437 



turn embrace those next in succession, and so on until the central 

 layer is reached, an arrangement which may in part explain alike 

 the rolling movements and powerful action of the ventricles. 



The Lecturer next drew attention to the manner in which the 

 external fibres pass into the interior of the ventricle to form the 

 musculi papillares. He first remarked that when the external fibres 

 get into the interior they are necessarily confined to a smaller area, 

 and are therefore crowded into a mass of greater thickness, which 

 contributes to form the papillary muscles. He then showed that the 

 external fibres, entering at the apex and forming the "vortex," pass 

 inwards in two principal parcels or bundles, one of which comes 

 chiefly from the posterior surface of the ventricle, and winds for- 

 wards to enter the apex anteriorly, whilst another comes from the 

 anterior surface, and winds backwards to enter the apex posteriorly, 

 a fact which the Lecturer believes has been hitherto overlooked. On 

 entering the cavity, the anterior bundle crosses to the posterior wall, 

 and forms the posterior papillary muscle, whilst the posterior bundle 

 forms the anterior papillary muscle. The fact of this double en- 

 trance, and its relation to the papillary muscles, was shown in various 

 preparations ; and it was remarked that, but for this double entrance, 

 which applies to all the external layers, the apex of the ventricle 

 would be like the barrel of a pen cut slantingly, or, in fact, lop- 

 sided ; whereas, by the arrangement described, it is rendered bilate- 

 rally symmetrical. 



To bring this bilateral entrance and symmetry into harmony with 

 the description already given of the succession of layers, and with 

 the illustration of the conically rolled sheet, the Lecturer explained 

 that we must regard the primary sheet as having split into two, or 

 we must suppose a second one superadded, and rolled up along with 

 the first. In fact, if a second sheet of net with parallel threads 

 be laid on the first, so that the threads upon it intersect those of 

 the first at an acute angle, and the two are then rolled up together 

 in the way already described, the result will be that the opening at 

 the apex will have two symmetrical lips, as it were, representing the 

 two parcels of fibres forming the vortex in the natural heart. 



It is well known that the wall of the left ventricle is thickest at 

 about a third of its length from the base, and that from this point 

 it decreases in thickness towards the base, and still more towards the 



