The temporal bones are situated at the sides and base of the skull. Each consists of five parts, viz. the squamous, mastoid, petrous, and tympanic parts, and the styloid process. These represent four morphologically distinct elements which have become fused with one another. The squamous part is a dermal bone, developed to assist in the protection of the cerebrum. The petromastoid portion is preformed in cartilage as a protecting capsule for the membranous labyrinth. The tympanic part, formed in membrane, is homologous with the angular bone, which constitutes a part of the composite lower jaw of many reptilians and bony fishes ; it has become incorporated in the skull and adapted to play a part in the provision of a satisfactory mechanism for the transmission of sound-waves in an air medium. The styloid process represents the dorsal end of the skeletal element of the hyoid arch.
The squamous part of the temporal bone forms the anterior and upper part of the bone, and is scale-like, thin and translucent. Its temporal or external surface (fig. 352) is smooth and slightly convex ; it forms part of the temporal fossa and gives origin to the temporal muscle; above the opening of the external auditory meatus it is marked by a vertical groove for the middle temporal artery. A curved line, termed the supramastoid crest, courses backwards and upwards across its posterior part; it serves for the attachment of the temporal fascia and limits the origin of the temporal muscle.
The boundary between the squamous and mastoid portions of the bone lies about 1-5 cm. below the supramastoid crest and is frequently indicated by traces of the original squamomastoid suture; the external surface of this lower part is convex, and from its anterior part the auricularis posterior takes origin. Between the anterior end of the supra-mastoid crest and the posterosuperior sector of the opening of the external auditory meatus there is a depression, termed the suprameatal triangle ; this triangle is an important landmark for the tympanic antrum, which lies media l to it, at a depth of about 1.25 cm. The anterior part of the depression is usually marked by a small projection, termed the suprameatal spine.
A long, arched process termed the zygomatic process, or zygoma, projects from the lower portion of the squamous part. The posterior part of this process is triangular in shape and springs from a broad base; it is directed laterally, and its surfaces are superior and inferior. The process is then twisted forwards and medially and runs forwards, and the surfaces of this anterior portion are therefore medial and lateral. The superior surface of the posterior part is concave, and continuous with the temporal surface of the squamous part ; the inferior surface is bounded by two roots, a posterior and an anterior, which converge as they approach the anterior part of the process. At the meeting point of the two roots the tubercle of the root of the zygoma gives attachment to the temporomandibular ligament. The posterior root is prolonged forwards from the surface of the squamous part immediately above the opening of the external auditory meatus ; its upper border is continuous behind with the supramastoid crest. The anterior root juts almost horizontally from the side of the squamous part; its inferior surface, convex from before backwards, is smooth for articulation with the articular disc of the mandibular joint, and the whole root presents the form of a short semicylindrical bar, named the articular eminence (tubereulum articulare). The articular eminence forms the anterior boundary of the articular (mandibular) fossa. Very rarely the squamous part is perforated just above the posterior root of the zygoma. When present, this squamosal foramen transmits the petrosquamous sinus.
The superior border is thin, beveled at the expense of the internal surface, and overlaps the inferior border of the parietal bone, forming with it the squamosal suture. Posteriorly the superior border forms an angle (B.N.A. parietal notch) with the mastoid portion of the bone. The antero-inferior border, thin above and thick below, articulates with the greater wing of the sphenoid bone; its upper part is beveled at the expense of the inner, its lower at the expense of the outer, surface.
The mastoid portion of the temporal bone forms the posterior part of the bone. Its outer surface (fig. 352) is rough and gives attachment to the occipital belly of the occipitofrontalis, and the auricularis posterior muscle. It is frequently perforated near its posterior border by the mastoid foramen, which gives exit to a vein from the sigmoid sinus and entrance to a small branch of the occipital artery to the dura mater, the position and size of this foramen are very variable ; it may be situated in the occipital bone, or in the suture between the temporal and the occipital bones. The mastoid portion is continued below into a conical projection, named the mastoid process, the size and form of which vary somewhat; it is larger in the male than in the female. The lateral surface of this process gives insertion to the sternomastoid, splenius capitis, and longissimus capitis; on its medial side there is a deep groove, termed the mastoid notch for the attachment of the posterior belly of the digastric muscle; medial to this notch the shallow occipital groove lodges the occipital artery.
A section of the mastoid process (fig. 354) exhibits a number of spaces, termed the mastoid air cells, which vary greatly in size and number. At the upper and front part of the process they are large and irregular, but towards the lower part they diminish in size, while those at the apex of the process are frequently quite small; occasionally they are entirely absent, and the mastoid is then solid throughout. In addition, a large irregular air-sinus, termed the tympanic antrum, is situated at the upper and front part of the mastoid portion of the bone; it is lined with a prolongation of the raucous membrane of the tympanic cavity. It is bounded above by a thin plate of bone, termed the tegmen tympani, which separates it from the middle fossa of the base of the skull, laterally by the portion of the squamous part which lies below the supramastoid crest ; its medial wall is related to the lateral semicircular canal of the internal ear. Below and behind, the tympanic antrum communicates with the mastoid air cells ; in front, it opens into that portion of the tympanic cavity which is known as the epitympanic recess. The tympanic antrum is a cavity of some considerable size at the time of birth; the mastoid air cells originate as diverticula from the antrum, and begin to appear at or before birth; by the fifth year they are of considerable size, but their development is not completed until puberty.
The petrous portion (or pyramid) of the temporal bone is wedged between the sphenoid and occipital bones at the base of the skull (figs. 322, 327). It is directed medially, forwards, and a little upwards, it has a base, an apex, three surfaces and three margins. The essential parts of the organs of hearing and equilibration are placed within it.
The base is fused with the squamous and mastoid portions, and is partially separated from them by the tympanic antrum.
The apex, rough and uneven, is received into the angular interval between the posterior border of the greater wing of the sphenoid bone and the basilar part of the occipital bone; it is pierced by the anterior orifice of the carotid canal and forms the posterolateral boundary of the foramen lacerum.
The anterior surface helps to form the floor of the middle fossa of the skull and is continuous with the inner surface of the squamous part, to which it is united by the pelrosquamosal suture. Remains of this suture are often distinct even at a late period of life.
The whole surface is marked by impressions for the gyri of the inferior surface of the temporal lobe of the brain. Immediately behind the apex a slight hollow lodges the trigeminal (semilunar) ganglion and it is termed the trigeminal impression. The bone anterior arid slightly lateral to the impression forms the roof of the anterior part of the carotid canal; it is often deficient in this situation. An irregular ridge separates the trigeminal impression posteriorly from a second hollow, which forms part of the roof of the internal auditory meatus and covers the cochlea. This concavity is limited behind by an elevation, termed the arcuate eminence (fig. 353), which is raised by the superior semicircular canal and, in its lateral part, roofs in the vestibule and the beginning of the facial canal. Between the squamous part on the lateral side and the arcuate eminence and the hollows just described on the medial side the surface is formed by the tegmen tympani (fig. 327). This thin plate of bone forms the roof of the tympanic antrum behind and extends forwards above the tympanic cavity and the canal for the tensor tympani muscle. Its lateral margin meets the squamous part at the petrosquamosal suture and turns downwards in front to form the lateral wall of the canal for the tensor tympani and the bony part of the pharyngotympanic (auditory) tube; the lower edge of this down-turned portion has already been observed in the floor of the squamotympanic fissure. Anteriorly the tegmen tympani presents a narrow groove, which runs backwards and laterally and enters the bone through an opening placed in front of the lateral part of the arcuate eminence. This hiatus transmits the greater superficial petrosal nerve, which runs forwards to the foramen lacerum. A second groove may mark the bone on the lateral side of the one just described; it transmits the lesser superficial petrosal nerve from the tympanic plexus. The posterior slope of the arcuate eminence covers the posterior and lateral semicircular canals, and lateral to it the posterior part of the tegmen tympani roofs in the posterior part of the tympanic antrum.
The posterior surface (fig. 353) forms the anterior part of the posterior fossa of the skull and is continuous with the inner surface of the mastoid portion. Near the centre of this surface is an orifice of varying size which leads into the internal auditory meatus, a canal about 1 cm in length, which runs laterally. It transmits the facial and auditory nerves and the internal auditory artery. The lateral end of the internal auditory meatus is separated from the internal ear by a vertical plate, which is divided by a horizontal crest, termed the transverse crest, into two unequal portions (fig. 355). Below the posterior part of the crest, and situated in the inferior veslibular area, are several openings for the transmission of the nerves to the saccule ; below and behind this area is the foramen singulare, which gives passage to the nerve to the posterior semicircular duct. Below the anterior part of the transverse crest is the tractus spiralis foraminosus, consisting of a number of small spirally arranged openings, which encircle the central canal of the cochlea; these openings together with the central canal transmit the nerves to the cochlea. The portion above the transverse crest displays, behind, the superior vestibular area, pierced by a series of small openings for the passage of the nerves to the utricle and the superior and lateral semicircular ducts, and in front, the facial area, with one large opening which is the commencement of the facial canal and transmits the facial nerve. Behind the opening of the meatus there is a small slit almost hidden by a thin plate of bone ; it leads to a canal, named the aquwduct of the vestibule, which contains the ductus endolymphaticus together with a small artery and vein. Above and between these two openings an irregular depression, termed the subarcuate fossa, lodges a process of the dura mater and transmits a small vein; in the infant this depression is represented by a large fossa which extends as a short, blind tunnel under the superior semicircular canal.
The inferior surface (fig. 356), rough and irregular, forms part of the external surface of the base of the skull. It presents the following parts for examination. (1) near the apex there is a quadrilateral rough surface, which serves partly for the attachment of the levator palati (levator veli palatini) muscle and the cartilaginous portion of the pharygotympanic (auditory) tube, and partly for connection with the basilar part of the occipital bone through the intervention of some dense fibrous tissue; (2) behind this a large, nearly circular aperture, leads into the carotid canal ; this canal runs at first vertically, and then, making a bend, is directed horizontally forwards and medially: it transmits into the cranium the internal carotid artery and the carotid plexus of nerves ; (3) behind this opening there is a deep depression, termed the jugular fossa, of variable depth and size in different skulls ; it lodges the superior bulb of the internal jugular vein; (4) in front of the medial part of the jugular fossa and directly below the internal auditory meatus, the hone is marked by a, triangular depression, which lodges the inferior (petrous) ganglion of the glossopharyngeal nerve ; at the apex of this notch a small opening leads into the cochlear canaliculus, which lodges the aqueduct of the cochlea and a tubular prolongation of the dura mater and transmits a vein from the cochlea to join the internal jugular vein; through the aqueduct the perilymph of the labyrinth is enabled to drain away into the subarachnoid space; (5) in the bony ridge dividing the carotid canal from the jugular fossa is the small canaliculus for the tympanic nerve, which is derived from the glossopharyngeal nerve; (6) in the lateral part of the jugular fossa the bone is pierced by the mastoid canaliculus for the entrance of the auricular branch of the vagus nerve; (7) behind the jugular fossa the jugular surface forms a rough quadrilateral area covered with cartilage in the recent state and articulating with the jugular process of the occipital bone; (8) extending laterally from the carotid canal is the sharp lower border of the tympanic part of the bone ; the lateral part of this border splits to ensheath the root of the styloid process and is therefore named the sheath of the styloid process (vagina processes slyloidei) ; (9) emerging from its sheath the styloid process, which is about 2.5 cm in length, is directed downwards and forwards; (10) between the styloid and mastoid processes the stylomastoid foramen forms the lower end of the facial canal and transmits the facial nerve and stylomastoid artery.
At the angle of junction of the petrous and squamous parts two canals are placed one above the other, and separated by a thin plate of bone. Both canals lead into the tympanic cavity; the upper transmits the tensor tympani ; the lower forms the canal of the pharyngotympanic (auditory) tube,
The tympanic part of the temporal bone (fig. 356) is a curved plate lying below the squamous part and in front of the mastoid process. Internally, it is fused with the petrous portion, and appears in the angle between it and the squamous part, where it lies below and lateral to the orifice of the pharyngotympanic (auditory) tube. Behind, it fuses with squamous part and the mastoid process and forms the anterior boundary of the tympanomastoid fissure. Its posterior surface is concave and forms the anterior wall, the floor, and a part of the posterior wall of the bony external auditory meatus ; at the medial end of this surface there is a narrow furrow termed the tympanic sulcus, for the attachment of the circumference of the tympanic membrane. Its anterior surface, quadrilateral and slightly concave, constitutes the posterior wall of the articular fossa and is sometimes in contact with a part of the parotid gland. Its lateral border, free and rough, forms a large part of the margin of the opening of the external auditory meatus and gives attachment to the cartilaginous part of the meatus. The lateral part of the upper border is fused with the back of the postglenoid tubercle; its medial part forms the posterior boundary of the petrotympanic fissure. The lower border is sharp; its lateral part splits to enclose the root of the styloid process and is therefore named the sheath of the styloid process. The central portion of the tympanic part of the temporal bone is thin, and in a considerable percentage of skulls is perforated by a foramen.
The styloid process of the temporal bone, slender: pointed, and averaging about 2.5 cm in length, projects downwards and forwards, from the under surface of the bone. Its proximal part (tympanohyal) is surrounded by a bony sheath, derived from the tympanic plate and best marked on its anterolateral aspect, while its distal part (stylohyal) gives attachment to the stylohyoid and stylomandibular ligaments, and to the styloglossus, stylobyoid and stylopharyngeus muscles. The process is covered by the parotid gland, and the facial nerve crosses its base and the external carotid artery its tip, as they lie within the gland. On its deep aspect the process is separated from the commencement of the internal jugular vein by the origin of the stylopharyngeus muscle.
Structure.-The structure of the squamous part is like that of the other cranial. bones the mastoid portion is spongy, and the petrous portion dense and hard.
Ossfication.-The temporal bone is ossified from eight centres (exclusive of those for the internal ear and the tympanic ossicles)-viz. one each for the squamous and the tympanic part, four for the petrous and mastoid parts, and two for the styloid process. Just before the close of fetal life the bone consists of three principal parts, viz. : the squamous, the petromastoid part, and the tympanic ring (fig. 357). The squamous part is ossified in membrane from a single centre which appears in the region of the root of the zygomatic process about the seventh or eighth week of fetal life and soon extends in an upward direction. The petromastoid part is developed from four centres, which make their appearance in the cartilaginous ear-capsule about the fifth or sixth month of fetal life.
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