Temporal Bone

Figure 2 : Left temporal bone. Inner surface.

The internal surface of the squama [Fig. 2] is concave; it presents depressions corresponding to the convolutions of the temporal lobe of the brain, and grooves for the branches of the middle meningeal vessels.

The inner surface of the mastoid portion presents a deep, curved groove, the sigmoid sulcus, which lodges part of the transverse sinus; in it may be seen the opening of the mastoid foramen. The groove for the transverse sinus is separated from the innermost of the mastoid air cells by a very thin lamina of bone, and even this may be partly deficient.

A section of the mastoid process [Fig. 3] shows it to be hollowed out into a number of spaces, the mastoid cells, which exhibit the greatest possible variety as to their size and number. At the upper and front part of the process they are large and irregular and contain air, but toward the lower part they diminish in size, while those at the apex of the process are frequently quite small and contain marrow; occasionally they are entirely absent, and the mastoid is then solid throughout. In addition to these a large irregular cavity is situated at the upper and front part of the bone. It is called the tympanic antrum, and must be distinguished from the mastoid cells, though it communicates with them. Like the mastoid cells it is filled with air and lined by a prolongation of the mucous membrane of the tympanic cavity, with which it communicates. The tympanic antrum is bounded above by a thin plate of bone, the tegmen tympani, which separates it from the middle fossa of the base of the skull; below by the mastoid process; laterally by the squama just below the temporal line, and medially by the lateral semicircular canal of the internal ear which projects into its cavity. It opens in front into that portion of the tympanic cavity which is known as the attic or epitympanic recess. The tympanic antrum is a cavity of some considerable size at the time of birth; the mastoid air cells may be regarded as diverticula from the antrum, and begin to appear at or before birth; by the fifth year they are well-marked, but their development is not completed until toward puberty.

1. near the center, an eminence (eminentia arcuata) which indicates the situation of the superior semicircular canal
2. in front of and a little lateral to this eminence, a depression indicating the position of the tympanic cavity: here the layer of bone which separates the tympanic from the cranial cavity is extremely thin, and is known as the tegmen tympani
3. a shallow groove, sometimes double, leading lateralward and backward to an oblique opening, the hiatus of the facial canal, for the passage of the greater superficial petrosal nerve and the petrosal branch of the middle meningeal artery
4. lateral to the hiatus, a smaller opening, occasionally seen, for the passage of the lesser superficial petrosal nerve
5. near the apex of the bone, the termination of the carotid canal, the wall of which in this situation is deficient in front
6. above this canal the shallow trigeminal impression for the reception of the semilunar ganglion.

Figure 4 : Diagrammatic view of the fundus of the right internal acoustic meatus. (Testut.) Crista falciformis. Area facialis, with (2’) internal opening of the facial canal. Ridge separating the area facialis from the area cribrosa superior. Area cribrosa superior, with (4’) openings for nerve filaments. Anterior inferior cribriform area, with (5’) the tractus spiralis foraminosus, and (5’’) the canalis centralis of the cochlea. Ridge separating the tractus spiralis foraminosus from the area cribrosa media. Area cribrosa media, with (7’) orifices for nerves to saccule. Foramen singulare.

The inferior surface [Fig. 5] is rough and irregular, and forms part of the exterior of the base of the skull. It presents eleven points for examination:

1. near the apex is a rough surface, quadrilateral in form, which serves partly for the attachment of the Levator veli palatini and the cartilaginous portion of the 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 is the large circular aperture of the carotid canal, which ascends at first vertically, and then, making a bend, runs horizontally forward and medialward; it transmits into the cranium the internal carotid artery, and the carotid plexus of nerves;
3. medial to the opening for the carotid canal and close to its posterior border, in front of the jugular fossa, is a triangular depression; at the apex of this is a small opening, the aquæductus cochleæ, which lodges a tubular prolongation of the dura mater establishing a communication between the perilymphatic space and the subarachnoid space, and transmits a vein from the cochlea to join the internal jugular;
4. behind these openings is a deep depression, the jugular fossa, of variable depth and size in different skulls; it lodges the bulb of the internal jugular vein;
5. in the bony ridge dividing the carotid canal from the jugular fossa is the small inferior tympanic canaliculus for the passage of the tympanic branch of the glossopharyngeal nerve;
6. in the lateral part of the jugular fossa is the mastoid canaliculus for the entrance of the auricular branch of the vagus nerve;
7. behind the jugular fossa is a quadrilateral area, the jugular surface, covered with cartilage in the fresh state, and articulating with the jugular process of the occipital bone;
8. extending backward from the carotid canal is the vaginal process, a sheath-like plate of bone, which divides behind into two laminæ; the lateral lamina is continuous with the tympanic part of the bone, the medial with the lateral margin of the jugular surface;
9. between these laminæ is the styloid process, a sharp spine, about 2.5 cm. in length;
10. between the styloid and mastoid processes is the stylomastoid foramen; it is the termination of the facial canal, and transmits the facial nerve and stylomastoid artery;
11. situated between the tympanic portion and the mastoid process is the tympanomastoid fissure, for the exit of the auricular branch of the vagus nerve.

Figure 5 : Temporal bone. Inferior surface.

At the angle of junction of the petrous part and the squama are two canals, one above the other, and separated by a thin plate of bone, the septum canalis musculotubarii (processus cochleariformis); both canals lead into the tympanic cavity. The upper one (semicanalis m. tensoris tympani) transmits the Tensor tympani, the lower one (semicanalis tubæ auditivæ) forms the bony part of the auditory tube.

The tympanic cavity, auditory ossicles, and internal ear, are described with the organ of hearing.

The external acoustic meatus is nearly 2 cm. long and is directed inward and slightly forward: at the same time it forms a slight curve, so that the floor of the canal is convex upward. In sagittal section it presents an oval or elliptical shape with the long axis directed downward and slightly backward. Its anterior wall and floor and the lower part of its posterior wall are formed by the tympanic part; the roof and upper part of the posterior wall by the squama. Its inner end is closed, in the recent state, by the tympanic membrane; the upper limit of its outer orifice is formed by the posterior root of the zygomatic process, immediately below which there is sometimes seen a small spine, the suprameatal spine, situated at the upper and posterior part of the orifice.

Figure 6 : The three principal parts of the tempora bone at birth. 1. Outer surface of petromastoid part. 2. Outer surface of tympanic ring. 3. Inner surface of squama.
Figure 7 : Temporal bone at birth. Outer aspect.	Figure 8 : Temporal bone at birth. Inner aspect.

1. The squama is ossified in membrane from a single nucleus, which appears near the root of the zygomatic process about the second month.
2. The petromastoid part is developed from four centers, which make their appearance in the cartilaginous ear capsule about the fifth or sixth month. One (proötic) appears in the neighborhood of the eminentia arcuata, spreads in front and above the internal acoustic meatus and extends to the apex of the bone; it forms part of the cochlea, vestibule, superior semicircular canal, and medial wall of the tympanic cavity. A second (opisthotic) appears at the promontory on the medial wall of the tympanic cavity and surrounds the fenestra cochleæ; it forms the floor of the tympanic cavity and vestibule, surrounds the carotid canal, invests the lateral and lower part of the cochlea, and spreads medially below the internal acoustic meatus. A third (pterotic) roofs in the tympanic cavity and antrum; while the fourth (epiotic) appears near the posterior semicircular canal and extends to form the mastoid process (Vrolik).
3. The tympanic ring is an incomplete circle, in the concavity of which is a groove, the tympanic sulcus, for the attachment of the circumference of the tympanic membrane. This ring expands to form the tympanic part, and is ossified in membrane from a single center which appears about the third month. The styloid process is developed from the proximal part of the cartilage of the second branchial or hyoid arch by two centers: one for the proximal part, the tympanohyal, appears before birth; the other, comprising the rest of the process, is named the stylohyal, and does not appear until after birth. The tympanic ring unites with the squama shortly before birth; the petromastoid part and squama join during the first year, and the tympanohyal portion of the styloid process about the same time [Fig. 7, 8]. The stylohyal does not unite with the rest of the bone until after puberty, and in some skulls never at all.

1. The tympanic ring extends outward and backward to form the tympanic part. This extension does not, however, take place at an equal rate all around the circumference of the ring, but occurs most rapidly on its anterior and posterior portions, and these outgrowths meet and blend, and thus, for a time, there exists in the floor of the meatus a foramen, the foramen of Huschke; this foramen is usually closed about the fifth year, but may persist throughout life.
2. The mandibular fossa is at first extremely shallow, and looks lateralward as well as downward; it becomes deeper and is ultimately directed downward. Its change in direction is accounted for as follows. The part of the squama which forms the fossa lies at first below the level of the zygomatic process. As, however, the base of the skull increases in width, this lower part of the squama is directed horizontally inward to contribute to the middle fossa of the skull, and its surfaces therefore come to look upward and downward; the attached portion of the zygomatic process also becomes everted, and projects like a shelf at right angles to the squama.
3. The mastoid portion is at first quite flat, and the stylomastoid foramen and rudimentary styloid process lie immediately behind the tympanic ring. With the development of the air cells the outer part of the mastoid portion grows downward and forward to form the mastoid process, and the styloid process and stylomastoid foramen now come to lie on the under surface. The descent of the foramen is necessarily accompanied by a corresponding lengthening of the facial canal.
4. The downward and forward growth of the mastoid process also pushes forward the tympanic part, so that the portion of it which formed the original floor of the meatus and contained the foramen of Huschke is ultimately found in the anterior wall.
5. The fossa subarcuata becomes filled up and almost obliterated.

Articulations