624 



BLOOD-VASCULAR SYSTEJI OF THE HOHSE 



Ihe anterior part of the orifice is usually only about an inch or two (ca. 3-5 cm.) above the 

 level of the ventral end of the first rib, while the posterior part is four or five inches (ca. 10-12 cm), 

 higher than the sternal end of the fifth rib. 



It is guarded by the tricuspid valve (Valvula tricuspidalis) ; of th>3 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 chordae ten- 

 dineae. The auricular surfaces are smooth. The ventricular surfaces are rough and 

 furnish attachment to interlacing branches of the chordae tendineae. The valves 

 are folds of the endocardium, strengthened by fibrous tissue and at the periphery 



Musculi pedinati 



Pulmonary artery 



Aortic valve -^ 



Great coronary^ I 

 vein 

 Left coronary 



artery 

 Bicuspid Dalve — 



Chorda' iendinecs ,-^—' 

 Papillary muscle 



Musculi pectinati 



V I '~~ Right coronary 



<, / artery 



"Tricuspid valve 

 Papillary muscle 



~*^ Traheriilfc carnece 



Fig. 548. — Section of Heart of 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 chordae tendineae 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 chordae tendineae 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 



