ON THE DIGESTIVE FUNCTION, &c. 133 



distinguished from the rest belonging to the animal, not less by their colour 

 than by their prodigious strength ; and the internal cuticle with which they 

 are covered is peculiarly callous, and often becomes quite horny from pres- 

 sure and friction. 



The gizzard of grazing birds, as the goose and turkey, differs in some de- 

 gree in the formation of its muscles from that of granivorous. They have 

 also " a swell in the lower part of the esophagus, which answers the purpose 

 of a reservoir, in which the grass is retained, macerated, and mixed with the 

 secretions poured out by the* glandular surfaces surrounding it, in this respect 

 corresponding to the first and second stomachs of ruminating animals, in 

 which the grass is prepared for mastification,"* though essentially lighter. 



In most birds, indeed, we meet with an approach towards this, in a cavity 

 situated above the muscular stomach, and called the crop, or craw. This 

 first receives the food from the mouth, and slightly softens it by a mucous 

 fluid secreted from its interior ; and thus prepared, a part of it is given back 

 to the young, where there are young to partake of it, and the rest is sent to 

 the gizzard or proper stomach, whose muscular mechanism, in conjunction 

 with its gastric juice, soon comminutes it into the most impalpable pulp. 

 There are several kinds, ho.wever, that, like the ostrich, endeavour to assist 

 the muscular action by swallowing pebbles or gravel ; some of which find 

 this additional aid so indispensable, that they are not able to digest their food, 

 and grow lean without it. Spallanzani attempted to prove that these stones 

 are of no use, and are only swallowed by accident; but their real advantage 

 has been completely established by Mr. J. Hunter, who has correctly ob- 

 served, that the larger the gizzards, the larger are the pebbles found in them. 

 In the gizzard of a turkey he counted two hundred ; in that of a goose, a thousand. 



Reaumur and Spallanzani have put the prodigious power of this muscular 

 stomach to the test, by compelling geese and other birds to swallow needles, 

 lancets, and 'Other hard and pointed substances ; which, in every experiment, 

 were found, a few hours afterward, on killing and examining the animal, or 

 on its regorging them, to be broken off and blunted, without any injury to 

 stomach whatever. 



Yet, as all animals are not designed for all kinds of food, neither the force 

 of the strongest muscular fibres, nor the solvent power of the most active 

 gastric juice, will avail in every instance. The wild-boar and the vulture 

 devour the rattlesnake uninjured, and fatten upon it ; but there are many 

 kinds of vegetables which neither of these are capable of digesting. The 

 owl digests flesh and bone, but cannot be made to digest grain or bread; and 

 in one instance died, under the experiments of Spallanzaui, when confined to 

 vegetable food. The falcon seems as little capable of dissolving vegetables; 

 yet the eagle dissolves bread and bone equally ; and wood-pigeons may, in 

 like manner, be brought to live, and even to thrive, on flesh meat. The pro- 

 cellaria pelagica, or stormy petrel, lives entirely on oil, as the fat of dead 

 whales and other fishes, whenever he can get it ; and if not, converts every 

 thing he swallows into oil. He discharges pure oil from his mouth at objects 

 that offend him ; and feeds his young with the same substance. This is the 

 most daring of all birds in a tempest, though not more than six inches long. 

 As soon as the clouds begin to collect, he quits his rocky covert, and enjoys 

 the gathering and magnificent scenery: he rides triumphantly on the whirl- 

 wind, and skims with incredible velocity the giddiest peaks and deepest 

 hollows of the most tremendous waves. His appearance is a sure presage 

 of foul weather to the seaman. 



There are some tribes of animals that appear capable of subsisting on water 

 alone, and a few on mere air, incapable as these substances seem to be, at 

 first sight, of affording any thing like solid nutriment. Leeches and tadpoles 

 present us with familiar proofs of the former assertion, and there are various 

 kinds of fishes that may be added to the catalogue. Rondelet kept a silver 

 fish in pure wat,er alone for three years ; and at the end of that period it had 



* Home, On the Gizzards of Grazing Birds, Phil. Trans. 1810, p. 183 



