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Lateral semicircular canal
facial canal. Reaching forwards, it extends to the apex of the petrous part; whilst laterally it forms part of the medial wall of the tympanum, surrounds the fenestra vestibuli, and encloses within its substance portions of the cochlea, vestibule, and superior semicircular canal. Another centre, the Opisthotic, appears in the vicinity of the promontory on the medial wall of the tympanum, surrounds the fenestra cochleæ, forms the floor of the vestibule, and extends medially to complete the floor of the internal acoustic meatus. Surrounding the cochlea inferiorly and laterally, it completes the floor of the tympanum, and ultimately blends with the anterior and inferior part of the tympanic ring. The carotid canal at first grooves it, and is then subsequently surrounded by it. According to Lambertz the lamina spiralis of the cochlea ossifies in membrane. The roof of the tympanum is formed from a separate centre, the Pterotic, which extends backwards towards the superior semicircular canal, and encloses the tympanic part of the facial canal; laterally this centre unites by suture with the squamosal, and sends down a thin process, which appears between the lips of the petro - tympanic fissure, and forms the lateral wall of the auditory tube. Nuclei, either
Styloid process. broken off
fossa Groove for membrana
acoustic meatus Mastoid air-cells
single or multiple, Epiotic, appear in the base of the petrous part, and envelop the
posterior and lateral semicircular canals. It is by extension from this part that the mastoid process is ultimately developed. The styloid process, an independent development from the upper end of the cartilage of the second visceral arch, is ossified from two centres. The upper or basal appears before birth, and rapidly unites with the petromastoid, the tympanic plate encircling it in front. This represents the tympanohyal of comparative anatomy. At birth, or subsequent to it, another centre appears in the cartilage below the above: this is the stylohyal. Ankylosis usually occurs in adult life between the tympanohyal and stylohyal, the union of the two constituting the socalled styloid process of human anatomy.
FIG. 141.-VERTICAL TRANSVERSE SECTION THROUGH THE LEFT
Osseous part or the auditory tube
meatus -Vestibule, fenestra vestibuli cut across Superior semicircular canal
Opening leading into tympanic
Lateral semicircular canal
FIG. 142.-HORIZONTAL SECTION THROUGH THE LEFT TEMPORAL BONE
The centre from which the squamo-zygomatic develops appears in membrane about Situated near the root of the zygoma, it extends forwards medially to form the floor of the infra-temporal fossa, and From this latter there is a downward and backward exten
sion, which forms the post-auditory process; this ultimately blends with the posterior limb of the tympanic ring, being separated from it in the adult by the petro-mastoid fissure. It forms the lateral wall of the tympanic antrum, and constitutes the anterior and upper part of the mastoid process in the adult. About the third month a centre appears in the outer membranous wall of the tympanum: from this the tympanic ring is developed. Incomplete above, it displays two free extremities. Of these, the anterior is somewhat enlarged, and unites in front with the mandibular portion of the squamo-zygomatic, being separated from it by the petro-tympanic fissure and the downgrowth from the tegmen tympani ; the posterior joins the post-auditory process of the squamo-zygomatic above mentioned. Below, it blends medially with the portion of the petro-mastoid which forms the floor of the tympanum and ensheathes the tympanohyal behind. From the medial surface of the ring below there is an extension medially and forwards which forms the floor of the osseous part of the auditory tube, as well as the lateral wall and half the floor of the carotid canal. From the lateral side of the lower part of this ring two tubercles arise;
The tympanic ring is left uncoloured.
The squamo-zygomatic part is coloured blue; the petro-mastoid red. Fr. 143.-A. THE PARIETAL SURFACE OF THE RIGHT TEMPORAL BONE AT BIRTH. B. THE SAME WITH THE SQUAMO-ZYGOMATIC PORTION REMOVED. (The lettering is the same in both A and B.) a, Tympanic ring. b, Medial wall of tympanum. c, Fenestra cochleæ. d, Fenestra vestibuli. e, Tympanic antrum. f, Mastoid process. g. Masto-squamosal suture, with foramen for transmission of vessels. h, Squamo-zygomatic, removed in figure B to show how its descending process forms the lateral wall of the tympanic antrum.
C. CEREBRAL SURFACE OF THE RIGHT
a, Squamo - zygomatic. b, Petrosquamosal suture and foramen (just above the end of the lead line). c, Subarcuate fossa. d, Aquæductus vestibuli. e, Aquæductus cochleæ. f, Internal acoustic meatus. Upper end of carotid canal.
these grow laterally, and so form the floor of the external acoustic meatus. The interval between them remains unossified till about the age of five or six, after which closure takes place. This deficiency may, however, persist even in adult life (see Appendix B, Temporal).
At birth the temporal bone can usually be separated into its component parts. The lateral surface of the petrous part not only forms the medial wall of the tympanum, but is hollowed out behind and above to form the inner side of the tympanic antrum, the outer wall of which is completed by the post-auditory process of the squamo-zygomatic. As yet the mastoid process is undeveloped. It only assumes its nipple-like form about the second year. Towards puberty its spongy substance becomes permeated with air spaces, which are in communication with and extensions from the tympanic antrum. Occasionally this pneumatic condition is met with in early childhood. The external acoustic meatus is unossified in front and below, the outgrowth from the tympanic ring occurring subsequent to birth. The mandibular fossa is shallow and everted; the jugular fossa is ill-marked; whilst the subarcuate fossa is represented by a deep pit, the so-called floccular fossa of comparative anatomy. The hiatus of the facial canal is an open groove, displaying at either end the openings of the medial and lateral portions of the facial canal.
The sphenoid bone lies in front of the basilar part of the occipital medially, and the temporals on either side. It enters into the formation of the cranial, orbital, and nasal cavities, as well as the temporal, infra-temporal, and pterygo
palatine fossæ. It consists of a body with three pairs of expanded processes, the great wings, the small wings, and the pterygoid processes.
The corpus (body), more or less cubical in form, is hollow, and contains within it the two large sphenoidal air sinuses. These are separated by a partition, which
FIG. 144. THE SPHENOID SEEN FROM BEHIND.
is usually deflected to one or other side of the median plane. Each sinus extends laterally for a short distance into the root of the great wing, and downwards and laterally towards the base of the pterygoid process of the same side. They com
Hamulus of medial pterygoid lamina
Superior orbital fissure
Hamulus of medial - pterygoid lamina
municate by apertures with the upper and posterior part of the nasal cavities. In the adult the posterior aspect of the body displays a sawn surface due to its separation from the basi-occipital, with which in the adult it is firmly ankylosed. The superior surface, from the anterior angles of which the small wings arise, displays an appearance comparable to that of an oriental saddle (sella turcica). Over its middle there is a deep depression, the fossa hypophyseos, in which is lodged the hypophysis (O.T. pituitary body). Behind, this is overhung by a sloping ridge, the dorsum sellæ, the posterior surface of which is inclined upwards, and is in continuation with
the basilar groove of the occipital bone, supporting the pons and the basilar artery. Anteriorly and laterally the angles of this ridge project over the fossa hypophyseos in the form of prominent tubercles, called the processus clinoidei posteriores (posterior clinoid processes). To these are attached the tentorium cerebelli and interclinoid ligaments. In front of the fossa hypophyseos there is a transverse elevation, the tuberculum sellæ, towards the lateral extremities of which, and somewhat behind, there are oftentimes little spurs of bone, the processus clinoidei medii (middle clinoid processes). In front of the tuberculum sellæ is the sulcus chiasmatis, which passes laterally on either side to become continuous, between the roots of the small wings, with the optic foramina.
This groove is liable to considerable variations, and apparently does not always serve for the lodgment of the optic chiasma. (Lawrence, "Proc. Soc. Anat.," Journ. Anat. and Physiol. vol. xxviii. p. 18.)
In front of the sulcus chiasmatis, from which it is often separated by a thin sharp edge, the superior surface continues forwards on the same plane as the upper surfaces of the small wings, and terminates anteriorly in a ragged edge, which articulates with the lamina cribrosa of the ethmoid, and has often projecting from it, in the median plane, a pointed process, the sphenoidal spine. The lateral aspects of the body are fused with the great wings, and in part also with the roots of the pterygoid processes. Curving along the side of the body, above its attachment to the great wing, is an-shaped groove, the sulcus caroticus (carotid groove), which marks the position and course of the internal carotid artery. Posteriorly, the hinder margin of this groove, formed by the salient lateral edge of the posterior surface of the body, articulates with the apex of the petrous portion of the temporal bone, and is hence called the petrosal process; just above this, on the lateral border of the dorsum sellæ, there is often a groove for the abducent nerve.
The anterior surface of the body displays a vertical, median crista sphenoidalis (sphenoidal crest), continuous above with the sphenoidal spine, and below with the pointed projection called the sphenoidal rostrum. This crest articulates in front with the perpendicular plate of the ethmoid. On each side of the median plane are seen the irregular openings leading into the sphenoidal air sinuses, the thin anterior walls of which are in part formed by the absorption of the sphenoidal conchæ (O.T. turbinated bones) with which in early life they are in contact. With exception of a broad groove leading downwards from the apertures above mentioned, which enters into the formation of the roof of the nasal cavity of the corresponding side, the lateral aspects of this surface of the bone are elsewhere in articulation with the labyrinths of the ethmoid and the orbital processes of the palate bones. The sphenoidal rostrum is continued backwards for some distance along the inferior surface of the body, where it forms a prominent keel which fits into the recess formed by the ale of the vomer. The edges of the alae serve to separate the rostrum from the incurved vaginal processes at the roots of the medial plates of the pterygoid processes. Posteriorly, the inferior surface of the body of the sphenoid is rougher, and covered by the mucous membrane of the roof of the pharynx; here, occasionally, a median depression may be seen which marks the position of the inferior extremity of a foetal channel, called the canalis craniopharyngeus.
Alæ Parvæ. The small wings are two flattened triangular plates of bone which project forwards and laterally from the anterior and upper part of the body of the bone, with which they are united by two roots which enclose between them the optic foramina for the transmission of the optic nerves and ophthalmic Of these roots, the posterior springs from the body just wide of the tuberculum sellæ, separating the carotid groove behind from the optic foramen in front; laterally this root is confluent with the recurved posterior angle of the small wing, forming the projection known as the processus clinoideus anterior (anterior clinoid process), which overhangs the anterior part of the body of the bone and affords an attachment to the tentorium cerebelli and interclinoid ligaments. The anterior root, broad and compressed, unites the upper surface of the small wing with the anterior and upper part of the body. Laterally, the lateral
angle terminates in a pointed process which reaches the region of the pterion and there articulates with the frontal, and may come in contact with the great wing. The superior aspect is smooth, and forms, in part, the floor of the anterior cranial fossa. The inferior surface constitutes part of the posterior portion of the upper wall of the orbit, and also serves to roof in the superior orbital fissure (O.T. sphenoidal fissure), which separates the small wing from the great wing below. The anterior edge is ragged and irregular, and articulates with the orbital parts of the frontal. The posterior margin, sharp and sickle-shaped, separates the anterior from the middle cranial fossa, and corresponds to the position of the stem of the lateral cerebral fissure on the inferior surface of the cerebrum.
Alæ Magnæ.-The great wings, as seen from above, are of a somewhat crescentic shape and form a considerable portion of the floor of the middle cranial fossa. If the medial convex edge of the crescent be divided into fifths, the posterior fifth extends backwards and laterally beyond the body of the bone, presenting a free posterior edge, which forms the anterior boundary of the foramen lacerum. This border ends behind in the horn of the crescent, from which a pointed process projects downwards, called the spina angularis; this is wedged into the angle between the petrous and squamous parts of the temporal bone. The medial surface of the posterior border and spine is furrowed for the cartilaginous part of the auditory tube (sulcus tube), whilst on the medial side of the spine the course of the chorda tympani nerve is indicated by a groove (Lucas). The second fifth of the convex border of the crescent is fused to the side of the body and united below with the root of the pterygoid process. The angle formed by the union of the great wing with the side of the body posteriorly corresponds to the posterior end of the carotid groove, the lateral lip of which is formed by a projecting lamina called the lingula. The remaining three-fifths of the convex border is divisible into two nearly equal parts; the medial is a free, curved, sharp margin, which forms the inferior margin of the superior orbital fissure, the cleft which separates the great wing from the small wing, and which establishes a wide channel of communication between the middle cranial fossa and the cavity of the orbit, transmitting the oculomotor, trochlear, ophthalmic division of the trigeminal, and the abducent nerves, together with the ophthalmic veins. Wide of the superior orbital fissure this edge becomes broad and serrated, articulating with the frontal bone medially, and at the part corresponding to the anterior horn of the crescent, by a surface of variable width, it unites with the sphenoidal angle of the parietal bone. The lateral border corresponds to the concave side of the crescent, and is serrated for articulation with the squamous part of the temporal, being thin and bevelled at the expense of its parietal surface above and laterally, and broad and thick behind as it passes towards the angular spine.
The internal or cerebral surface is concave from behind forwards, and, in its anterior part, from side to side also; it forms a considerable part of the floor of the middle cranial fossa, and bears the impress of the gyri of the extremity of the temporal lobe of the cerebrum, which rests upon it; towards its lateral side it is grooved obliquely by an anterior branch of the middle meningeal artery.
The following foramina pierce the great wing: Close to and in front of the alar spine is the foramen spinosum, for the transmission of the middle meningeal artery and its companion vein, together with the nervus spinosus from the mandibular division of the trigeminal nerve. In front of and medial to this, and close to the posterior free border, is the foramen ovale, of large size and elongated form. This gives passage to the motor root and mandibular division of the trigeminal nerve, and admits the accessory meningeal branch of the middle. meningeal artery; a small emissary vein from the cavernous sinus usually passes through this foramen, and occasionally also the lesser superficial petrosal nerve. Near the anterior part of the root of the great wing, and just below the sphenoidal fissure, is the foramen rotundum, of smaller size and circular form. Through this the maxillary division of the trigeminal nerve escapes from the cranium. Occasionally there is a small canal—the foramen of Vesalius-which pierces the root of the great wing to the medial side of the foramen ovale. This opens below into the scaphoid fossa at the base of the medial pterygoid lamina, and transmits a