AVES. 



377 



liver ordinarily consists of two lobes into the fissure ' between which the 

 apex of the heart is received. There are always two, and sometimes three 

 bile-ducts : they open separately into the duodenal region of the intestine, 

 and on one of them a gall-bladder is usually developed. It is absent in 

 many Pigeons, some Parrots, &c. A large and compact pancreas with two 

 or three ducts is always to be found in the concavity of the duodenal loop. 

 A ductus vitello-intestinalis, the duct of the yolk-sac of embryonic, and of 

 early life subsequent to the hatching, is often to be found on the small 

 intestine, especially in aquatic birds and waders. In Ratitae remains of the 

 yolk may be found in it for a long time or during life. Two caeca, rarely 

 absent, e. g. some Parrots, are appended to the intestine at the junction of 

 the ilium with the large intestine. The Herons have but one. Their size 

 varies much, and they are largest in grain-eating birds, Lamellirostres and 

 Ratitae. In the Ostrich they are very large and contain a spiral valve. In 

 all young birds, and in some adults, a Bursa Fabricii opens on the dorsal 

 aspect of the cloaca close to its external aperture. This bursa is produced 

 originally as a solid growth which subsequently acquires a cavity. It 

 usually atrophies away : its function is unknown, but its walls contain a 

 rich vascular supply, and numerous follicles derived from the epithelium of 

 the intestine and surrounded by adenoid (reticular) tissue (see pp. 54-5). 



In addition to the points already noted in the heart, the right auricle is 

 larger than the left : the left ventricle is of great size, with strong muscular 

 walls, whilst the right has thin walls and embraces the left : the left auri- 

 culo-ventricular valve consists of two membranous flaps with attached 

 chordae tendineae and musculi papillares. The fourth aortic arch on the 

 left side is converted into the innominate artery, instead of forming a 

 second or left systemic aorta as in Reptilia, though its homology with this 

 latter vessel is shown in many birds, especially Accipitrine, by the reten- 

 tion of a compact fibrous prolongation onwards to the dorsal aorta. The 

 aorta appears to divide close to its origin into three great trunks, as it 

 gives off two sub-equal innominate arteries. In birds of powerful flight 

 these arteries are often of larger calibre than the aorta itself. The two 

 carotids run up the ventral aspect of the neck and are sometimes united 

 for a portion of their course, or the aortic origin of one of the two may be 

 lost There are various arterial plexuses, and one especially is notable on 

 the surface of the abdomen which enlarges during the period of incubation. 

 There are as a rule two superior venae cavae, but the cardiac extremity of 

 the left is sometimes much reduced or even obliterated. They open 

 separately from each other and from the vena cava inferior into the right 

 auricle, the sinus venosus of Reptilia having disappeared by absorption into 

 the auricle. The two jugular veins are united under the base of the skull 

 by an anastomotic vein. The vena cava inferior is formed by the con- 

 fluence of the iliac veins. These veins are in their turn formed by the union 



