322 



DISSECTION OF THE THORAX. 



Fig. 102. 



Behind the aortic aperture, between it and the auriculo-ventricular 

 orifices, is a piece of fibre-cartilage, with which the fibrous rings are 

 united. 



The muscular fibres belong to the involuntary class, though marked 

 witli transverse stria?, and form concentric layers, which inclose the 

 cardiac cavities. In the auricles the fibres are separate from those in the 

 ventricles. 



In the wall of the auricles the fibres are mostly transverse (fig. 102, a), 

 and are best marked at the lower part, though they form there but a thin 

 layer; and some of the fibres dip into the septum between the auricular 

 cavities. Besides this set there are annular fibres around the appendages 

 of the auricles, and the endings of the different veins. Lastly a few 

 oblique fibres (c and d) pass upwards over the auricles both in front and 

 behind. 



Dissection. The auricles having been learnt, separate them from the 

 ventricles by dividing the fibrous auriculo-ventricular rings. Next clean 



the fleshy fibres of the ventricles by 

 removing all the fat from the base of 

 the heart around the two arteries 

 (aorta and pulmonary), and from the 

 anterior and posterior surfaces. 



Before cutting into the heart let the 

 student note that the anterior surface 

 is to be recognized by the fibres turning 

 in at the septum, with the exception 

 of a small band above about half an inch 

 wide ; and that at the posterior aspect 

 the fibres are continued from the left 

 to the right ventricle aross the septum. 

 Separate partly the ventricles in front 

 along the septum by dividing the band 

 near the base, and sinking the knife 

 for about an inch into the groove be- 

 tween them. Disjoin then the aorta 

 and pulmonary artery along the mid- 

 dle line, so as to leave one attached to 

 each ventricle as in fig. 103. 



To show the laminar composition of 

 the left ventricle divide its fibres in 

 front longitudinally near the septum, 

 and transversely about half an inch 

 below the left auriculo-ventricular 

 opening ; but the cut is to be very 

 shallow, because seven layers, eaeli 

 about as thick as the thin end of the 

 scalpel, are to be demonstrated. From 



the lines of incision reflect the different layers downwards to the apex, 

 upwards to the auriculo-ventricular aperture, and backwards into the 

 septum. As the layers are raised the fleshy fibres will he seen to change 

 their direction ; and the outer three to be thinner than the three internal. 

 The LEFT VENTRICLE is a hollow cone, and its wall is formed of layers 

 of fibres, as if a flat muscle had been rolled up into a conical figure (fig. 

 103). Seven layers enter into the construction of the wall ; and they are 



MUSCUI.A'R 'FIBRES OF THE AURICLES. 



/ Inferior cava, and e, superior cava of the 

 right auricle. 



g* Right, and 7t, left pulmonary veins of the 

 left auricle. 



a. Transverse fibres of the right, and &, 

 transverse fibres of the left auricle, 

 entering the septum auricularum. 



c. Oblique fibres of the left, and d, of the 

 right auricle. Annular fibres surround 

 the auriculae and the veins. 



