248 AVES. 



sacrum, and are divided by indentations into a number of lobes. 

 The ureters run behind the rectum and open into tha cloaca 

 internally to the genital apertures. The urinary secretion is not 

 fluid, as in the Mammalia, but is a white semi-fluid mass, which 

 soon hardens. 



The heart is completely divided into a right and left half, and lies 

 in the median line, enclosed by the pericardium. As a peculiarity 

 of the heart may be mentioned, the special development of the right 

 atrio ventricular valve, which, unlike the tricuspid valve of the 

 Mammalian heart, is a simple strong muscular fold. Since the 

 diaphragm is rudimentary, the thoracic cavity is directly continuous 

 with the abdominal. The pulsations of the heart, in correspondence 

 with the more active respiration, are repeated more rapidly than in 

 Mammalia. The right aortic arch persists. The veins open by two 

 superior and one inferior vena cava into the right auricle. The 

 renal-portal circulation still persists in Birds, though it is but 

 slightly developed. 



The lymphatic system opens by two thoracic ducts (ductus tho- 

 racici) into the superior venae cavse, but also very generally com- 

 municates with the veins of the pelvic region. Lymph hearts are 

 only found at the side of the coccygeal bone in the Ostrich and 

 Cassowary, and in some wading and swimming Birds. They are, 

 however, often replaced by vesicular non-contractile dilatations. 



Respiratory organs. The glottis is placed behind the root of the 

 tongue, and leads into a long trachea, which is supported by bony 

 rings. The trachea is not unf requently longer than the neck, and 

 in such cases, principally in the male sex, is thrown into a number 

 of coils, which either lie beneath the skin (Capercally) or even 

 penetrate into the hollow crest of the sternum (Whooper Swan). 



The lower larynx or syrinx. Except in the Ostrich, the Stork, 

 and some Vultures, the vocal organ is developed at the point where 

 the trachea divides into the bronchi, so that both divisions take part 

 in its formation (fig. 657). The last tracheal rings and the anterior 

 bronchial rings have a modified form, and. are often intimately con- 

 nected with each other ; the end of the trachea and the beginning 

 of the bronchi are compressed or dilated into a vesicular form 

 and transformed into the so-called tympanum, which in the males 

 of many Ducks and Divers is dilated into unsymmetrical secondary 

 cavities (tympanic cavity and labyrinth), which serve as resonating 

 apparatuses. The part of the trachea from which the bronchi pass off 

 (i.e., tympanum) is traversed in a horizontal direction by a projecting 



