THE INNER TISSUES OP ANIMALS. 325 



enlarged into a kind of pouch; and the inner surface of this 

 pouch begins to secrete juices that produce in the food a kind 

 of rude digestion. Again, stricture of the intestine, when it 

 arises gradually, is followed by hypertrophy of the muscular 

 coat of the intestine above the constricted part : the ordinary 

 peristaltic movements being insufficient to force the food 

 forwards, and the lodged food serving as a constant stimulus 

 to contraction, the muscular fibres, habitually more exercised, 

 become more bulky. The deduction from general principles 

 being thus inductively enforced, we cannot, I think, resist 

 the conclusion that the direct actions and reactions between 

 the food and the alimentary canal have been largely instru- 

 mental in establishing the contrasts among its parts. And 

 we shall hold this view with the more confidence on observ- 

 ing how satisfactorily, in pursuance of it, we are enabled to 

 explain one of the most striking of these differentiations, 

 which we will take as a type of the class. 



The gizzard of a bird is an expanded portion of the alimen- 

 tary canal, specially fitted to give the food that trituration 

 which the toothless mouth of a bird cannot give. Besides 

 having a greatly-developed muscular coat, this grinding- 

 chamber is lined with a thick, hard cuticle, capable of bear- 

 ing the friction of the pebbles swallowed to serve as grind- 

 stones. This differentiation of the mucous coat into a ridged 

 and tubercled layer of horny matter — a differentiation which, 

 in the analogous organs of certain Mollusca, is carried to the 

 extent of producing from this membrane cartilaginous plates, 

 and even teeth — varies in birds of different kinds, according 

 to their food. It is moderate in birds that feed on flesh and 

 fish, and extreme in granivorous birds and others that live 

 on hard substances. How does this immense modification of 

 the alimentary canal originate? In the stomach 



of a mammal, the macerating and solvent actions are united 

 with that triturating action which finishes what the teeth 

 have mainly done; but in the bird, unable to masticate, these 

 internal functions are specialized, and while the crop is the 



