178 NATURAL THEOLOGY. 



soon fill their craw ; but it does not immediately pass thence 

 into the gizzard : it always enters in very small quantities, 

 in proportion to the progress of trituration ; in like manner 

 as, in a mill, a receiver is fixed above the two large stones 

 which serve for grinding the corn ; which receiver, although 

 the corn be put into it in bushels, allows the grain to dribble 

 only in small quantities into the central hole in the upper 

 miU-stone. 



But we have not done with the alimentary history. 

 There subsists a general relation between the external or- 

 gans of an animal by which it procures its food, and the 

 internal powers by which it digests it. Birds of prey, by 

 their talons and beaks, are qualified to seize and devour 

 many species both of other birds and of quadrupeds. The 

 constitution of the stomach agrees exactly with the form of 

 the members. The gastric juice of a bird of prey, of an owl, 

 a falcon, or a kite, acts upon the animal fibre alone ; it will 

 not act upon seeds or grasses at all. On the other hand, 

 the conformation of the mouth of the sheep or the ox is 

 suited for browsing upon herbage. Nothing about these 

 animals is fitted for the pursuit of living prey. Accordingly 

 it has been found, by experiments tried not many years ago, 

 with perforated balls, that the gastric juice of ruminating 

 animals, such as the sheep and the ox, speedily dissolves 

 vegetables, but makes no impression upon animal bodies. 

 This accordancy is still more particular. The gastric juice 

 even of granivorous birds, will not act upon the grain while 

 whole and entire. In performing the experiment of digest- 

 ing with the gastric juice in vessels, the grain must be 

 crushed and bruised before it be submitted to the menstru- 

 um ; that is to say, must undergo by art, without the body, 

 the preparatory action which the gizzard exerts upon it 

 within the body, or no digestion will take place. So strict, 

 in this case, is the relation between the ofllces assigned to 

 the digestive organ — ^between the mechanical operation and 

 the chemical process. 



