BIRD. 



465 



depress and raise the wing in flight, are applied ; that, 

 (though its tendon is longer, from passing through 

 the pulley) the body of the middle pectoral is shorter 

 than that of the grand, having a lighter labour to 

 perform, but having to perform it more quickly ; and 

 that birds which have this habit have the wings, and 

 indeed the whole frame, more solid and heavy than 

 other birds. But we find that in those birds which 

 make no rush, but capture their food in the air at the 

 speed of ordinary tlight, as in the swallows and bee- 

 eaters, or which flit about, not in chace of their food, 

 but simply in quest of it, as the humming-birds, the 

 sternum is narrowed in these parts so as to adapt it to 

 the action of shorter muscles, without the load of an 

 additional portion of tendon ; and that, in proportion 

 as the habit of these birds requires the action of the 

 wing to be more powerful, the keel is deepened to 

 admit the insertion of a greater number of muscular 

 fibres, or fasciculi. 



In birds which fly with equal speed on a level, or 

 an ascent or a descent, the raising of the wing with 

 great rapidity and considerable force is much more 

 necessary than in those which get their most rapid 

 motions upon a descent ; and it is especially necessary 

 in the group of birds under consideration, which 

 always have to hold the wings in readiness for being 

 expanded, while they are adhering by the feet to 

 upright or sloping surfaces. Accordingly, they have 

 the middle pectoral larger in its volume, and also in 

 the surface to which its origin is attached, in pro- 

 portion to the grand pectoral, than birds which have 

 a descent in their rushing motions. 



The different style and direction of the flight are 

 not, however, the only circumstances to which the 

 muscles have to be accommodated. The bird requires 

 to be able to make its way through the air the 

 more easily the greater length of time that it spends 

 on the wing, and this easiness of the motion has 

 reference to the respiratory and circulating systems 

 of the bird, as well as to the fatiguing or not fatiguing 

 of the muscles. Now, if we may judge from the 

 analogies, which, as far as we can trace them in all 

 living nature, agree, we must conclude, that, up to a 

 certain rate of speed, the short muscle, moving the 

 comparatively light member, is that which can con- 

 tinue longest in action without fatigue to the muscles, 

 or derangement to any part of the system ; but that, 

 beyond a certain rate of motion, the long muscle is 

 the best for speed, though the continuation of its 

 action is shorter than that of the other. We find 

 this in the limbs of the mammalia, and in the feet of 

 birds, whether used in walking or in swimming, as 

 well as the wing, and the principle is one of considerable 

 importance in the economical use of animal power. 



The hoopoe, which we shall select as a specimen of 

 anisodactylic feet, with the habit different the most 

 from that of the swimming birds, has the tarsi much 

 longer and stronger, and instead of walking with 

 difficulty on the ground, it walks with a sort of strut, 

 as if it had more power there than what is absolute!}' 

 necessary for moving it along. The following figures 

 represent the sternal apparatus of the natural size. 



It will be seen that the general form of the sternum 

 is nearly t'he same as that of the humming-bird, but 

 that there are very marked differences in particular 

 places ; the keel is lower, but the anterior part of the 

 sternum is better developed ; the lateral processes are 

 much larger, and there is a process in front which 

 approaches the furcal bone. The posterior angles 

 NAT. HIST. VOL. I. 



are weakened or rendered flexible by two deep 

 notches, and there are only four ribs on each side. 



Hoopoe. 



Thus the body of the bird is much more yielding 

 than that of the humming-bird, and bears nearly the 

 same relation to it as that of the swift bears to that 

 of the goat-sucker. The anterior part of the keel is 

 much rounded, and the furcal bone is much bent 

 toward the coracoids, though it does not proceed so 

 straight backward as in the parrot tribe. There is, 

 however, a considerable resemblance in the anterior 

 part of the sternum to that of the parrots, while in 

 the posterior part there is a slight approach to the 

 character of the ground birds. The style of their 

 flight corresponds. It is unequal and jerking, per- 

 formed with much flutter of the wings, as if the down 

 stroke of these were not very effective, which might 

 be inferred from the rounding away of the anterior 

 part of the keel and the feebleness of the furcal 

 bone. But they can raise the wing with great quick- 

 ness ; and. though their flight is far from being so 

 graceful as their form, they make way with con- 

 siderable speed. They are birds of the margins of 

 the waters, and especially resort to rivers which are 

 subject to flooding; but they also sometimes hunt 

 for beetles by running along the bark of decayed 

 trees, and often choose the holes of these for nestling 

 places. 



Sterna of the Zygodactyli. Of these we shall give 

 as specimens, the sternal apparatus of the common 

 green woodpecker, as being chiefly a bank bird, and 

 resembling in some of its habits the anisodactylic 

 birds, and of the common jacko, or grey parrot, wriich 

 is one of the most scandent. The cut on next page, 

 that of the woodpecker, is of the natural size. 



This is a very peculiar sternum ; and upon looking 

 back to what is said of the foot, or, far better, by 

 watching the habits of the bird itself, it will be found 

 to be one of very peculiar action. The bird, while 

 it seeks its food in the holes of trees, or excavates 

 them in making a resting hole for its young, rests 

 chiefly with the sternum pressed against the troe ; 

 and it will be seen, upon examining the profile, that 

 the bones are fully as much suited for answering 

 this purpose as they are for flight, though the great 

 length of the sternum, and the production of the keel 

 for the whole of that length, though not very high at 

 any point, still enable the bird to use its wings with 

 considerable effect. 



The posterior angles of the sternum are divided by 

 two deep notches on each side, the processes between 

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