370 THE HEART. 



It has been thought that a similar continuity was the rule at the base of the heart 

 also, and that few if any of the bundles are attached to the tendinous rings. But 

 although some bundles may turn round at the auriculo-ventricular openings, this is by 

 no means general, and most of the muscular fasciculi are attached to the fibrous and 

 fibro-cartilaginous structures at the base, either directly or through the medium of 

 the chordse tendineae and segments of the valves. 



The greater part of the thickness of the wall of the left ventricle is formed of 

 fibres which are attached above, like those just described, to the fibrous rings at the 

 base. From there they pass obliquely downwards in the posterior and left wall, 

 and more directly downwards in the anterior wall towards the apex. But before 

 reaching this they are inclined, especially the anterior fibres, somewhat abruptly 

 towards the front of the septum, which they enter near its lower end. They may 

 now be divided into three sets. (1) Many pass at once obliquely upwards in the 

 septum to be attached to the central fibro-cartilage, so that these may be regarded 

 as forming simple Y-shaped muscular loops around the cavity of the left ventricle. 

 (2) Others cross in the septum in their passage upwards and pass to the posterior 

 wall of the right ventricle, being partly continuous with its posterior papillary 

 muscle. (3) Others again pass nearly horizontally into the posterior wall of the 

 left ventricle, and take an annular course in it. But many of these annular 

 fibres of the left ventricle (which are found about the middle of its thick- 

 ness) are continuous with the deeper fibres of the right ventricle. 



Fig. 322. VIEW OP THE FIBRES OF THE SHEEP'S 



HEART, DISSECTED AT THE APEX TO SHOW THE 



"VORTEX." (Pettigrew.) 



a, a, fibres entering the apex posteriorly at b 

 c, c, fibres entering the apex anteriorly at d. 



The right ventricle so far as regards 

 the arrangement of its muscular fasciculi 

 may be looked upon as an appendage of 

 the left. Its superficial fibres are directly 



continuous, as we have seen, with the papillary muscles of the left ventricle. Its 

 deeper fibres also mostly pass into continuity with those of the left ventricle. 

 Attached above to the fibrous structure encircling the right auriculo-ventricular and 

 the pulmonary apertures, either directly or (in the case of the papillary muscles) 

 indirectly through the chordse tendinese they pass towards the septum. The posterior 

 fibres enter this behind and pass forwards in it to the front of the left ventricle (as 

 above described under 2) ; the anterior enter the septum in front and pass back- 

 wards in it, intercrossing with those just mentioned, to the posterior part of the left 

 ventricle ; while the middle set of fibres, derived chiefly from the external wall of 

 the right ventricle, enter the lower part of the septum and are directed upwards in 

 it to be attached to the central fibro-cartilage. Finally a considerable number of 

 fasciculi pass straight across the septum without turning into it and appear to 

 encircle both ventricles. There are besides certain sets of fibres which appear not 

 readily assignable to any of those above described : some, for instance, which 

 encircle the pulmonary orifice, and others which, as Sibson has shown, radiate 

 upwards from the base of the anterior papillary muscle of the right ventricle, to be 

 attached to the tendinous structures at the base of the heart, especially to the 

 pulmonary ring, opposite the two anterior sinuses of Yalsalva. 



In the middle of the thickness of the ventricular wall the fibres are, as before 

 said, annular and transverse (fig. 323, 4, 4') ; but, as Ludwig showed, they pass by 



