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bevelled at the expense of its inner table, except in front, where the margin is thick and stout. There it articulates with the great wing of the sphenoid, its union with that bone extending to near the anterior part of the summit of the curve, behind which it is united to the parietal, overlapping the squamous border of that bone; posteriorly the free margin of the squamous part ends at an angle formed between it and the mastoid process called the incisura parietalis.
Pars Tympanica.-The tympanic part of the temporal bone forms the anterior, lower, and part of the posterior wall of the external acoustic meatus. Bounded in front and above by the petro-tympanic fissure, it forms the posterior wall of the non-articular part of the mandibular fossa. Fused medially with the petrous part, its lower edge, sharp and well defined medially, splits to enclose the root of the projecting styloid process, and is hence called the vagina processus styloidei (sheath of the styloid process). Laterally it unites with the anterior part of the
'mastoid process, and higher up with the descending process of the squamous part, from both of which it is separated by the tympano-mastoid fissure, through which the auricular branch of the vagus escapes. Its free border, which forms the anterior, lower, and part of the posterior border of the external acoustic meatus, is usually somewhat thickened and rough, and serves for the attachment of the cartilaginous part of the external acoustic meatus.
The meatus acusticus externus (external acoustic meatus) 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, close to its orifice, is nearly vertical, but, as it passes inwards, inclines somewhat forwards so as to give a twist to the canal. The depth of the canal to the attachment of the membrana tympani averages from 14 to 16 mm. The superior 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 superior wall of the osseous canal only exceeds the length of the lower wall by one or two millimetres.
Pars Petrosa et Pars Mastoidea.-The petro-mastoid part of the temporal
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 convexity 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 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 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 musculotubarius) 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 tubæ. The upper compartment, the smaller of the two (semicanalis m. tensoris tympani), lodges the tensor tympani muscle, whilst the lower (semicanalis tubæ auditivæ) 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. Posteriorly 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. Running 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.
On the inferior surface of the petrous part, which is bounded in front by the anterior border medially, the tympanic plate laterally, and behind by the posterior border, the following structures are to be noted:-Springing from and surrounded by its sheath is the slender and pointed processus styloideus, the length of which varies much. Projecting downwards and slightly forwards and medially, it affords attachments for the stylo-glossus, stylo-hyoid, and stylo-pharyngeus muscles, as well as the stylo-hyoid and stylo-mandibular ligaments. Just behind it, and between it and the mastoid process, is the foramen stylomastoideum, which lies at the anterior end of the mastoid groove, and transmits the facial nerve and the stylo-mastoid artery. Just medial to the styloid process there is a deep, smooth, excavated hollow, the fossa jugularis, which is converted into a foramen (jugular) by articulation with the occipital bone. Behind and lateral to the fossa there is a small quadrilateral surface of variable size, which is united to the extremity of the jugular process of the exoccipital by a synchondrosis. Inside the fossa, on its lateral part, or placed on its lateral border, is the opening of a small canal (canaliculus mastoideus), which passes laterally to Tuberculum open into the canalis facialis, and transmits the auricular branch of the vagus, which ultimately escapes through the petro-mastoid fissure (vide ante). In front of the jugular fossa and separ- Petro-mastoid fissure
ated from it by a sharp
Groove for inferior petrosal sinus Aqueduct of cochlea (external orifice of) Canal for the tympanic nerve Jugular fossa
FIG. 138. THE RIGHT TEMPORAL BONE SEEN FROM BELOW. The squamo-zygomatic part is coloured blue; the petro-mastoid, red. The tympanic portion and styloid process are left uncoloured.
apex of the bone corresponds to a rough quadrilateral surface which forms the floor of the carotid canal, and also serves for the attachment of the cartilaginous part of the auditory tube as well as the origin of the levator veli palatini muscle; elsewhere it has attached to it the dense fibrous tissue which fills up the cleft (petro-basilar fissure) between it and the basilar part of the occipital bone.
The anterior surface of the petrous part bears the impress of the gyri of the lower surface of the temporal lobe of the cerebrum, which rests upon it; in addition, there is a distinct but shallow depression (impressio trigemini) near the apex, corresponding to the roof of the carotid canal; in this is lodged the semilunar ganglion on the sensory root of the trigeminal nerve. Lateral to the middle of the anterior surface, and close to its superior border, is the elevation (eminentia arcuata), more or less pronounced, which marks the position of the superior semicircular canal, here developed within the substance of the bone. A little
Preparation to display the position and relations of the tympanic antrum. The greater part of the posterior wall of the external acoustic meatus has been removed, leaving only a bridge of bone at its medial extremity; under this a bristle is displayed, passing from the tympanic antrum through the iter to the cavity of the tympanum.
in front of this, and in line with the angle formed by the anterior border and the squamous part, is the slit-like opening of the hiatus canalis facialis, within the projecting lip of which two small orifices can usually be seen. These are the openings of the canalis facialis; if a bristle is passed through the more medial of the two openings it will be observed to pass into the bottom of the internal acoustic meatus, if into the more lateral, it will pass through the facial canal, and, provided the channel be clear, will appear on the inferior surface of the bone at the stylo-mastoid foramen. Leading forwards and medially from the hiatus towards the anterior border is a groove; in this lies the greater superficial petrosal nerve, which passes out of the hiatus. A small branch of the middle meningeal artery also enters the bone here. A little lateral to the hiatus is another small opening (apertura superior canalis tympanici), often difficult to see; from this a groove runs forwards which channels the upper surface of the roof of the canal for the tensor tympani muscle. Through this foramen and along this groove passes the lesser superficial petrosal nerve. Behind this, and in front of the arcuate eminence, the bone is usually thin (as may be seen by holding it up to he light falling through the external acoustic meatus), roofing in the cavity
within the bone called the tympanum and forming the tegmen tympani. the line of fusion of the petrous with the squamous part is often indicated by a faint and irregular petro-squamous fissure.
Posterior Surface. The most conspicuous object on the posterior surface of the petrous part of the bone is the meatus acusticus internus (internal acoustic meatus), about 8 mm. deep in the adult. This has an oblique oval aperture, and leads laterally and slightly downwards into the substance of the bone, giving passage to the acoustic and facial nerves, together with the nervus intermedius and the auditory branch of the basilar artery. The canal appears to end blindly; but if it is large, or still better, if part of it is cut away, its fundus will be seen to be crossed by a horizontal ridge, the falciform crest, which divides it into two fossæ, the floors of which (laminæ cribrosæ) are pierced by numerous small foramina for the branches of the acoustic nerve and the vessels passing to the membranous labyrinth, whilst in the anterior and upper part of the higher fossa the orifice of the canalis facialis, through which the facial nerve passes, is seen leading in the direction of the hiatus canalis facialis (ride supra). Lateral to the internal acoustic meatus and above it, close to the superior border, an irregular depression, often faintly marked, with one or two small foramina opening into it, is to be noticed. This is the fossa subarcuata, best seen
in young bones (see Fig. 143 C), where it forms a distinct recess, which is bounded above by the bulging caused by the superior semicircular canal, within the concavity of which it is placed; it lodges a process of the dura mater. Below and lateral to this, separated from it by a smooth, elevated curved ridge, is the opening of the apertura externa aquæductus vestibuli (aqueduct of the vestibule), often concealed in a narrow curved fissure overhung by a sharp scale of bone. In this is lodged the saccus endolymphaticus, Internal developed as an evagination from the otocyst, together with a small vein. The ridge above it corresponds to the upper half of the posterior semicircular canal.
External acoustic meatus Osseous part of the auditory tube
Superior opening of the canal for the tympanic branch of glosso-pharyngeal
Fenestra vestibuli cut across
FIG. 140.-VERTICAL TRANSVERSE SECTION THROUGH THE LEFT
Connexions. The temporal bone articulates with the zygomatic, sphenoid, parietal, and occipital bones, and by a movable joint with the mandible. Occasionally the temporal articulates with the frontal, as happens normally in the anthropoid apes; although the region of the pterion is characterised by an X-like form, in the lower races of man there is no evidence that the occurrence of a fronto-squamosal sature is more frequent in the lower than the higher races, its occurrence being due to the manner of fusion of the so-called epipteric ossicles with the surrounding bones.
Ossification. The temporal bone of man represents the fused periotic, squamosal, and tympanic elements; the two latter are membrane or investing bones, whilst the former is developed in cartilage around the auditory capsule. The cartilages of the I. and II. visceral arches are also intimately associated with its development, as will be elsewhere explained (Appendix E). The human temporal bone is characterised by the large proportionate size of the squamosal, the comparatively small size of the tympanic, the absence of an auditory bulla, and the exceptional development of the mastoid process.
Ossification commences in the ear capsule in the fifth month, and proceeds so rapidly that by the end of the sixth month the individual centres are more or less fused. Of these, one, the Pro-otic (Huxley), 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 form is developed, which covers in the medial limb of the superior semicircular canal, and forms the roof of the internal acoustic meatus, together with the commencement of the