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 vasorum, supply the Heart itself. 
The right atrium occupies the upper right quarter of the organ, 
its thin walls having numerous muscular ridges (muscult pectinati) 
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 vw ¢ sup. 
sinistra more dorsally, and (3) the v. ¢ inferior more to the right 
and below. ‘The entrance of the last is guarded by two prominent 
valves, corresponding functionally with the valvula Eustachi 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 ostiwm atrio-ventriculare deatrwm, 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 wlwm, papillary muscles, or 
chordx tendinew. ‘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 cavity into two portions—that on 
the right having smooth walls and receiving through one orifice the 
two pulmonary veins, and that on the left with 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 chordx tendinex, 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 carnex are often numerously developed. 
