CHAPTER VIII. 



DIFFERENTIATIONS AMONG THE INNER TISSUES OF 

 ANIMALS. 



§ 297. The change from the outside of the lips to their 

 inside, introduces us to a new series of interesting and in- 

 structive facts, joining on to those with which the last chapter 

 closed. They concern the differentiations of those coats of 

 the alimentary canal which, as we have seen, are physiologi- 

 cally outer, though physically inner. 



These coats are greatly modified at different parts; and 

 their modifications vary greatly in different animals. In 

 the lower types, where they compose a simple tube running 

 from end to end of the body, they are almost uniform in their 

 histological characters; but on ascending from these types, 

 we find them presenting an increasing variety of minute 

 structures between their two ends. The argument will be 

 adequately enforced if we limit ourselves to the leading 

 modifications they display in some of the higher animals. 



The successive parts of the alimentary canal are so placed 

 with respect to its contents, that the physical and chemical 

 changes undergone by its contents while passing from one 

 end to the other, inevitably tend to transform its originally 

 homogeneous surface into a heterogeneous surface. Clearly, 

 the effect produced on the food at any part of the canal by 

 trituration, by adding a secretion, or by absorbing its nutri- 

 tive matters, implies the delivery of the food into the next 

 part of the canal in a state more or less unlike its previous 



323 



