DISSECTION OF THE DOG 53 



These are least well marked in the conus arteriosus. Some of the trabeculae 

 are ridges simply ; others are in the form of cords attached at their two ends ; 

 whereas three or more projections are of considerable size and are attached 

 to the wall of the cavity by one extremity only. These conical, muscular 

 processes are known as the musculi papillares, and have filamentous tendinous 

 cords (chordae tendineas) attached to their apices. One or more slender, and 

 often branched, muscular cords pass across the cavity of the ventricle from 

 the septum to the outer wall. 



The opening (ostium venosum) from the atrium into the ventricle is guarded 

 by the tricuspid valve (valvula tricuspidalis), composed of three triangular, 

 membranous cusps. The bases of the cusps are attached to the margin of the 

 atrio-ventricular orifice, and each cusp has chordae tendinece affixed to its 

 margins and ventricular surface. Most usually smaller cusps, of similar form, 

 intervene between the three major cusps. 



The exit from the ventricle, i.e. the opening (ostium arteriosum) into the 

 pulmonary artery, is provided with three pocket-shaped semilunar valves so 

 arranged as to prevent the backward flow of blood from the artery into the 

 ventricle, but offering no obstacle to the passage of blood from the ventricle 

 into the artery. Of the three valves, one is cranial, one is to the right, and one 

 is to the left. By slitting open a part of the pulmonary artery the character 

 of the valves may be more easily observed. Each segment has an attached 

 convex border and a concave free margin looking towards the pulmonary 

 artery. Behind each valve there is a slight bulging of the wall of the artery, 

 which is here thinner than in other parts of the vessel. 



Dissection. — Now proceed to examine the left side of the heart, commencing 

 with the left atrium. To open this chamber it is only necessary to 

 make one incision, beginning far back and cutting forwards to the 

 extremity of the auricular appendage. The incision should be made at 

 no great distance from the coronary sulcus, in order to avoid injury to the 

 terminations of the pulmonary veins. 



Atrium sinistetjm. — In its outward characters the left atrium differs 

 somewhat from the corresponding chamber on the right side of the heart. 

 The auricle is more ventral in position, blunter at its tip, and more or less 

 dentated along its border. In the interior of the cavity the wall is smooth, 

 except in the auricle, in which part alone are there musculi pectinati. The 

 number of openings from the pulmonary veins into the atrium is variable (three 

 to five). The atrio-ventricular orifice (ostium venosum) is oval in outline and 

 somewhat smaller than the corresponding opening in the right chamber. 



Dissection. — In order to open the left ventricle an incision should com- 

 mence near the coronary sulcus and be carried parallel to one 

 longitudinal sulcus, round the apex and up the other side parallel to 

 the other longitudinal sulcus. 



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