96 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
fibres wliicli form the posterior pillars, and consecpiently have the same rigid skeletal 
attachments as the latter. Traced dorsally from the antero -lateral and lateral walls' 
the fibres converge in each half of the dorsal wall towards the crescentic process of 
the tripus (figs. 18 , 20 , 21 , in.sf.). The fibres derived from each antero -lateral wall 
are inserted, at first into the outer margin of the ventral ridge of the tripus, and 
more posteriorly into the outer margin of the crescentic process and the apex of its 
heel-like projection. The fibres derived solely from each lateral wall have their final 
insertions from before backwards into the hinder margin of the heel-like process, and 
into the whole extent of the posterior margin of the crescentic process. From an 
inspection of figs. 18 and 19 it will be apparent that tlie convergence of these 
transversely or rather obliquely arranged fibres in each half of the dorsal Avail to 
their connection with the corresponding tripus causes the greater part of the dorsal 
Avail of the chamber to present the appearance of being formed of tAvo broad and 
Avell-defined triangular sheets of fibres, with the. apex of each sheet connected with 
the tripus of its side, and the broad base continued outAAm-ds into the antero-lateral 
and lateral walls. Reference to the same figures Avill also shoAv that the oblique 
posterior margins of the tAvo sheets slightly overlap the skeletal insertions of the 
posterior pillars into the dorsal lamina. 
The fibres forming the outer stratum of the tunica externa in the lateral and 
antero-lateral Avails (figs. 19 , 20 , 22 , o.st.) if traced into the dorsal Avail also coiiA’erge 
towards the corresponding tripus, and, in so doing, form the outer of the tAvo strata 
of AA'hich each triangular sheet is composed. For the latter part of their course, that 
is, as they approach the tripus, these fibres coincide in direction Avith the corre- 
sponding fibres of the inner stratum (compare figs. 19 and 20 , o.st., in.st.). 
Eventually their inner extremities, like the analogous fibres of the inner stratum are 
successively inserted from before backwards into the oblique ventral ridge, the heel- 
like projection, and the posterior margin of the crescentic jrrocess of the tripus 
(figs. 20, 22, o.st.). The insertion of these fibres into the different regions of 
the crescentic process, although practically identical with the corresponding inser- 
tions of the fibres of the inner stratum, is, of course, external to the latter. Traced 
outAvards from either tripus the precise course Avhich the fibres of the outer stratum 
pursue varies someAvhat according to their points of insertion into that ossicle. The 
most anterior fibres, that is, those starting from the anterior end of the ventral ridge, 
after traversing the anterior part of the dorsal wall curve doAvn wards into the antero- 
lateral Avail, and finally extend into the ventral Avail, Avhere their disposition is 
mainly longitudinal or oblique (fig. 19 ). The bulk of the fibres, however, as they 
successively pass from the regions of the tripus already indicated, on reaching the 
antero-lateral and lateral walls loop backAvards, and in doing so describe a character- 
istic curvature, the convexity of which is directed toAvards the ventral surface, and 
the concavity towards the dorsal surface, and then, as they are traced backAvards, 
extend into the lateral, and finally into the dorsal Avail of the lateral compartment of 
