AVES. 
161 
of incubation, and the function of which is to secrete a lacteal substance, with 
which the young are at first nourished. The craw of Birds generally is situate on 
the right side only; but in the Pigeons it is double, and fig. 70 represents the ordi- 
nary aspect of that on one side when inflated (a), and the thickened glandular appear- 
ance of that on the other (b), as noticeable in Pigeons that have newly-hatched young. 
In other Birds, the craw merely serves as a reservoir for such food as cannot be imme- 
diately taken into the stomach; though grain is generally moistened there and 
softened, by macerating in fluid sipped for the purpose] . 
The liver voids its bile into the intestine by two ducts, which alternate with the two 
or three by which the pancreatic fluid passes. The pancreas of Birds is large, but their 
spleen is small ; they have no epiploon, the functions of which are in part fulfilled by 
the partitions of the air-cavities. The coecal appendages [when present] are placed near 
the origin of the rectum, and at a short distance from its outlet ; these are more or less 
long, according to the regimen of the bird. * The Herons [as also the Smew Mer- 
ganser] have only one, which is minute ; in other genera, as that of the Woodpeckers, 
|i they are wanting altogether. 
The cloaca is a pouch in which the rectum, the ureters, and the spermatic ducts — 
or, in the female, the oviduct — terminate ; it opens externally by the anus. As a 
general rule, Birds do not urinate ; the secretion of the kidneys being mingled with 
their solid excrement. The Ostriches alone have the cloaca sufficiently dilated to 
allow of an accumulation of the urine. [In the majority of Water-fowl, there is a 
small accessory pouch to the cloaca, termed the bursa Fabricii: its use has not been 
clearly ascertained.] 
In most of the genera, coition is effected by the simple juxta-position of the anus ; 
the Ostriches and many aquatic Birds [those which copulate in water] , however, have 
a penis furrowed with a groove, along which the seminal fluid is conducted. The 
testicles are situate internally above the kidneys, and near the lungs ; [they attain an 
enormous developement towards the season of propagation;] only one oviduct is 
developed, the other [with its ovary] being reduced to minute size. 
The egg, detached from the ovary, where only the yolk is perceptible, imbibes in the 
upper part of the oviduct that exterior fluid termed the white, and becomes invested 
with its shell in the lower part of the same canal. The chick is developed by incuba- 
tion, unless where the heat of the climate suffices, as in the case of the Ostrich [in 
some localities] . The young bird has on the tip of its beak a horny point, which 
serves to rupture the shell, and falls off a few days after exclusion. 
Every one knows the varied industry which Birds exhibit in the construction of their 
nests, and the tender care which they take of their eggs and young ; it is the 
principal part of their instinct. With regard to the rest, their rapid passage through 
different regions of the air, and the intense and continued action of that element upon 
them, renders them presensible of the variations of the atmosphere, to an extent of 
* Some difficulties occur in the way of this explanation, unless 
duly qualified in reference to the normal characters of particular 
g:roups, or subtypes of form. Thus, the Hawks and the Owls subsist 
pretty nearly on the same regimen ; the coeea being in the former in- 
stance constantly minute, and in the latter as invariably of consider- 
able size, but with the same proportional dimensions in every species : 
nor can this diversity be explained on another principle that has been 
advanced, equally correct in its application to groups ; viz., that the 
somnolent inactive Owls require to have more complex digestive 
organs (which should retain the chyme longer in its passage), than 
the more energetic tribe of Falcons ; inasmuch as the rapidly-flying, 
active Harfang, or Snowy Owl, which on the wing can scarcely be 
distinguished from the Jer Falcon, possesses ceeca — as before gene- 
rally intimated — proportionally quite as large as those of the light- 
flapping Barn Owl ; while the lazy, smooth-sailing Buzzard, the 
floating Kite, and the buoyantly-skimming Harrier, present no further 
developement of these appendages than the darting Hawks, or the 
impetuous, far-rushing Falcons. A variety of analogous instances 
might be enumerated.-— Ed. 
M 
