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BONES. 61

front of the diga-stric fossa, is the stvlo-mastoid foramen, which is the
termination of the aqueductus Fallopii, and transmits the facial nerve
and the stylo-mastoid artery. Beneath tlie semicircular curved line are
one or more foramina of variable size which admit veins to the lateral
sinus. When these foramina are large, wounds in this region are dan-
gerous, as the blood would flow freely from the sinus. Sometimes these
veins enter the sinus through the suture.
The internal surface is marked by a deep groove, the sigmoid groove,
for the accommodation of the lateral sinus. Frequently these sinuses
vary greatly in depth in the same skull.
Development.—The temporal bone is developed from four centres
of ossification. Tlie squamous, the zygomatic, and the tympanic por-
tions are developed from
membrane, tlie petrous por- Fig. 20.
tion and the styloid pro-
cess from cartilage. The
1 for S^uamoUyS
squamo-zygomatic portion
fOTlwn inclttcliimi
commences to ossify in the
lower part of the squamous Zijqoma,
portion at the latter part of 2'^? mo.
tlie second or the beginning
of the third month of em-
bryonic life. Ossification
extends upward into the I for Auditor)!
squamous and outward into jtrnetss
the zygomatic portions.
Shortly afterward an os-
sific centre appears in the
lower part of the membran- foT Petrous
h Mcustuid
ous tympanum, ossification
portions
spreading upward and in-
ward until it joins the petro-
mastoid portion behind and If on' StyloiH proo.
the squamo-zygomatic in Development of the Temporal Bone by Four Centres.
front, forming the incom-
plete tympanic ring. Ossification of the petro-mastoid portion com-
mences much later, usually about the end of the fifth or the beginning
of the sixth month of foetal life. The osseous deposits are made at
many points in the cartilage, being all united, however, at birth. The
styloid portion is the last to ossify, remaining cartilaginous until after birth.
The temporal bone is composed of four separate pieces at birth. The
mastoid process has not appeared, and does not commence to develop
until the second year ; from this period it increases in size until adult
life, the air-cells appearing about the age of puberty. The external
auditory meatus at first is shallow, but increases in depth by the out-
growth of the united squamous and petro-mastoid portions above and
behind, and the tympanic portion in front and below.
The glenoid fossa is superficial, the articulating eminence being slight.
By the growth of the tympanic portion downward the depth of the
fossa is increased.