Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/744

Rh 726 BIRDS [ANATOMY. tions above the intestine ; the latter is divisible into small and large, and the last always terminates in a cloaca. It is invested by a peritoneal coat, which follows the curva tures of the intestine. In most Birds, as in the Croco diles, the pyloric and cardiac apertures are approximated. In many Crocodilia and Aves (e.g., Ardeidce) there is a pyloric dilatation before the commencement of the duo denum. In the Aleetoromorphag, in Eagles and Hawks amongst the Aetomorphae, and in Pigeons, the oesophagus is enlarged into a &quot; crop.&quot; In the latter it is bilobate and symmetrical (Macg., op. cit., vol. i. plate 7). In the Crocodilia and in Aves the walls of the stomach are very muscular, and the muscular fibres of each side radiate from a central tendon or aponeurosis. _ The thick ening of the muscular tissue of the stomach attains its maximum in the Graminivorous Birds ; and it is accom panied by the development of the epithelium into a dense and hard coat, adapted for crushing the food of these ani mals. Birds commonly aid the triturating power of this gastric mill by swallowing stones ; but the habit is not confined to them, Crocodiles having been observed to do the same thing. 1 Birds are further remarkable for the development of a broad zone of glands in the lower part of the oesophagus, which is usually dilated, and forms a proventriculus, connected by a narrow neck with the gizzard (gigeriwn). In Sula alba and Phalacrocorax carbo, the writer, long ago, saw this zone to be imperfect. In these birds the gullet is very capacious from the pharynx down wards, but the proventriculus is still more so, it is a large &quot; paunch.&quot; Some Ophidia have a caecum at the junction of the small intestine with the large ; and two such caeca, which some times attain a large size, are generally developed in Aves. In this class, also, the small intestine not unfrequently presents a caecal appendage, the remains of the vitelline duct. The writer s drawings show this in Gallinula chlo- ropus, Ardea cinerea, and Colymbus septentrionalis. The duodenum of Birds constantly makes a loop, within which the pancreas lies, as in the Mammalia. The liver in the Sauropsida almost always possesses a gall bladder, which is visually attached to the under surface of the right lobe, but in Ophi iia is removed to some dis tance from it. A peculiar glandular sac, the Imrsa Fabricii, opens into the anterior and dorsal region of the cloaca in Birds. 2 THE HEAET. In Birds, the venous and arterial blood currents com municate only in the pulmonary and systemic capillaries. The auricular and ventricular septa are complete (see Owen, &quot;Aves,&quot; p. 330), as in the Crocodilia; but the right ven tricle only gives off the pxilmonary artery, the left aortic arch has disappeared, and the right arch (the 4th of that side in the embryo) becomes the most important of all the arches. The septum of the cavum pidmonale becomes a great muscular fold, and takes- on the function of an auri- culo-ventricular valve. At the origin of the pulmonary artery, and at that of the aortic arch, three semilunar valves are developed 1 See Sir S. Baker s Ismuilia, vol. i. p. 295 &quot; The stomach, con tained about five pounds weight of pebbles (in a Crocodile 12 feet 3 inches long in its entire length), as though it had fed upon flesh resting upon a gravel bank, and had swallowed the pebbles that adhered. This intrepid traveller seems to be unaware that the Crocodile has a strong gizzard. 2 Besides copious unpublished materials on this subject from his own dissections, the writer is largely indebted to Macgillivray s most valuable work, so full of illustrations of the digestive organs of Birds also to Prof. Owen s article &quot;Aves&quot; (op. cit.) ; tc Prof. Huxley he owes form and order. For the development of these parts the reader is still directed to Foster and Balfour s work, as also of the parts yet to be described. In Ares there is no renal portal system, and the anterior abdominal vein opens into the inferior vena cava, close to the heart. Nevertheless, a median trunk, which is given off from the caudal vein, carries a considerable proportion of its blood directly into the hepatic portal system. The pericardium of the Bird is thin, but of a firm texture, and adheres by its external surface to the surrounding air-cell*. (Owen, &quot; Aves,&quot; p. 330.) RESPIRATORY AND VOCAL ORGANS. 3 &quot;In Birds there are distinct thyroid, cricoid, and arytenoid car tilages, which may be more or less completely ossified. Sometimes an epiglottis is added. 4 The voice of Birds, however, is not formed in the larynx, but in the syrinx or lower larynx, which may be de veloped in three positions : 1. At the bottom of the trachea, from the trachea alone ; 2. At the junction of the trachea and bronchi, and out of both ; 3. In the bronchi alone. The syrinx may be altogetlu r absent, as in the llatitcc and the Cathartidce or American Vultures. The commonest form of syrinx is the second mentioned above, or the bronchi-tracheal syrinx. It is to be met with iu all our common Song Birds, but is also completely developed in many Birds, such a.s the Crows, which have no song. In its commonest condition this form of syrinx presents the following characters : The hindermost rings of the trachea coalesce, and form a peculiarly formed chamber, the tympanum. Immediately beyond this the bronchi diverge, and from their posterior wall, where one bronchus passes into the other, a vertical fold of the lining membrane rises in the middle line towards the tympanum, and forms a vertical septum between the anterior apertures of the two bronchi. The anterior edge of this septum is a free and thin membrana scmilunaris, but in its interior a cartilaginous or osseous frame is developed, and becomes united with the tympanum. The base of the frame is broad, and sends out two cornua, one along the ventral, and the other along the dorsal edge of the inner wall of the bronchus of its side, which iu this part of its extent is membranous and elastic, and receives the name of the membrana tympaniformis internet. &quot; The bronchial rings opposite this are necessarily incomplete internally, and have the form of arches embracing the outer moiety of the bronchus. The second and third of these bronchial arcs are freely movable, and elastic tissue accumulated upon their inner surfaces gives rise to a fold of the mucous membrane, which forms the outer boundary of a cleft, bounded OIL the inner side by the membrana semilunaris. &quot;The air forced through these two clefts from the lungs sets these elastic margins vibrating, and thus gives rise to a musical note, the character of which is chiefly determined by the tension of the elastic margins and the length of the tracheal column of air. The muscles, by the contraction of which these two factors of the voice are modified, are extrinsic and intrinsic. The former are possessed by Birds in general, and are usually two pairs, passing from the trachea to the furcula and to the sternum (Macg., vol. ii. plate 12, fig. 8, d.d., e.e.; and vol. iii. plate 15, m.m., n.n.) Some Birds possessing a broncho-tracheal syrinx such as has been described, as the Alcctoromorphce (see Macg., vol. ii. plate 12, fig. 8, /.), C hcno- morphce, and Dysporomorphce, have no intrinsic muscles. Most others have one pair, attached on one side to the rings of the trachea above, and to the tympanum, or the proximal bronchial arcs below (Macg., vol. ii. plate 12, figs. 1, 2; and vol. iii. plate 19). The majority of the Coracomorphce (Macg., vol. ii. plates 10, 11) have five or six pairs of intrinsic syringeal muscles, which pass from the trachea and its tympanum to the movable bronchial arcs. 5 The Parrots have no septum, and only three pairs of intrinsic muscles. &quot;The tracheal syrinx only occurs in some American Coraco morphce. The hinder end of the trachea is flattened, and six or seven of its rings above the last are interrupted at the sides, and held together by a longitudinal ligamentous baud. These rings are excessively delicate, so that the part of the trachea is in great part membranous. The bronchial syrinx occurs only in Steatornis and C rotophaga. 3 We shall here give Professor Huxley s excellent abstract of what is known upon this subject up to this time ; but the reader is referred to Joh. Muller s work, &quot; Researches on the Comparative Anatomy of the Vocal Organs of Birds,&quot; Berlin Acad., June 1845, and Ann. and Mag. N. H., vol. xvii. p. 499. Macgillivray has many excellent illus trations and descriptions of these parts ; and the writer followed him step by step many years ago. 4 For a clear description of the exquisite structure of the tracheal rings in Birds, see Macg., vol. ii. p. 34. They are often thoroughly ossified, and are notched above and below, both before and behind ; and alternate ridges allow a marvellous amount of overlapping, the edges being well bevelled ; each ring is an ellipse. 5 Macgillivray (op. cit., vol. ii. pp. 26,28) was afraid to bethought overstating the number of these intrinsic muscles. He understated them, not thoroughly making out their divisional lines.