Class 2. Div. 1. VERTEBRATE ANBIALS.— AVES. 149 



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 ccecal 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, 

 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 bemg mingled with 

 their solid excrement. The Ostriches alone have the cloaca sufficiently dUated 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 eflfected 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 waf of this explanation, unless 

 duly qualified in reference to the normal characters of particular 

 groups, or subtypes of form. Thus, the Hawks and the Owls subsist 

 pretty nearly on the same regimen ; the c(Eca being in the former in- 

 stance constantly minute, and in the latter as invariably of consider- 

 able size, but with the same proportional nimensiuns in every species : 

 nor can this diversity be explained on another principle that has been 

 advanced, equally correct in its application to ^oups ; viz., that the 

 somnolent inactive Owls require to have more complex digestive 

 organg (which should retain the chyme longer in its passagel , than 



the more energetic tribe of Falcons ; inasmuch as the rapidly-flying, 

 active Harfang, or Snowy Owl, which on the uing can scarcely be 

 distinguished from the Jer Falcon, possesses coeca — 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 instancea 

 might be enumeratca.^En. 



