166 COMPEN^SAl-lOl^f* 



the grinding teeth complete at their leisure the trituration 

 \vhich is necessary, but which was before left imperfect. 

 I say the trituration which is necessary ; for it appears 

 from experiments that the gastric fluid of sheep, for example^ 

 has no effect in digesting plants, unless they have been 

 previously masticated ; that it only produces a slight ma- 

 ceration, nearly as common water would do in a like degree 

 of heat ; but that, when once vegetables are reduced to 

 pieces by mastication, the fluid then exerts upon them its 

 specific operation. Its first effect is to soften them, and to 

 destroy their natural consistency ; it then goes on to dis- 

 solve them ; not sparing even the toughest parts, such as 

 the nerves of the leaves.* 



I think it very probable that the gratification also of the 

 animal is renewed and prolonged by this faculty. Sheep, 

 deerj and oxen, appear to be in a state of enjoyment whilst 

 they are chewing the cud. It is then, perhaps, that they 

 best relish their food* 



II. In birds, the compensation is still more striking. 

 They have no teeth at all. What have they then to make 

 up for this severe want? I speak of granivorous and 

 herbivorous birds ; such as common fowls, turkeys^ ducks, 

 geese, pigeons, &/C. for it is concerning these alone that 

 the question need be asked. All these are furnished with 

 a peculiar and most powerful muscle, called the gizzard; 

 the inner coat of which is fitted up with rough plates, which, 

 by a strong friction against one another, break and grind 

 the hard aliment as eflfectually, and by the same mechani- 

 cal action, as a coffee-mill would do. It has been proved 

 by the most correct experiments, that the gastric juice 

 of these birds will not operate upon the entitle grain ; not 

 even when softened by water or macerated in the crop. 

 Therefore without a grinding machine within its body, 

 without the trituration of the gizzard, a chicken would 

 have starved upon a heap of corn. Yet why should a bill 

 and a gizzard go together ? Why should a gizzard never 

 be found where there are teeth 1 



Nor does the gizzard belong to birds as such. A giz- 

 zard is not found in birds of prey. Their food requires 

 not to be ground down in a mill. The compensatory con- 

 trivance goes no further than the necessity. In both class- 

 es of birds, however, the digestive organ within the body, 

 bears a strict and mechanic^ relation to the external in- 



^ Spal. Dis. HI. sec. 140. 



