THE THORAX 2053 



tricular septum. The tricuspid valve is next to receive attention, and its 

 three main segments are to be carefully studied, the smcdl secondary seg- 

 ments l>Tng between their basal parts being noted. The chordae tendineae 

 and their relations to the segments of the tricuspid valve should be closely 

 examined, and the function of that valve is to receive careful consideration. 

 The study of the pulmonary valve, which guards the orifice of the pulmonary 

 artery, is to be postponed till a later period. 



On account of the posterior position of the left auricle, and preparatory to 

 opening it, the inferior vena cava may be divided in order to liberate the 

 heart from the diaphragm. The heart is then to be turned over to the right 

 side, and also upwards. A transverse incision may then be made above and 

 parallel to the auriculo-ventricular groove, and any other which may suggest 

 itself to the dissector, who is supposed to know by previous study the intemzd 

 arrangement of the parts. The interior having been cleansed, it will be at 

 once apparent that the musculi pectinati are confined entirely to the region 

 of the auricular appendix. On examining the left surface of the interauri- 

 cular septum the position of the foramen ovale of the foetal heart, and the 

 remains of the valve which originally closed it, will be recognised as a slight 

 depression, which is limited inferiorly by a crescentic border with the con- 

 cavity' directed upwards. It may be compared to the imprint of a finger- 

 nail. 



The orifices of the pulmonary veins on the posterior wall — two right and 

 two left — are to be examined, and the absence of valves noted. The left 

 auriculo-ventricular orifice is next to receive attention. If two fingers, held 

 side by side, are passed through the orifice, it will be found, if normal, to 

 admit them readily. 



The left ventricle is to be opened by two incisions, one carried transversely 

 across the ventricle, just below and parallel to the auriculo-ventricular 

 groove, and the other extending from the right end of the preceding incision 

 to the lower part of the ventricle on the left side of the apex. The ventricle 

 having been cleansed, the great thickness of its wall is to be noted, and 

 the dissector is to observe (1) that its cavity extends quite to the apex of 

 the heart, and (2) that the interventricular septum recedes from it, so as 

 to be concave on this aspect. The portion of the cavity adjacent to the 

 aortic orifice is to receive attention, this part, which is known as the aortic 

 vestibule of Sibson, having fibrous walls. The columnae cameae and their 

 varieties are to be studied as on the right side, their very intricate arrange- 

 ment being noted, and special attention is to be directed to the musculi 

 papillares, which are anterior and posterior. The two large cusps of the 

 mitral valve, with two small cusps placed between their basal parts, are to 

 be carefully studied, and the relation of the anterior or aortic cusp to the 

 aortic orifice is to be noted. The chordae tendineae, and their relations to 

 the segments of the valve, should be closely examined, and the function of 

 the vadve is to receive careful consideration. The auriculo-ventricular and 

 aortic orifices are to be studied, but the aortic valve is to be left over for 

 future consideration. Before leaving the study of the interior of the heart, 

 the relations of its auriculo-ventricular and arterial valvular orifices to the 

 thoracic wall should be thoroughly mastered. 



Deep Cardiac Plexus. — To expose this plexus the ascending acrta should 

 be divided near its upper end, and the superior vena cava is to be divided a 

 Uttle below the point where it receives the right azygos vein. The arch of 

 the aorta is then to be displaced towards the left side, and kept out of the 

 way by hooks. If necessary, the ligamentum ductus arteriosi should be 

 divided, but the superficial cardiac plexus is to be left intact. This dissec- 

 tion will expose the thoracic portion of the trachea, in front of which the 

 deep cardiac plexus is situated, just above the bifurcation into the two 

 bronchi. By very careful dissection the nerves forming this plexus cire to 

 be shown passing dowTiwards and inwards on the sides of the trachea to its 

 anterior aspect, where they end in the plexus. The deep cardiac plexus is 

 arranged in two halves, right and left, which are in free communication with 

 each other. The contributory nerves to the right half of the plexus are 



