128 OSTEOLOGY. 



bone, of pyramidal form, is fused to the medial aspect of the tympanic and 

 squamosal portions, extending behind them, however, to form the well-marked and 

 prominent mastoid process, which lies posterior to the external acoustic meatus. 

 This process 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 

 lateral surface and inferior border serve for the insertions of the sterno-mastoid, 

 splenius capitis, and longissimus 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 

 medial 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 the course 

 of the occipital artery. The medial surface of the mastoid portion forms, in part, the 

 side wall of the posterior cranial fossa, in which the cerebellar hemispheres 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 transverse venous sinus. Anteriorly 

 che 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 acoustic meatus and the cavity of the tympanum. Above, 

 its free margin is rough and serrated, and articulates with the mastoid angle 

 of the parietal; behind and below it articulates by a jagged suture with the 

 occipital. Traversing this suture, or near it, is the mastoid foramen, which 

 transmits a vein from the transverse sinus to the cutaneous occipital vein, together 

 with a small branch of the occipital artery. 



The petrous part (pyramis) of the petro-mastoid is of the form of an elongated 

 three-sided pyramid. By its base it is united obliquely to the inner sides of the 

 squamosal and tympanic parts. Its apex is directed medially, forwards, and a little 

 upwards. Its three surfaces are arranged as follows : The anterior looks upwards, 

 slightly forwards, and a little laterally, and forms part of the floor of the middle 

 cranial fossa. The posterior is directed backwards and medially, 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 margins or angles 

 are named respectively anterior, superior, and posterior. 



The anterior margin is short, and forms an acute angle with the anterior part of 

 the squamous part; within this angle is wedged the spinous part of the great wing 

 of the sphenoid. Here, too, the osseous part of the auditory tube (canalis musculo- 

 tubarius) may be seen leading backwards and laterally from the summit of the angle 

 to reach the anterior part of the cavity of the tympanum in the interior of the bone. 

 On looking into it, the canal is seen to be divided into two unequal parts by 

 an osseous partition, the septum tubse. The upper compartment, the smaller of 

 the two (semicanalis m. tensoris tympani), lodges the tensor tympani muscle, 

 whilst the lower (semicanalis tubse auditivae) forms the osseous part of a channel 

 (the auditory tube), which serves to conduct air from the pharynx to the 

 tympanic cavity. 



The posterior margin is in part articular and in part non-articular. Pos- 

 teriorly and laterally it corresponds to the upper margin of an area on the inferior 

 surface with which the extremity of the jugular process of the occipital articulates. 

 In front of that it is irregularly notched, and forms the free anterior edge of the 

 jugular foramen, medial to which it has a sharp curved border, often grooved, 

 reaching to the apex. This groove, which is completed by articulation with the 

 side of the basi-occipital, lodges the inferior petrosal venous sinus. 



The superior margin is a twisted edge which is continuous with the upper 

 margin of the sulcus for the transverse sinus posteriorly, and anteriorly and 

 medially reaches the apex of the bone. Eunning along it there is usually a 

 well-marked groove for the superior petrosal venous sinus, and near its medial 

 extremity it is slightly notched for the passage of the trigeminal nerve. Along 

 the entire length of this border the tentorium cerebelli is attached. 



