SANGUIFEROUS SYSTEM IN THE VERTEBRATA. 89 



of the carnese columnse, of which few exist in the ventricle. In 

 the left auricle a mere rudiment of valve is found between the sinus 

 and the appendix. The parietes of the left ventricle are very thick, 

 and the auriculo-ventricular opening 1 is guarded by two mem- 

 branous folds corresponding to the mitral valve in mammalia. The 

 pulmonary artery and aorta are provided at their origins with three 

 semilunar valves, those of the former vessel being thicker and 

 stronger than those of the latter. The extremities of these valves 

 will be found by careful dissection to be attached to cartilaginous 

 or osseous points imbedded in the fibrous tunic of the vessels. 



The pulmonary artery having arisen from the right ventricle at 

 once divides into two branches, one for each lung; from these or- 

 gans the blood is returned by two veins, which unite before they 

 reach the left auricle. From the left ventricle arises the aorta, 

 which immediately sends off two branches analogous to arteriae 

 innominatae, for the supply of the head and wings. It may be 

 observed that birds possess no palmar arches, nor, strictly speaking, 

 a radial artery. The cerebral, orbital, temporal, and spermatic 

 arteries are remarkable for their free and plexiform anastomoses. 

 But of all the arterial plexuses, that of the organ of incubation 

 requires special notice. It is formed by branches from the posterior 

 thoracic, abdominal, cutaneous, and ischiadic arteries, immediately 

 under the integuments of the abdomen. This plexus becomes 

 enormously enlarged during the period of incubation. 



The venous system in birds is remarkable for the frequency of 

 its communications, especially one which exists between the united 

 caudal, hemorrhoidal, and iliac veins and the vena porta, by means 

 of which the blood from the viscera and posterior parts of the body 

 may flow either into the cava or porta, a disposition obviously de- 

 signed to guard against congestions: as yet it is a question con- 

 cerning which contradictory opinions prevail, as to whether the 

 branches of the pulmonary artery extend to the air-cells distributed 

 over the body, or whether the blood in the systemic capillaries un- 

 undergoes any change tending to its purification in the parietes of 

 these cells. 



MAMMALIA. 



The form of the heart, and the distribution of the blood vessels in 

 the whole of this class so closely resemble the human type, that it 

 becomes necessary to notice only a kw individual peculiarities ; in 

 the dugong and rytina for instance, among the cetacea, the heart is 

 cloven by the deep separation of its two ventricles, and the orifice 

 of the inferior cava is guarded by a fleshy Eustachian valve, which 

 is wholly absent in the lion, the bear, and the dog. In the mono- 

 tremata, the marsupiata, the porcupine, and the elephant, the right 

 auricle receives one inferior and two superior cavae as in birds, and 

 the coronary vein terminates in the left superior cava. In the 

 ourang-outang and the mole, only, is the apex of the heart inclined 



