In such birds as nourish their young from the crop the glands swell very much at the hatching season, and secrete a greater quantity of fluid than ususl. In the pigeon, which thus feeds its young, there is a spherical bag formed on each aide of the oesophagus, a specimen of which may be seen in the museum of the College of Surgeons. It is not improbable that the banter about pigeon's milk' took its rise from this part of the economy of the bird.
In those birds which feed on flesh, fish, or worms, and which con sequently do not require so powerful an apparatus, the muscles of the gizzard are reduced to an extreme weakness, and that organ appears to make only a part of the same membranous bag with the ventricule snecenttnid.
The food being thus reduced into a sort of chyme passes through the remainder of the intestinal canal, where all the nutritious parts are taken into the system, and the remainder is at length expelled by the cloaca, where the urinary ducts terminate and the organs of generation are situated. It may be worth mentioning that the liver becomes much larger in domesticated birds than in wild ones (a pro pensity which can be increased by artificial means, as the gourmand who revels in his 'foies gran' well knows), and that the gall-bladder is entirely wanting in some birds, the parrot and pigeon for instance. Hence no doubt the saying, "He has no more gall than a pigeon." The pancreas (sweet-bread) is of considerable size in birds, but the spleen is small.
Organs of Circulation, Respiration, and Voice.
The heart in this class is of peculiar structure. Instead of the membranous valve which is present in both ventricles of the heart of maramifers, and in the left ventricle in birds, the right ventricle of the heart in the latter is furnished with a strong muscle which assists in driving the blood with greater impetuosity from the right side of the heart into the lungs ; a structure rendered necessary from the want of expansion of the lungs in breathing consequent upon their connection with the numerous air-cells. The lungs are small and flattened, and adhere to the back of tho chest in the intervals of the ribs, and a considerable part of the abdomen as well as of the chest is occupied by membranous air-cells with which the lungs communicate by considerable apertures. In addition to these, a great portion of the skeleton in most birds becomes a receptacle for air. Instead of marrow the larger cylindrical bones contain air, and form large tubes, interrupted only towards the ends by transverse bony fibres. The broad bones present internally a reticulated bony texture, pervaded by the same fluid, communicated from the lungs by small air-celle. The enormous bills of the toucan and of the hornbill are supplied with air from the same quarter.
The effect of this structure in lightening the body of the bird, and facilitating its motions whether in flying, swimming, or running, is obvious. Where the demand is greatest (as in birds of the highest and most rapid flight) the supply is largest. Thus, in the eagle, we find the bony cells of great size and very numerous. The section of a head of the Hornbill (Buceros Ithinocero4 here represented, will convey some idea of the structure of these air-cells.
The organs of the voice in birds bear a striking resemblance to certain musical wind-instruments. The larynx is double, or rather made up of two parts : one, the proper rime glottidia, situated at the upper end of the windpipe ; and the second, the bronchial, or lower larynx, which contains a second rime glottidis, furnished with tense membranes that perform in many birds (and especially in those which are aquatic) the same part as a reed does in a clarionet or hautboy, while the upper rima, like the ventage or hole of the instrument, gives utterance to the note.
The length of the windpipe and the structure of the lower larynx vary much in different species and even in the sexes, particularly among the water-birds. In the domestic or dumb swan the windpipe is straight ; in the vials wild swan the windpipe is convoluted in the hollow of the breast-bone, like the tube of a French horn.
The following are the conclusions of M. Jacquemin from his observa tions on the respiration of birds. After observing that the air enters not only into the lungs and about the parietes of the chest, but that it also penetrates by certain openings (foramina) into eight pneumatic bags or air-cella, occupying a considerable portion of the pectoro abdominal cavity, and thence into the upper and lower extremities, he concludes :—Ist, That the pneumatic bags are so situated as to be ready conductors of the air into the more solid parts of the body ; and that the air, by surrounding the meet weighty viscera, may sup port the bird in flight, and contribute to the facility of its motions when so employed. 2nd, That the quantity of air thus introduced penetrates the most internal recesses of their bodies, tending to dry the marrow in the bones and a portion of the fluids ; a diminution of specific gravity is the result, the true cause of which has been, in his opinion, vainly sought in the quantity alone of permeating air. 3rd, That in birds the oxidation of the nourishing juices is not entirely effected in the lungs, but is much promoted also in the pneumatic bags above mentioned, for their contained air operates through the membranes upon the blood-vessels and lymphatics in contact with them ; a more complete and speedy oxidation is the result. 4th, That not only the skeleton, but all the viscera are much more permeable by air in birds than in any of the other vertebrated animals. 5th, That the air-reservoirs are not always symmetrical, their shape and extent depending entirely upon the form and situation of the organs among which they occur; but the supply is so modified that the total quantity received into the pneumatic bags on the right side of the body is equal to that which enters into those on the left ; and indeed without the maintenance of this condition the act of flying would be impossible, and that of walking difficult. 6th, That no portion of a bird's struc ture is impervious to air ; it reaches even the last joints (phalanges) of the wings and feet, and the last caudal vertebra`, or rump-bones. The quill of the feathers is not excepted, as has been sometimes asserted. 7th, That the air within the bead has a separate circulation, and does not directly communicate with the air-pipes of the rest of the body. 8th, That in no instance does the air come into direct contact with the viscera or nourishing juices, but invariably through the medinm of a membrane, however fine and transparent. 9th, That the volume of air which birds can thus introduce into their bodies, and the force with which they can expel it, offer the only explanation how so small a creature as a singing-bird (the nightingale, for example) is able to utter notes so powerful, and without any apparent fatigue to warble so long and so musically.