36 COATS OF THE ALIMENTARY CANAL. 



then suddenly contracted, it continues onward with va- 

 rious degrees of capacity. In man, its length is five or 

 six times that of the whole body. In the inferior ani- 

 mals, we find it larger, longer, and more complicated 

 in those whose food is entirely of a vegetable nature, 

 than in those which are supported by an animal nour- 

 ishment exclusively. 



Emily. fs not this because food which has just for- 

 med part of an animal body requires less preparation to 

 fit it for nutrition, than that which is of a vegetable na- 

 ture ? 



Dr. B. Your explanation is probably correct. The 

 walls of the alimentary canal, differ in its different 

 parts, in appearance and capacity, and are found through- 

 out their whole extent, to be formed by two membra- 

 nous coats, which though they do not together amount 

 jto the thickness of one eighth of an inch, are exceedingly 

 strong, and by their toughness and elasticity are capa- 

 ble of resisting a great degree of force. The inner of 

 these coats or that with which the food comes into con- 

 tact, is called the mucous coat. Its appearance is dif- 

 ferent in different parts of the alimentary canal, being 

 thin and smooth in the mouth which it completely lines, 

 thicker and more loosely applied in the stomach, and 

 collected into transverse wrinkles, or folds in the intes- 

 tines. . It is formed by cellular tissue and provided with 

 numerous minute glands from which a fluid is poured 

 out to keep it constantly moist. The next coat is cal- 

 led the muscular, and is formed by muscular fibres run- 

 ning in two different directions, one layer being longitu- 

 dinal, the other, circular. The third coat called the 

 serous or peritoneal and external to these two, is found 

 only in some portions of the alimentary canal. 



Emily. I am afraid I shall obtain but a poor idea of 

 these parts that form the alimentary tube without some 

 plate or model. 



Dr. B. And yet, I suspect, you have seen more 

 than once, what is better than either plates or models 

 the object itself. 



