340 CLASS XVI. 



The heart of birds is, like that of mammals, divided into two 

 perfectly distinct chambers. It rests on the sternum, included in 

 a thin pericardium, lies along the middle of the body in a straight 

 direction and has an elongated conical form, terminating in a point 

 backwards. The left ventricle has very thick and strong walls, 

 and, since it is longer than the right, the apex of the heart is 

 entirely formed by it. In the right ventricle of the heart the 

 walls are much thinner; there is here, in place of the tricuspid 

 valves of the heart of mammals, a single valve, very muscular and 

 thick, which is situated in the upper part of the ventricle in front 

 of the venous aperture or the opening of the cardial sinus (auricle) ; 

 it has a triangular form, and descends obliquely with its free 

 inferior margin from the left side to the right. In the left ventricle 

 there are usually three or two membranous valves (valvulce mi- 

 trales) situated in front of the entrance from the auricle 1 . 



From the heart arises the large artery (aorta), which, after 

 having given off at its origin the coronary arteries of the heart, 

 divides almost immediately into two principal branches. The 

 right branch, which is the widest, presently divides into a de- 

 scending artery, which is situated more downwards, bends to 

 the right and runs backward under the vertebral column, and into 

 an innominate or subclavian artery, destined for the anterior parts 

 of the body. The left branch is the left innominate or left sub- 

 clavian artery. Commonly there arises a carotid artery on each 

 side; yet in the singing birds and some others one only arises 

 on the left side and the right artery is then subclavian alone. In 

 the flamingo (Pkcem'copterus) the single carotid artery is, on the 

 contrary, the branch of a right innominate artery 2 . The carotid or 

 the two carotid arteries ascend upon the inferior surface of the 

 cervical vertebras; when there is only one then it divides on 

 reaching the head into a right and left branch. The particularly 

 large size of the external thoracic artery (art. ihoracica s. mammaria\ 



entirely refuted by accurate investigations, particularly of LAUTH Ann. des Sc. na 

 in. 1824, pp. 381 410, PL 21 25 and of PANIZZA, 1. 1. 



1 Compare on the heart of birds R. LOWER Tractatus de corde, Lugd. Bat. 1701 

 8vo, pp. 60, 6r, BLUMENBACH Kleine Schriften, s. 29 32, TIEDEMANN Zoologie, i 

 s. 557 559, and the general works on comparative anatomy of CUVIER, MECKEL, & 

 See also OWEN, 1, 1, pp. 330, 33 r. 



2 Compare MECKEL in his Arckiv f. Anat. u. Physiol. 1826, s. 19, 20, and als 

 C. L. NITZSCH Observationes de Avium arteria carotide communi, Halae, 1829, 4to. 



