THE TEMPORAL BONES. ai 
that by the end of the sixth month the individual centres are more or less fused. Of 
these, one which appears in the vicinity of the eminentia arcuata is the most definite in 
position and form ; from this 
a lamina of bone of spiral ANY 
form is developed, which eth 
covers in the inner limb H 
of the superior semicircular Pee ao, 
canal, and forms the roof of rit y "ites 
the internal auditory meatus, tyrant ne Me tates 
together with the commence- ef 
ment of the Fallopian aque- \ 
duct. Reaching forwards, it — styloid process 
extends to the apex of the Wels OS 
petrous part; whilst exter- 
Glenoid fossa —_\; 
nally it forms part of the Groove for : Tyniggnun 
j - Ww r ‘ membrana 4! EH = ; 
inner wall of the tympanum, teiupaie oo a Gochien 
surrounds the fenestra ovalis, ; 
: Maes ce External _ Internal auditory 
and enclosés within its sub- Oy ; meatus 
meatus } 
stance portions of the coch- 
lea, vestibule, and superior 
semicircular canal. Another 
centre appears in the vicinity 
of the promontory on the 
inner wall of the tympanum, \ 
surrounds the fenestra ‘ 
rotunda, forms the floor of 
the vestibule, and extends in- 
wards to complete the floor 
of the internal auditory 
mals. z Surrounding the Fic. 90.—HorizontaL SECTION THROUGH LEFT TEMPORAL BONE 
cochlea inferiorly and exter- (Lower Half of Section). 
nally, it completes the floor 
of the tympanum, and ultimately blends with the fore and under part of the tympanic 
ring. The carotid canal at first grooves it, and is then subsequently surrounded by 
it. According to Lambertz the lamina spiralis of the cochlea ossifies in membrane. 
The roof of the tympanum is formed from a separate centre, which extends backwards 
towards the superior semicircular canal, and encloses the tympanic part of the 
aqueduct of Fallopius; externally this centre unites by suture with the squamosal, 
and sends down a thin process, which appears between the lips of the Glaserian fissure, 
and forms the outer wall of the Eustachian tube. Nuclei, either single or multiple, 
appear in the base of the petrous part, and envelop the posterior and external semicircular 
canals. It is by extension from this part that the mastoid process is ultimately deve- 
loped. To these centres the terms pro-otic, opisthotic, pterotic, and epiotic, respectively, 
have been applied by Huxley and others. The styloid process, an independent develop- 
ment from the upper end of the cartilage of the second visceral arch, is ossified from two 
centres. The upper or basal appears before birth, and rapidly unites with the petro- 
mastoid, the tympanic plate encircling it in front. This represents the tympanohyal of 
comparative anatomy. At birth, or subsequent to it, another centre appears in the 
cartilage below the above: this is the stylohyal. Ankylosis usually occurs in adult life 
between the tympanohyal and stylohyal, the union of the two constituting the so-called 
styloid process of human anatomy. 
The centre from which the squamo-zygomatic develops appears in membrane about 
the end of the second month. Situated near the root of the zygoma, it extends forwards 
and outwards into that process, inwards to form the floor of the glenoid fossa, and 
upwards into the squamosal. From this latter there is a downward and backward exten- 
sion, which forms the post-auditory process ; this ultimately blends with the posterior limb 
of the tympanic ring, being separated from it in the adult by the auricular fissure. It forms 
the outer wall of the mastoid antrum, and constitutes the fore and upper part of the 
mastoid process in the adult. About the third month a centre appears in the outer 
membranous wall of the tympanum: from this the tympanic ring is developed. Incom- 
plete above, it displays two free extremities. Of these, the anterior is somewhat enlarged, 
and unites in front with the glenoid portion of the squamo-zygomatic, being separated 
from it by the Glaserian fissure and the downgrowth from the tegmen tympani; the 
8b 
i 
1 
Mastoid air-cells-- 
: Pee fenestra 
Se ovalis cut across 
"~Superior semicircular 
“canals 
\ 
Aqueductus Fallopii 
\ External semicirenlar canal 
