AVES. IGl 



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 wlren inflated (a), and the thickened glandular appear- 

 ance of that on the other (b), as noticeable in Pigeons that have newly-hatched young. 

 In otlier 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 regmien 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 liy 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 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 Fubricii: its use has not been 

 clearly ascertained.] 



In most of the genera, coition is eff'ected 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 wdiite, and becomes invested 

 with its shell in the lower part of the same canal. The chick is develojjed by incuba- 

 tion, unless where the heat of the climate suffices, as in the case of tlie 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 

 ditferent 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 ilifficulties occur in the way of tliis explanation, unless 

 duly (|ualilieil In reference to the normal eliaraeters of partieuliir 

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

 pretty nearly on the same re^'imen ; the ctEca beinjj in the former in- 

 stance constantly minute, anil in tlie latter as invariably of eonsiiler- 

 able size, but with the same proportional liimensions in every species ; 

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

 advanced, cijually correct in its application to groups ; viz., that the 

 somnolent inactive Owls rcijuire to have more eoniplex digestive 

 organs (which should retain the cbyme longer in its passage], than 



ihemore energetic tribe of Falcons; inasmuch as the rapidly flyini^, 

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

 disiiiiguislicd from the Jer Falcon, possesses cffic.a— as before gene- 

 rally intimated — proportionally quite as large as those of the light- 

 dapping Barn Owl; while the lazy, smooth-sailing Bi.zzard, tiie 

 Uoatiiig Kite, and the buoyantly-skiminiiig Harrier, present no further 

 developemeiit of these appendages than the darting Hawks, or the 

 impetuous, far-rushing Falcons. A variety of analogous iiiatances 

 inight be enuinermed.^Eo. 



