THE HARVEIAN ORATION. 741 



if not the sole criterion of homology— may be spoken of as being 

 but trabeeulae flaked off" from the inner surface of the wall of a 

 muscular sac, and subsequently made more or less membranous in 

 the way of specialisation and its correlative economy. Thus, as Ge- 

 genbaur (Vergleichende Anatomic, 2nd edition, p. 836) has remarked, 

 the intervalvular space in these animals corresponds to the entire 

 cavity of the spongy-walled heart of fishes and amphibia ; and the 

 sinuous intertrabecular cavities in the spongy walls of these latter 

 animals correspond with the chief part — viz. the extravalvular part of 

 the ventricular space — in mammals, birds, and Crocodilina. Now, the 

 musculi papillares represent the disposal or destination of the inner- 

 most layer of the right ventricle, according to Dr. Pettigrew (see 

 his paper, Phil. Trans. 1864, p. 479) ; and I would submit that the 

 moderator band is but a specialisation of the next layer in order 

 from within outwards — to wit, Dr. Pettigrew's sixth layer, which 

 he has figured (Plate XIV. fig. ^^) as proceeding in a spiral direc- 

 tion from right to left, much as the fibres of the moderator bands I 

 have figured do. A study of the heart of the rabbit will put this 

 matter in a very clear light, and further open our eyes to see and 

 recognise the rudimentary representation of this moderator band in 

 our own hearts. If we look at the outer aspect of that very con- 

 stant musculus papillaris, which passes in man from the outer and 

 movable wall of the right ventricle to distribute its chordae ten- 

 dineae to the two more anteriorly placed of the three segments of 

 its auricular valve, we shall frequently see that its longitudinal 

 fibres are crossed nearly or quite at right angles by a slender fibrous 

 band, so that we have before us an appearance not wholly nor 

 essentially unlike that presented by the striae longitudinales of 

 Lancisi and the fibres of the corpus callosum when viewed in their 

 mutual connection. This band of fibres can sometimes be traced up 

 towards the conus arteriosus, and be seen not to die away until 

 close upon the point of origin of the most anteriorly or upwardly 

 placed of the chordae tendineae which arise from the septum to pass 

 to the hindermost of the three segments of the tricuspid. The 

 points between which this line of fibres lies may be observed to be 

 the very same as those between which the moderator bands in the 

 cassowary and the sheep stretch as free columns in the diagrams 

 before you. It is not altogether rare to see this band raise itself 

 from the position of fusion, like the ventricular wall, and assume 



