94 
PROFESSORS T. W. BRIDGE AND A. C. HADDON 
the component fibres of the latter stratum separate on the ventral face of the fifth 
vertebral centrum into two thick diverging sheets which pass dorsally, one on each 
side of the centrum, and, at the same time, curve slightly forwards into the dorsal 
wall of the anterior chamber ; eventually, after separating from the rest of the dorsal 
rvall, the fibres of each sheet become firmly inserted, from the median line outw'ards, 
into the ventral surface and obliquely along the sides of the centrum in question, and 
into the ventral surface of the transverse process of the fourth vertebra (figs. 19-20, 
p.p.). The line of attachment of each sheet coincides with the ridge-like dorsal 
margin of each superficial ossification and the posterior margin of the dorsal lamina, 
which, as we have previously described, form by their convergence anteriorly a 
characteristic A-shaped ridge (fig. 3). As these divergent sheets of fibres firmly 
attach the whole extent of the dorsal margin of the posterior wall to the axial 
skeleton, and physiologically must have the effect of preventing any distension or 
contraction of the anterior chamber in the posterior direction, they may not inaptly be 
termed its “posterior pillars.” 
{h.) If the fibres of the anterior layer of the transverse septum which form the 
posterior pillars by their dorsal attachment to the skeleton are traced ventrally 
instead of dorsally, they will be found to curve forwards into the ventral wall of the 
anterior chamber, where they assume a direction which varies according as they are 
mesially or more laterally situated, and, at the same time, represent the inner stratum 
of the tunica externa of this region. The more mesial fibres pursue a straight course 
forwards in the median line of the ventral wall, and ultimately curve upwards into 
the mesial portion of the anterior Avail, where they form a vertically disposed stratum 
of fibres, and also blend with the corresponding portion of the outer stratum of the 
tunica externa, the fibres of which are also more or less vertically arranged. Beneath 
the anterior third of the complex centrum the confluent mesial portions of the two 
strata split into two slightly divergent bundles of fibres, which pass dorsally, one on 
each side of the centrum, to the ventral surface and sides of which, and to the radial 
nodules, they become closely adherent, leaving, hoAvever, a slip of fibres from each 
bundle to be continued dorsally to an insertion into the ventral ridge of the tripus 
(figs. 19 and 20, a.p>.). As these bundles probably have much the same physiological 
significance with regard to the anterior wall that the posterior pillars bear to the 
posterior wall, and, by reason of their attachments to the com})lex centrum, render 
the anterior wall incapable of partaking in any enlargement or contraction of the 
anterior chamber, they may be designated the “ anterior pillars.” Their rigid skeletal 
attachments are not so extensive as in some other Siluridse {e.g., Arius), but, taken 
in conjunction with the transverse membrane and the post-temporal plates, they are 
obviously competent to prevent any forward bulging of the anterior wall. As we shall 
subsequently point out, the more laterally situated fibres of the inner stratum of the 
tunica externa of the ventral wall pursue an oblique course forwards, outwards, and 
upwards, and are then traceable into the antero-lateral and lateral wall of each half 
