DIGESTIVE CANAL. 



27 



With respect to the causes of nausea they 

 may be reduced to two heads ; those that act 

 immediately on the stomach, and those that 

 act, in the first instance, on the system at large. 

 Of the first class the most active in their opera- 

 tion are the medicinal substances which are 

 specifically styled emetics, from their peculiar 

 tendency to produce nausea and subsequent 

 vomiting. Besides these certain kinds of food, 

 or food of any description, if it remain in an 

 undigested state, and various substances of an 

 acrid or stimulating nature frequently produce 

 nausea and vomiting. In the second class of 

 causes we have to enumerate various circum- 

 stances, which act upon parts of the body, 

 sometimes very remote from the stomach, but 

 which, either by direct nervous communica- 

 tion, by sympathy, or association, produce the 

 effect in question. One of the most powerful 

 of these is the motion of a vessel at sea, giving 

 rise to the well-known and most distressing 

 sensation of sea-sickness, certain morbid affec- 

 tions of the brain, particular odours and flavours, 

 renal and biliary calculi, hemiae or other affec- 

 tions of the intestinal canal, and lastly, certain 

 causes which can act only through the medium 

 of the mind or imagination. These various 

 circumstances, although so extremely different 

 in their nature and origin, agree in producing 

 a similar effect on the stomach, which may be 

 explained by referring to the nervous com- 

 munications which exist between the organ 

 and every part of the system, and more espe- 

 cially with the other abdominal viscera and the 

 brain.* 



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(J. Bostock.) 



DIGESTIVE CANAL (Comp. Anat.) 

 The digestive canal is that cavity of the body 

 vrhich is destined to receive the food of animals 

 and to retain it until its nutritious part has been 

 separated or absorbed. It is termed also the 

 alimentary or the intestinal canal. As it is the 

 part into which foreign matter is first conveyed 

 for the nutriment of the system, its forms and 

 structure are most intimately related to the kind 

 of food, and consequently to the living habits 

 and instincts, and the whole mechanism of 

 animals. The most universal organs in the 

 animal kingdom are the digestive, and most of 

 the others may be considered as secondary or 

 subservient to these. The lowest animals pre- 

 sent us with no other organs than those sub- 

 servient to digestion, and almost all the organs 



