118 ~ OSTEOLOGY 
posterior joins the post-auditory process of the squamo-zygomatic above mentioned. 
Below, it blends internally with the portion of the petro-mastoid which forms the floor of 
the tympanum, and ensheathes the tympanohyal behind. From the outer side of the 
lower part of this ring two tubercles arise ; these grow outwards, and so form the floor 
of the external auditory meatus. The interval between them remains unossified till 
about the age of five or six, after which closure takes place. This deficiency may, how- 
ever, persist even in adult life (see ante, Variations). 
At birth the temporal bone can usually be separated into its component parts. The 
outer surface of the petrous part not only forms the inner wall of the tympanum, but is 
Fic. 91.—C. INNER SURFACE OF THE 
Fic. 91.—A. THE OUTER SURFACE OF THE RIGHT TEMPORAL BONE AT RiauT TEMPORAL BONE AT BIRTH. 
Brrata' B. THE SAME WITH SQUAMO-ZYGOMATIC PORTION REMOVED. 
a, Squamo-zygomatic. 06, Petro- 
(The lettering is the same in both Aand B.) a, Tympanicring. 6, Inner wall squamosal suture and foramen (just 
of tympanum. c, Fenestra rotunda. d, Foramen ovale. e, Mastoid. above the end of the lead line). 
J, Mastoid process. g, Masto-squamosal suture, with foramen for c, Subarcuate fossa. d, Aquieductus 
transmission of vessels. h, Squamo-zygomatic, removed in figure B to vestibuli. e, Aqueductus cochlee. 
show how its descending process forms the outer wall of the mastoid J, Internal auditory meatus. 4g, 
antrum. Upper end of carotid canal. 
hollowed out behind and above to form the inner side of the mastoid antrum, the 
outer wall of which is completed by the post- auditory process of the squamo-zygomatic. 
As yet the mastoid process is undeveloped. It only assumes its nipple-like form about the 
second year. ‘Towards puberty its cancellous tissue becomes permeated with air spaces, 
which are in communication with and extensions from the mastoid antrum. The external 
auditory meatus is unossified in front and below, the outgrowth from the tympanic ring 
occurring subsequent to birth. The glenoid fossa is shallow and everted ; the jugular 
fossa is ill-marked ; whilst the subarcuate fossa is represented by a deep pit, the so-called 
floccular fossa of comparative anatomy. The hiatus Fallopii is an open groove, displaying 
at either end the openings of the inner and outer portions of the Fallopian aqueduct. 
THE SPHENOID BONE. 
The Sphenoid bone (os sphenoidale) lies in front of the basioccipital mesially, 
and the temporals on either side. It enters into the formation of the cranial, 
orbital, and nasal cavities, as well as the temporal, zygomatic, pterygoid, and 
spheno-maxillary fosse. It consists of a body with three pairs of expanded pro- 
cesses, the great wings, the lesser wings, and the pterygoid processes. 
The body (corpus), more or less cubical in form, is hollow, and contains within 
it the two large sphenoidal air sinuses. These are separated by a partition, which 
is usually deflected to one or other side of the middle line. Each sinus extends 
outwards for a short distance into the root of the great wing, and downwards and 
outwards towards the base of the pterygoid process of the same side. They 
communicate by apertures with the upper and back part of the nasal fossze. In the 
adult the posterior aspect of the body displays a sawn surface due to its separation 
from the basioccipital with which in the adult it is firmly ankylosed. The superior 
surface, from the fore angles of which the lesser wings arise, displays an appearance 
comparable to that of an oriental saddle. Over its middle there is a deep depression, 
the sella turcica or pituitary fossa (fossa hypophyseos), in which is lodged the 
