496 SANGUIFEROUS SYSTEM. 



habitual great muscular exertions, the blood is determined 

 from the deep-seated to the superficial parts, by which 

 their surface-temperature is elevated for incubation, their 

 down and plumage are developed to equalize their tem- 

 perature or to aid their flight, and their perspiration 

 is increased to moderate their occasional heats. 



The heart of birds, of a tapering conical form, with thick 

 muscular parietes, is placed longitudinally on the median 

 plain, anterior to the liver, enclosed in a thin transparent 

 vascular pericardium, surrounded by air-cells prolonged from 

 the bronchi, and still occupies, as in the inferior vertebrata, 

 a more advanced position in the trunk than in the mammi- 

 ferous tribes. The right auricle and ventricle, especially in 

 diving birds, have more capacious cavities with thinner pa- 

 rietes, than the corresponding parts on the left side, and the 

 venous blood of the system is received into the right auricle 

 from the two anterior and the larger single posterior, vense 

 cavee, by three distinct orifices provided with strong semilunar 

 valves to direct the currents and check regurgitation. The thin 

 muscular outer parietes of the capacious right ventricle en- 

 velope the greater part of the exterior of the strong left 

 muscular cavity, and the right auriculo- ventricular orifice 

 is here defended by a thick fleshy valvular fold extended 

 from the base of the ventricle, which probably aids in 

 forcing the blood through the fixed lungs of birds, where 

 the diaphragm is almost wanting. The arterialized blood 

 arrives from two pulmonary veins by a single orifice in the 

 left auricle, which is provided, like those of the superior 

 venae cavse, with only one semilunar valve, and the walls of 

 this auricle are more muscular and columnar than the smooth 

 parietes of the right cavity, but neither of these cavities yet 

 presents auricular appendices developed from its margins, as 

 in mammalia. The left auriculo-ventricular orifice is pro- 

 tected, as in quadrupeds, by a membranous mitral valve, 

 composed of two folds, and connected by its irregular free 

 margin, with the thick fleshy columns of the ventricle, by 

 means of numerous tendinous cords. 



The arterial orifices of the right and left ventricles of birds 

 are single, and provided with three semilunar valves, as in all 

 the higher vertebrata, and all the parts of the heart, though 

 completely separated internally by the inter-auricular and 

 inter- ventricular septa, are now more intimately united 



