THE ALIMENTARY CANAL. [CH. xxvn. 



is very untrustworthy, as it is also given by many other organic 

 substances. 



In medico-legal cases it is often necessary to ascertain whether 

 or not a red fluid or stain upon clothing is or is not blood. In 

 any such case it is advisable not to rely upon one test only, but 

 to try every means of detection at one's disposal. To discover 

 whether it is blood or not is by no means a difficult problem, 

 but to distinguish human blood from that of the common mam- 

 mals is practically impossible. 



CHAPTER XXVII. 



THE ALIMENTARY CANAL. 



THE alimentary canal consists of a long muscular tube lined 

 by mucous membrane beginning at the mouth, and terminating 

 at the anus. It comprises the mouth, pharynx, ossophagus, 

 stomach, small intestine and large intestine. Opening into it are 

 numerous glands which pour juices into it ; these bring about 

 the digestion of the food as it passes along. Some of the glands, 

 like the gastric and intestinal glands, are situated in the lining 

 mucous membrane of the canal ; others like the salivary glands, 

 liver, and pancreas, are situated at a distance from the main 

 canal, and pour their secretion into it by means of side tubes or 

 ducts. 



The events that take place in the alimentary canal are, (i) 

 digestion, that is the conversion of the food into soluble sub- 

 stances; and (2) absorption, that is the passage of these soluble 

 materials into the blood or lymph in the vessels of the wall of 

 the canal. 



Digestion is a series of chemical actions produced by the 

 digestive juices on the food. We shall therefore have to study 

 the composition of the food as a preliminary to the consideration 

 of their digestion. In addition to chemical processes, there are a 

 number of mechanical actions such as mastication, deglutition, 

 peristalsis, which we shall reserve for a separate chapter. 



In the present chapter we shall take the structure of the 

 alimentary canal, reserving, however, a detailed study of the 

 glands until we consider the action of their secretions. 



