Class 5. Aves. 445 



part, the large glands are wanting, the lining becomes firmer, and 

 gradually passes into that of the muscular stomach. This is 

 short and saccular, and has muscular walls; it possesses glands of 

 one kind only, namely, simple, close-set glandular pits, like those 

 which secrete the mucous layer of the other chamber. Their 

 secretion is, however, very peculiar ; each gland forms a hard, horny 

 thread, which projects from its mouth, and adhering to neighbouring 

 threads, forms a horny lining;* it is continually being worn 

 away at its surface, and renewed by fresh secretion at the base of 

 the threads. On the outer surface of the muscular stomach, there 

 is a dorsal and a ventral tendinous disc, from which the muscular 

 elements arise. The muscles of the wall are especially strong in 

 herbivorous, notably in graminivorous species (e.gr.. Fowls and Ducks), 

 in which this organ is provided with a large muscle mass, plano- 

 convex from within outwards; whilst its cavity is very small. In 

 such, the muscular stomach forms a true gizzard, in which the 

 food may be ground by the two muscles just mentioned ; the horny 

 lining is very thick and hard, and sand and stones are swallowed 

 to assist in trituration. In insectivorous and predaceous forms the 

 muscular stomach, on the other hand, is thin walled, the musculature 

 feeble, and the cavity large. Its openings into the small intestine, 

 and into the glandular stomach, are close together. The small 

 intestine is well-developed, and longest in the herbivorous forms ; 

 it is continued into a rectum which is almost invariably short, and 

 which has a posterior widened portion, the cloaca. Two ceeca usually 

 open at the junction of the small intestine with the rectum, and in 

 many herbivorous and omnivorous Birds they are of considerable 

 length, whilst in the carnivorous forms they are usually quite short 

 or rudimentary (this may also be the case in others; Pigeons, for 

 instance, "have quite short caeca) . ' There is a large brownish-red 

 liver provided with a gall bladder, and an elongate whitish 

 pancreas lying in the first loop of the small intestine. t 



Many Birds disgorge the indigestible portions of their food, bones, hairs, 

 feathers, insect skeletons, etc., in small masses, the so-called pellets; the best 

 knpwn of such peUets are thrown, up by the Owls, and usually consist of skin and 

 bones of Mice, but various others disgorge similar ones : Swifts (insect remains), 

 Kingfishers (fish bones). Ravens, etc. 



Respiratory, organs. - A • longitudinal slit in the mouth 

 close behind the tongue leads into the larynx, which is continued 



' *, In Birds of Prey and other carniTorous forms, the lining of the musonlar stomach 

 has a softer character. . ^ 



t In most yomig Birds, there opens on the dorsal waU of the cloaca, a small 

 unpaired-sao-wi%h-a-narrow-aperture, the-iursa- Fairicii, in the-wall of which he small- 

 portions of epithelium, which are indeed constricted evagiuations .of the epithelium of 

 the, sad. ' In older forms, 'the .Bursa, of which the significance is unknown, is usually 

 degenerate orabsentj^^'-In' the-Struthious Birds, instead of this, there is a large, 

 undefined outgrowth of the cloaca. 



