This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(2041). The number of toes varies in different tribes of birds. Thus, in the Ostrich there are only two; in many genera there are three; in by far the greater number, four; and in the Gallinacea, five. But whatever the number of toes may be, the number of phalanges peculiar to each is remarkably constant: thus, the outermost toe always consists of five phalanges; the fourth toe invariably of four; the third as constantly of three; the second, when it exists, has only two; and, lastly, in the spur or innermost toe there is but a single piece.
(2042). So rapidly is the process of ossification accomplished in the skeleton of a bird, that it is only in very young animals the individual bones or elements composing the cranium can be identified, as the sutures speedily become obliterated: when, however, they are examined under very favourable circumstances, as for example in the skull of a young Ostrich, it is by no means difficult to distinguish them, and, by comparing them with those of other Vertebrata, to observe the modifications they have undergone both in form and position. In the annexed figure the principal pieces, both of the cranium and face, have been indicated by the same figures as were used to point out the correspondent bones in the skulls of the Crocodile (fig. 332) and the Serpent (fig. 335); so that it would be needless again to enumerate them in this place.
Fig. 356. Skull of a young Ostrich.
(2043). The muscular system of the feathered tribes, as far as activity and energy of motion is concerned, contrasts strikingly with that of the Vertebrata we have as yet considered; for, with the exception of Insects, no animals in creation are comparable to Birds, either in the vigour or velocity of their movements.
(2044). This perfection of muscular power, which is obviously essential to enable the bird to sustain itself in the air, and there perform the varied evolutions connected with flight, is no doubt mainly connected with the highly arterialized condition of the blood and the completeness of the respiratory apparatus. Neither is it uninteresting to observe that, while respiration was effected in the Insect by the admission of air to every part of the system by means of tracheal tubes, in Birds likewise the air freely penetrates to the interior of the body, and, as we shall afterwards find, is there most extensively diffused.
(2045). In the construction of the alimentary system, there are many interesting peculiarities to invite our notice. Their mouth constitutes the apparatus whereby the prehension of food is accomplished; it is in no instance provided with teeth, or adapted to masticate food, but forms a beak encased in a dense, horny sheath, which, from the varieties of form that it assumes in different genera, becomes adapted to very various purposes.
(2046). In the Rapacious tribes, for instance, the bill is a strong and formidable hook, calculated to tear in pieces the animals devoured. In Granivorous birds, it is a simple forceps for picking up the seeds of vegetables. In the Snipe and the Curlew it forms a probe, whereby insects are extracted from the soft and marshy ground. In the Parrot it is partially an assistant in climbing, as well as an organ for seizing food; and, not to mention innumerable other modifications, in the Flamingo and Duck tribes it constitutes a shovel, by the aid of which alimentary matters are obtained.
(2047). The sense of taste, even in these highly-gifted animals, is as yet but very imperfectly developed; and their tongue, instead of being soft and flexible, as in the Mammalia, is supported by one or two bony pieces, derived from the os hyoides (fig. 357), and covered with a horny sheath, obviously ill adapted to gustation, but simply assisting in the deglutition of food. We must not be at all surprised, therefore, if even in birds the tongue is convertible into various instruments assisting in the apprehension or preparation of nourishment: thus, in the Parrot it is a thumb opposable to the upper mandible, and eminently serviceable in holding and turning nuts or morsels of fruit; in the honey-eating tribes the tongue is armed at its extremity with a tuft of horny filaments, resembling a camel-hair pencil, which, being plunged into the bell of a flower, sucks up the nectar from the bottom; and in the Woodpecker it is absolutely converted into a harpoon, whereby the insect is speared in its lurking-place and dragged into the mouth.
Fig. 357. Hyoid apparatus of a bird.
(2048). In most birds, in consequence of the very small size of the cavity of the stomach, or gizzard, as it is generally called, some other receptacle for the aliment becomes indispensable; and accordingly various provisions have been made for lodging food in sufficient quantities in situations where it may be retained until the gizzard is ready to receive it. In birds that catch insects on the wing, this is most conveniently effected by dilating the fauces and upper part of the throat into a capacious chamber, wherein the insects as they are seized accumulate: this is remarkably the case in the Swifts. In the Pelican a very peculiar plan is adopted: the beak is amazingly prolonged, and beneath the lower jaw is suspended a white pouch, formed by the skin of the throat, wherein large quantities of fish may be contained and carried about. In other fishing birds the whole oesophagus is extraordinarily capacious, and will hold a considerable supply; but the most usual arrangement in birds requiring such a reservoir is the existence of a crop, or dilatation of some part of the gullet into a wide bag (ingluvies), wherein grain or other substances hastily picked up maybe stored preparatory to digestion.
After expanding into the crop in those birds that possess this cavity, the oesophagus again contracts to its former dimensions (fig. 358, a); but just before terminating in the gizzard, it again dilates to form a second but smaller cavity (b), called the proventriculus, or bulbus glandulosus, in which the food undergoes further preparation. The walls of the proventriculus are thickly studded with large glandular follicles, variously disposed, from whence a copious secretion of "gastric juice," as it is called, is poured out and mixed with the aliment. Having, therefore, undergone maceration in the juices of the crop, and become subsequently saturated with the gastric fluid, that constitutes so important an agent in digestion, alimentary substances are at length received into the gizzard (c), where further preparation is necessary.