ill Introduction. 



is correlated the width of the oesophagus, which often expands into 

 a crop. The liver ordinarily consists of two lobes, into the fis- 

 sure between which the apex of the heart is received. There are 

 always two, and sometimes three bile-ducts in Birds : they open 

 sej)arately into the intestine ; and on one of them a gall-bladder is 

 usually developed. A large and compact pancreas, with two or 

 three ducts, is always to be found in a fold of the duodenum. 

 With few exceptions, two coeca are appended to the intestine at the 

 junction of the ileum to the colon ; and a third, representing the 

 omphalo-mesenteric duct of embryonic, and of early life subsequently 

 to hatching, is occasionally present at a point higher up in the 

 small intestine. The urinary and genital ducts ordinarily open 

 separately into the cloaca, but there may be a short urogenital 

 cavity distinct from the lower segment of the digestive tube, but 

 opening directly into it. The characters of the beak, tongue, crop, 

 gizzard, and circum-oral salivary glands, vary much, in correspond- 

 ence with the nature of the food. 



The heart is quadrilocular, and the right auriculo-ventricular 

 valve muscular in all Birds. The fourth aortic arch of the right 

 side, instead of that of the left, as in Mammals, forms the single 

 systemic aorta ; the fourth aortic arch of the left side is converted 

 into the subclavian artery instead of forming a second, or left, sys- 

 temic aorta as in Beptiles, though its homology with this latter vessel 

 is spoken to in many Birds, especially Accipitres, by its retention of 

 a fibrous prolongation onwards to the functional aorta. The aorta 

 and pulmonary artery have each three semilunar valves. The aorta 

 divides after a very short course into three great trunks, by giving 

 off two subequal innominate arteries. In Birds of powerful flight, 

 these trunks are often of larger calibre than the continuation of the 

 aortic trunk itself. There are always two superior venae cavae, which 

 open separately from each other, and from the vena cava inferior, 

 the sinus venosus having disappeared by absorption into the right 

 auricle. The vena cava inferior is formed by the confluence of the 

 efferent renal veins, with which the veins from the lower extremities 

 are, as in Mammals, directly continuous; though, as in Reptiles, 

 the blood which these latter vessels cany can find its way also into 

 the kidney, forming thus a ^ renal portaF system, as well as into 

 the liver by anastomosis with factors of the true portal system. 

 Tlie trachea in Birds is always of considerable length ; it is often 



