, when these 

 parts are removed from the body, water, poured into the colon, is, owing 

 to the closure of the valve-segments, completely prevented from pass- 

 ing into the ileum. In the caecum, the still pulpy residue of the 

 processes of digestion and absorption undergoes further inspissation, 

 perhaps also further digestion. By the combined and comparatively 

 slow peristaltic action of the longitudinal bands between the sacculi, 

 and of the circular fibres spread over the sacculi themselves, it is 

 pressed upwards into the ascending colon, and, in like manner, on- 

 wards from sacculus to sacculus of the ascending, transverse, and 

 descending colon, and, yet more slowly, through the sigmoid flexure 

 of the colon into the rectum, acquiring, by gradual absorption, as it 

 descends, its final state of inspissation, before it is expelled from the 

 body. Undue pressure, or weight, is prevented by the sigmoid curve 

 of the intestine. The external and internal sphincters, which close 

 the rectum below, are kept contracted, in a reflex manner, by the 

 action of the spinal cord. In defecation, these muscles are relaxed, 

 whilst the intestine above contracts, the action being aided by expul- 

 sive efforts on the part of the abdominal and expiratory muscles gen- 

 erally, the diaphragm being fixed after closure of the glottis. The 

 fibres surrounding the cardiac opening of the stomach must also close 

 that aperture simultaneously. 



Vomiting. 



In the ordinary exercise of their functions in the digestive process, 

 the oesophagus, the stomach, and the intestinal canal, manifest, as we 

 have seen, movements of the so-called peristaltic kind, due to succes- 

 sive wave-like contractions of their muscular walls, excited partly 

 through the nervous system, but also, especially in the case of the 

 intestines, by the direct stimulation of the food upon them. But, 

 under certain conditions, an undue local stimulation of the muscular 

 fibres, or some wider irritation, operating through the nervous system, 

 excites these organs to reversed, or so-called anti-peristaltic action, 

 often accompanied with powerful associated movements of the abdom- 



