This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(2049). The gizzard in such birds as feed upon vegetable substances is an organ possessing immense strength, and constitutes, in fact, a crushing mill, wherein nutritive materials are bruised and triturated.
Fig. 358. Gizzard of a bird.
Its cavity is very small, and lined with a dense, coriaceous cuticular stratum; and its substance is almost entirely made up of two dense and enormously powerful masses of muscle, the fibres of which radiate from two central tendons (fig. 358, c), situated upon the opposite sides of the viscus. The action of these lateral muscles will obviously grind and crush with great force whatever is placed in the central cavity, - a process that is materially expedited by the presence of hard and angular pebbles, swallowed for the purpose, by the assistance of which the contained food is speedily comminuted.
(2050). Another and much feebler set of muscles (d) bounds the cavity of the gizzard in the intervals between the great lateral masses which, receiving the food from the proventriculus, perpetually feed this living mill, and retain the material to be ground within the influence of the crushers until it is properly prepared, when other fibres, acting the part of a pylorus, allow it to pass on into the duodenum (e).
(2051). The intestinal canal of Birds is, as in other classes, very variable in its relative length as compared with that of the body; its calibre is pretty equal throughout, and the division into large and small intestines can scarcely be said to exist. Commencing from the pylorus, the duodenum (fig. 359, d h) is always found to make a long and very characteristic loop, embracing the lobes of the pancreas (e e); and then, after sundry convolutions, the intestine is continued to its termination in the cloaca. The division between the large and small intestines is indicated by the presence of one, or more generally two, csecal appendages, which communicate with the cavity of the gut at no great distance from its cloacal extremity.
(2052). In Birds, the auxiliary secretions subservient to the digestive process are the salivary, the gastric, the hepatic, and the pancreatic.
(2053). The salivary apparatus varies much in structure and disposition in different tribes. In its simplest form it consists of distinct secerning follicles, placed immediately beneath the mucous membrane of the mouth, into which the secretion is poured by numerous orifices. In the Gallinaceous birds the glands assume a conglomerate character. In the Turkey there are two pairs *: the first pair forms a cone, having its apex directed towards the extremity of the beak; and the two glands of the opposite sides touch each other along the mesial line through almost their entire length, filling up anteriorly the angle of the lower jaw. These glands are situated immediately beneath the skin, but in front they touch the mucous membrane of the mouth; and their secretion is poured into the buccal cavity by several orifices. The second pair of glands is smaller, of an elongated form, and is placed above the posterior third of the former; this is immediately in contact with the mucous lining of the mouth.
(2054). In the Woodpeckers the glands that secrete the fluid whereby the tongue is lubricated are of very considerable size. They pass further back than the angle of the lower jaw, extending even to beneath the occiput; and their secretion, which is viscid and tenacious, enters the mouth by a single orifice situated under the point of the tongue.
* Cimer, Lecons d'Anat. Comp. tom. iii. p. 221.
(2055). In the generality of birds, however, there is only one pair of salivary glands; and these, in many cases, seem to be united into a single mass, separated posteriorly into two lobes, and situated beneath the palatine membrane, behind the angle of the rami of the lower jaw. From these glands a thick, white and viscid fluid is poured into the mouth through numerous orifices, principally disposed along the mesial line which separates the two glands.
(2056). We have already spoken of the gastric glands which densely stud the coats of the proventriculus, and furnish the "gastric juice," and therefore pass on to notice the other subsidiary chylopoietic viscera, namely, the liver, the pancreas, and the spleen.
(2057). The liver is a viscus of considerable magnitude, consisting of two principal lobes, and firmly suspended in situ by broad ligaments and membranous processes. The vena portae, supplying that venous blood from which the bile is elaborated, is formed by vessels derived from numerous sources, receiving not only the veins of the stomach, spleen, and intestines, as in Mammalia, but likewise the renal and sacral veins, - another proof, if any were wanting, that no arrangement by which the decarbonization of the blood can be facilitated has been omited in the organization of the class before us. The hepatic arteries and the hepatic veins present nothing remarkable in their disposition; but the course of the bile from the liver into the intestine merits our notice. Two sets of ducts are provided for this purpose: the first (fig. 359, i) carries the bile from the liver into the gall-bladder (g), from which another duct conveys the bilious fluid into the duodenum; but the second set of bile-vessels conducts the secretion of the liver at once into the intestine, by a wide canal (o) that has no communication whatever with the gall-bladder. There is, therefore, no arrangement like that of the "ductus communis choledochus" of Mammals: if the bile is wanted immediately, it passes at once into the intestine through the duct o; but if digestion is not going on, it is conveyed into the gall-bladder through the duct i, to be there retained until needed.
(2058). The pancreas (fig. 359, e e) is a conglomerate gland of considerable size, situated in the elongated loop formed by the duodenum: it generally consists of two portions more or less intimately connected, and from each portion an excretory duct (n) is given off; these two ducts terminate separately in the intestine, in the immediate vicinity of the openings of the biliary canals. In some birds even three pancreatic ducts are met with, as is the case in the common Fowl; but under such circumstances the third duct, instead of opening into the intestine at the same point as the other two, issues from the opposite extremity of the pancreas, and enters the middle of the duodenum at the place where the gut turns upon itself.