442 AVES. 



ever anastomose with the system of the femoral as in reptiles 

 and amphibians. 



The red blood corpuscles are oval and nucleated. 



The lymphatic system opens by two thoracic ducts (ductus 

 thoracici) into the superior venae cavae, but also very generally 

 communicates 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. 



A spleen, thymus, and thyroid glands are present. The thy- 

 roid is paired and placed at the base of the neck attached to the 

 carotid artery and jugular vein. The thymus is an elongated 

 body on each side of the neck along the jugular vein. The supra - 

 renals (adrenals) are a pair of elongated yellow bodies attached to 

 the ventral surface of the kidneys along the iliac veins. 



The respiratory organs * are perhaps the most remarkably 

 modified of all the organs. The lungs are small and but slightly 

 if at all distensible. They are prolonged into a number of thin- 

 walled air-sacs which extend amongst the viscera, into the bones, 

 and sometimes beneath the skin. The organ of voice is not in 

 the larynx but lower down on the trachea, usually at its bifur- 

 cation into the bronchi. These peculiarities are associated with 

 three remarkable properties, viz. flight, the extraordinary power 

 of voice production, and the intense activity of the metabolic 

 processes. 



The slitlike glossis is placed behind the root of the tongue, and 

 leads into a larynx, which is supported by cricoid, thyroid and 

 arytenoid cartilages, but is devoid of vocal chords. An epi- 

 glottis is absent or represented only by a small transverse fold. 

 The trachea is supported by bony rings which are usually com- 

 plete, and on reaching the thorax bifurcates into the two bronchi. 

 It is not unfrequently longer than the neck, and in such cases, 

 principally in the male sex, is thrown into a number of coils, 

 which either lie outside the thorax beneath the skin (capercally), 

 etc.) or even penetrate into the hollow crest of the sternum 

 (whooper swan). 



* Sappey, Recherches sur Vappareil respiratoire des Oiseaux, 1847, and 

 Compt. Rend. 22, pp. 250, 508. Huxley, On the respiratory organs of 

 Apteryx, Proc. Zool. Soc. 1882, p. 560. Strasser, Jen. Zeitschr., 19. pp. 

 174, 330. Butler, P. Z. S., 1889, p. 452. 



