THE TEMPORAL BONES. 113 
posterior border of the external auditory meatus, is usually somewhat thickened 
and rough, and serves for the attachment of the cartilaginous part of the canal. 
The external auditory meatus (meatus acusticus externus) is directed obliquely 
inwards and a little forwards, and describes a slight curve, the convexity of which 
is directed upwards ; of oval form, its long axis, near its orifice, is nearly vertical, but, 
as 1b passes inwards, inclines somewhat forwards so as to give a twist to the canal. 
The upper margin of the outer orifice overhangs considerably the lower edge, but 
owing to the obliquity of the inner aperture, to which the membrana tympani is 
attached, the upper and lower walls of the osseous canal are nearly equal in length. 
The petro-mastoid part (pars petrosa et mastoidea) of the temporal bone, of 
pyramidal form, is fused to the inner aspect of the tympanic and squamosal 
portions, extending behind them, however, to form the well-marked and prominent 
mastoid process, which lies pe sterior to the external auditory meatus. This process 
(pars mastoidea) forms a nipple-like projection, the size of which differs considerably 
in different individuals. Usually larger in the male than in the female, its rough 
outer surface and lower border serve for the insertions of the sterno- mastoid, 
trachelo-mastoid, and splenius capitis muscles. Within and below its pointed 
extremity there is a deep groove (incisura mastoidea), usually well marked, which 
gives origin to the posterior belly of the digastric muscle; whilst lying to the inner 
side of this, and separated from it, by a more or less well-defined rough ridge, there 
can oftentimes be seen a narrow, shallow furrow, which indicates ie course GE the 
occipital artery. The inner surface of the mastoid portion forms, in part, the lateral 
wall of the posterior cranial fossa, in which the cerebellar lobes are lodged. 
Coursing across this aspect of the bone there is a broad curved groove, the con- 
vexity of which is directed forwards and lies in the angle formed by the base of the 
petrous part, and its fusion with the mastoid portion. The depth to which the bone 
is here channelled varies considerably, and is important from a surgical standpoint, 
as herein lies the sigmoid portion of the lateral venous sinus. Anteriorly the 
mastoid is fused with the descending process of the squamosal above, and below, 
where it is united with the tympanic, it enters into the formation of the posterior 
wall of the external auditory meatus and the cavity of the tympanum. Above, its 
free margin is rough and serrated, and articulates with the posterior inferior angle 
of the parietal ; behind and below it ar ticulates by a jagged suture with the 
occipital. Traversing this suture, or near it, is the mastoid foramen (foramen 
mastoideum), which transmits a vein from the lateral sinus to the cutaneous 
occipital vein. 
The petrous part of the petro-mastoid is of the form of an elongated three-sided 
pyramid. By its base it is united obliquely to the imner sides of the squamosal 
and tympanic parts. Its apex is directed inwards, forwards, and a little upwards. 
Its three surfaces are arranged as follows :—The swpervor or anterior looks upwards, 
shghtly forwards, and a little outwards, and forms part of the floor of the middle 
cranial fossa. The postervor is directed backwards and inwards, and forms part of 
the anterior wall of the posterior cranial fossa. The inferior is seen on the under 
surface of the base of the skull, and is directed downwards. The borders are named 
respectively anterior, superior, and posterior. 
The anterior border is short, and forms an acute angle with the fore part of the 
squamous part within which is received the spinous part of the great wing of the 
* sphenoid. Here, too, the osseous Eustachian canal (canalis musculotubarius) may be 
seen leading backwards and outwards from the summit of the angle to reach the 
fore part of the cavity of the tympanum. On looking into it, the canal is seen to 
be divided into two unequal parts by an osseous partition, the cochleariform process 
(septum tubs). The upper compartment, the smaller of the two (semicanalis 
m. tensoris tympani), lodges the tensor tympani muscle, whilst the lower (semi- 
canalis tubee auditivee) forms the osseous part of a channel (the Eustachian tube), 
which serves to conduct air from the pharynx to the tympanum. 
The postervor border is in part articular and in part non-articular. Posteriorly 
and externally it corresponds to the upper margin of an area on the inferior 
surface with which the extremity of the jugular process of the exoccipital articulates. 
In front of that it is irregularly notched, and forms the free anterior edge of the 
