MORPHOLOGY OF THE VERTEBRATE SKULL aE 
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Fig. 19 Relations of the auditory ossicles to the auditory capsule and tubo- 
tympanal cavity. 
A, Cross section of the auditory region in a human embryo of three months. 
After Minot (lettering modified). 
The developing ossicula (St., Mal.) lie above the incipient tympanic cavity (cav. ty.) which is merely 
a dilatation of the Eustachian tube (tu.ews.), the supposed homologue of the spiracular gill cleft. 
m.a.e., external auditory meatus; mb.ty., tympanic membrane, separating the external auditory 
' meatus from the Eustachian tube. 
B, Hypothetical scheme of the, relations of the stapes (St’) to the reduced 
quadrate and articular and to the tubo-tympanal cavity in a pro-mammalian 
stage. 
The quadrate and articular should have been lowered to the region where the stapes joins the extra- 
columella. The essential idea is the upgrowth of the tubotympanal cavity around the quadrate and 
articular. 
prootic and the zygomatic branch of the squamosal; (b) in articu- 
lating with the articular (=malleus) by a convex-concave joint. 
6. The fact that the quadrate is attached to and partly coy- 
ered by the squamosal in Cynodonts (fig. 16) considered by all 
neontologists an insuperable objection to relationship with the 
mammals was clearly recognized (’10 a, p. 139); but it was hinted 
that as only the dorsal prolongation of the quadrate was covered 
by the squamosal, antero-posterior pressure on the lower end 
of the quadrate would tend to loosen the upper end from its 
squamosal attachment and thus to transform a monimostylic 
into a streptostylic condition.® 
8 Jt also seems reasonable to infer that as the new squamoso-dentary joint 
was being established the old quadrato-articular joint would be more or less 
wrenched by the pull of the temporal and other muscles. The matter is further 
discussed below, p. 36. 
