DISEASES OF THE ALIMENTARY TRACT. 245 



sists of a long niuseular tube of unequal caliber at differ- 

 ent parts, covered with an elastic serous membrane exter- 

 nally, which tends to prevent distention, and internally 

 with a mucous membrane well supplied with blood (vascu- 

 lar) and abounding in glands which secrete the various 

 digestive juices. The muscular tissue is necessary for the 

 movements essential to push on the food from place to 

 place throughout the tract. Nervous structures are 

 found in abundance, which explains how pain is caused in 

 disease, and how the nervous centers affect and are affected 

 by the condition of the tract. (Fig. 15.) 



The solid organs, as the pancreas and liver, are really 

 parts of the digestive tract, as already explained, and are 

 connected with the main tube by outlets {ducts) for their 

 secretions, which are, of course, emptied into the in- 

 testine. 



From this brief description alone it must be apparent 

 that a disease affecting, say, the last part of the whole tract, 

 while localized to a certain extent, must also be expressed 

 elsewhere, even at the very commencement ; and, as a 

 matter of fact, the appearance of the tongue is a pretty 

 fair index to the condition of the stomach, intestines, etc. 



While the causes of the disturbances of the digestive 

 organs must be sought in anything that will seriously dis- 

 order any part of the body, it is more especially to that 

 which the animal eats and its quantity, or to those condi- 

 tions that affect the general health — as cold, wet, bad 

 ventilation, etc. — that we must look for an explanation. 

 Of acute forms of inflammation due to irritant poisons, 

 nothing will be said till later, when treating the subject 

 of poisoning. 



