ORGANS OF CIRCULATION. 105 



ventricle there is found a peculiar valve of very great strength, and 

 formed of longitudinal muscular bundles, which pass obliquely from 

 the right wall of the ventricle to the interventricular septum, and 

 serves obviously to bring into forcible approximation the opposite 

 parietes of the ventricle, and thus to drive out the blood more effectu- 

 ally into the pulmonary artery. The septum ventriculorum is directed 

 convexly toward the right, concavely toward the left ventricle. In 

 the inter-auricular septum the fossa ovalis is found always completely 

 closed, and surrounded by a strong muscular ring (annulus ovalis). 

 The left auricle is smaller, but more muscular than the right, and the 

 openings within it of the two pulmonary veins are commonly provided 

 with valves. Three semilunar valves, provided in the middle of their 

 margin with a small ossified nodule, are found at the commencement 

 of the pulmonary artery and aorta. 



The trunk of the systemic circulation, the Aorta, is extremely 

 short ; from its root two coronary arteries arise to be distributed to 

 the heart, and it then divides at once into three main branches, 

 one of which forms the descending aorta ; the two others, a right 

 and left arteria innominata, giving off their corresponding carotid 

 and subclavian arteries. The carotids, which are of small propor- 

 tional size, exhibit many remarkable varieties, that are frequently 

 characteristic of the several orders. As a general rule, in the Ra- 

 paces, Gallinae, most of the Grallae and Palmipedes, the Ostrich, and 

 a few Scansorial birds, e. g. Kakadus, Psittacus builarius, passe- 

 rinus, &c., the two carotids traverse the canal formed by the trans- 

 verse processes of the cervical vertebrae. On the other hand, in all 

 the Passeres without exception, and many Scansores, e. g. Picus, 

 Sitta, Merops, and some of the Parrot-tribe, as also the genus Podi- 

 ceps, and the Rhea or American Ostrich, there is only a single, and 

 that the left, carotid present ; it is much more rare, as in the Fla- 

 mingo and Pelican, for the right carotid to exist and the left be 

 absent. In a few instances the two carotids adhere so closely to- 

 gether as to appear like a single trunk, as in the Bittern. A lesser 

 degree of variety is found in most of the Parrots where both the caro- 

 tids are present, but the left mounts upward, without entering the 

 canal of the cervical vertebrae, by the side of the jugular vein. The 

 subclavian artery runs beneath the furcular bones in the direction 

 outward, gives off the brachial artery to the wing, and a still larger 

 thoracic artery to the pectoralis major muscle. The descending 

 aorta curves over the left instead of the right bronchus, as in 

 Mammalia, passes along and in front of the vertebral column be- 



