624 BLOOD-VASCULAR SYSTEM OF THE HORSE 
The anterior part of the orifice is usually only about an inch or two (ca. 3-5 em.) above the 
level of the ventral end of the first rib, while the posterior part is four or five inches (ca. 10-12 em). 
higher than the sternal end of the fifth rib. 
It is guarded by the tricuspid valve (Valvula tricuspidalis); of the three large 
cusps of this valve, one is between the atrio-ventricular opening and the conus 
arteriosus, one is septal, and the third is on the right margin. Small intermediate 
cusps intervene between the large ones. The peripheral edges of the cusps are at- 
tached to the fibrous ring at the atrio-ventricular opening. The central edges are 
irregular and hang down into the ventricle; they give attachment to chorde ten- 
dine. The auricular surfaces aresmooth. The ventricular surfaces are rough and 
furnish attachment to interlacing branches of the chordee tendinee. The valves 
are folds of the endocardium, strengthened by fibrous tissue and at the periphery 
Pulmonary artery 
Musculi pectinati aN 
N P 
hy 
Musculi pectinati 
Great coronary 
vein 
Left coronary —__ 
artery 
Bicuspid valve -~ ~~~ 
Chorda@ tendinew _-- e i Vi > ~/ ~~ Right coronary 
55 artery 
Papillary muscle __--== 
~ “Tricuspid valve 
“s Papillary muscle 
Ps a "SS Trabecule carnee 
Fic. 548.—Section oF Heart or Horse. SPECIMEN HARDENED in situ. 
The section is cut nearly at right angles to the ventricular septum, and is viewed from the right and posteriorly. 
by muscular fibers also. The chorde tendinee are attached below to the three 
musculi papillares, which project from the ventricular wall; superiorly they 
divide into branches which are inserted into the ventricular surfaces and the free 
edges of the valves. Each cusp of the valve receives chords tendines from 
two papillary muscles. Of the latter, two are on the septum and the third and 
largest springs from the anterior wall. 
The pulmonary orifice (Ostium pulmonale) is circular and is situated at the 
summit of the conus arteriosus, opposite to the third rib and intercostal space. 
The position of the orifice is somewhat variable. Its anterior margin may not be quite so 
far forward as the posterior border of the third rib, and the posterior margin is often opposite to 
