898 



BIRD. 



for any length of time, unless the hold were above 

 the body, and kept by the pressure of that. All 

 perchers by the claws for repose perch with the back 

 undermost, or at least in such a way as that the weight 

 hangs upon the claws as upon hooks, the form of 

 which is maintained either by stops of bone, or by 

 the weight of the dependent body pulling the tendons, 

 and the firmness is given wholly by the latter. 



Those provisions for the secure maintenance of 

 their place, in animals, arising from structure merely, 

 and not requiring any muscular exertion, or other 

 effort on their part, which can in any way fatigue 

 them by its continuance, are among the most striking 

 instances of that superiority of design and adaptation 

 of which every thing that nature produces is an 

 example. We know, from our own case, that if even 

 a single muscle of the system, except those which 

 are employed in immediately carrying on the vital 

 functions, be exerted, not one part of the body, far 

 less the whole of it, can be in that state of repose 

 which is necessary for enabling the active system to 

 recover its tone. With us, an easy position on the 

 couch, in the chair, on the grassy sod, or on the bare 

 earth, according to the habit, and the need that there 

 is for rest, is the position for repose ; and if we were 

 to attempt to sleep, clinging by the hands, or in any 

 other way in which our mere weight is not the means 

 by which we retain our position, we should assuredly 

 fall. But birds have to repose in all varieties of 

 situation, and their means of keeping their places are 

 increased and varied accordingly. 



In their alimentary system birds are variously 

 formed, according to the general nature of their food. 

 If that food is wholly animal, their stomachs are 

 simple and membranous, and their intestinal canals 

 short, and without ciBca. If wholly vegetable, the 

 stomach is more complicated: one part, which is 

 styled the craw, and which is little else than an en- 

 largement of the inferior part of the gullet, being a 

 sort of receptacle into which they can take much 

 more food than the bare stomach can at once receive 

 for the purpose of digestion. In this respect it bears 

 some resemblance to the paunch, or first stomach of 

 the ruminant mammalia ; but as birds have no chewing 

 apparatus in the mouth, the food taken into the craw 

 does not return to the mouth, but proceeds into the 

 stomach portion by portion, as the progress of di- 

 gestion requires. The craw of these birds is generally 

 placed in front of the sternal bones, so that it can 

 admit of distension by a large quantity of food without 

 disturbing the other viscera, or requiring any enlarge- 

 ment of the bony cavity of the body to admit of its 

 distension. Even this position of the craw, which at 

 first seerns a very simple matter, is attended with con- 

 siderable advantage to the bird. The habits of birds 

 require that the size of their body should be as 

 small as possible, and that the capacity of it, as 

 determined by the bones of the trunk, should be as 

 uniformly the same as possible. Now, if this craw, 

 which, as a magazine of food, is sometimes empty and 

 sometimes much distended, were lodged within the 

 bones, it would either press in an inconvenient manner 

 upon the other viscera, when full, or it would leave a 

 vacant space in the cavity of the body when empty. 

 Either of these would be attended with inconvenience, 

 as the pressure on the viscera would disturb the 

 functions, and render the bird inactive, and the 

 additional space would require an enlargement in the 

 cross section, and thus cause the bird to oppose more 



resistance to the air in proportion to its power of 

 flying, and thereby to the same extent diminish that 

 power. 



The avoiding of the same inconveniences requires 

 the existence both of the craw and the true digestive 

 stomach as separate organs. Vegetable food contains 

 much less animal nourishment than animal food, and 

 in many birds there has to be as much and as severe 

 labour used in the procuring of it. The waste of 

 substance, and the necessity of food, is in all animals 

 in proportion to their activity ; therefore active 

 vegetable-feeding birds must take a proportionably 

 greater quantity of food than animal feeders of the 

 same activity. This is well exemplified in the case 

 of pigeons, and also in that of all the finch tribe 

 which are active birds, and also very vegetable in 

 their feeding. 



If this excess of food were taken into the stpmach 

 at once, and that stomach lodged within the cavity of 

 the body, the same inconveniences would arise as if 

 the craw were situated there ; and the stomach itself, 

 as containing a greater quantity at once, would either 

 require to be more powerful, or it would perform the 

 function of digestion less efficiently. But, with the 

 addition of the craw as a reservoir, the gizzard, or 

 muscular stomach, which may be said to perform the 

 operations of both mastication and digestion, can 

 afford to be smaller ; and this, besides making the 

 organ more efficient, with the same exertion of 

 muscular energy, leaves room for the greater length 

 of the intestinal canal, and for the ca?cal appendages 

 which are necessary in extracting the chyle from 

 the vegetable aliment. 



Thus we see that, in the structure of the alimentary 

 system in these birds, there is the same evidence of 

 wisdom of purpose, and perfection of execution, as in 

 that of their external organs. These advantages are 

 not confined to the mere convenience of that system 

 in which they are found. The advantage goes to all 

 the habits of the bird. In birds, as well as in mam- 

 malia, the vegetable feeders are the chief prey of the 

 carnivorous ones, and thus they require to be on the 

 alert, and capable of making their escape at all times 

 when their enemies are abroad. But if they took 

 into the true stomach, and had subjected to the 

 process of digestion at once the great quantity of 

 food which is required for their support, they would, 

 as in the case with birds of prey, be incapable of 

 flight, or, at all events, indisposed to it, after a hearty 

 meal. Their food they must take during the day, 

 and not after dusk, and yet it seems that they require 

 the digestive process to be going on constantly ; and 

 thus the moderately-sized gizzard, and the supply in 

 the craw, are remarkably well adapted to their habits ; 

 so that, after taking an abundant supply of food, they 

 are nearly as fit for flight as when they are empty. 



The carnivorous birds, especially vultures, and 

 other species which feed upon, the carcases of the 

 larger animals, when these are killed by other 

 casualties, are remarkable for the opposite structure 

 of the alimentary system, and for indolence after a 

 full meal. Their stomachs are membranous and 

 simple, and their intestines short, and without any 

 cffical appendages, so that the process of digestion is 

 with them a much more simple matter. But when 

 they have made a hearty meal, the full stomach presse? 

 upon and retards the action of the other parts of their 

 viscera, or, at all events, the process of digestion 

 monopolises the whole of their energy ; so that, 



