414 HEART 



marked externally by a transverse girdle of fat, indicating the 

 course of the coronary vessels, or those arteries and veins which, as 

 vasa vasonim, supply the Heart itself. 



The right atrium occupies the upper right quarter of the organ, 

 its thin walls having numerous muscular ridges {musculi pedinati) 

 projecting into its cavity and presenting a honeycombed appearance. 

 It receives the 3 great venous trunks of the body — namely (1) the 

 vena cava superior dextra to the right and above, (2) the v. c. sup. 

 sinistra more dorsally, and (3) the v. c. inferior more to the right 

 and below. The entrance of the last is guarded by two prominent 

 valves, corresponding functionally with the valvula JEustachii of 

 Mammals. The orifices of the other veins are in many Birds without 

 a valve, and are subject to many modifications. 



The right ventricle occupies the ventral portion of the organ 

 from the coronary sulcus to near the apex, and its walls are smooth 

 internally, except in the distal corner, where the ventral wall passes 

 into the septum ventriculorum, and sends out retiform muscular and 

 tendinous projections. This ventricle communicates with the right 

 atrium through the ostium atrio-ventriculare dextrum, which is furnished 

 with a peculiar valve that hinders the return of the blood. This 

 valve, valvula cardiaca dextra, represents the tricuspid valve of 

 Mammals in function but not in shape or structure, since it consists 

 chiefly of an oblique prominent reduplication of the muscles with 

 the endocardiac lining of the right ventricle, while the opposite 

 dividing wall is convex, and forms no velwn, papillary muscles, or 

 cJwrdse tendineae. The right anterior corner of the right ventricle 

 passes into the two pulmonary arteries, the short and still un- 

 divided stem of which is guarded by three semilunar valves. 



The left atrium is less capacious but more muscular than the 

 right. From its dorsal wall, a membranaceous and partly muscular 

 projection partially divides its cavit}^ into two portions — that on 

 the right having smooth walls and receiving through one orifice the 

 two pulmonary veins, and that on the left Avith numerous pectinate 

 muscles — this projection directs the arterialized pulmonary blood 

 towards the left ventricle. 



The left ventricle extends to the apex of the Heart and is 

 covered ventrally by the right ventricle, and anteriorly by the left 

 atrium. Its cavity is larger and its walls three or four times 

 thicker than those of the right ventricle. Two or three elaborate 

 membranaceous flaps, held by numerous chordse tendineas, form a 

 true mitral valve and allow the blood to pass through the left 

 ostium atrio-ventriculare and enter the root of the aorta through three 

 semilunar valves. 



The interventricular septum is always very thick, smooth and 

 complete. In the corner which it forms with the ventral walls of 

 the ventricles, trabecule carnese are often numerously developed. 



