Applied Anatomy (34)
The temporal bones are situated at the sides and base of the skull. Each consists of five parts, viz., the squamous, the petrous, mastoid, and tympanic parts, and the styloid process.
The Squamous -The squamous forms the anterior and upper part of the boneSurface- Its outer surface is smooth and convex; it affords attachment to the Temporalis muscle, on its hinder part is a vertical groove for the middle temporal artery. A curved line, the temporal line, or supramastoid crest, runs backward and upward across its posterior part; it serves for the attachment of the temporal fascia, and limits the origin of the Temporalis muscle. Projecting from the lower part of the squama is a long, arched process, the zygomatic process. The posterior end is connected to it by two roots, the anterior and posterior roots. Between the posterior wall of the external acoustic meatus and the posterior root of the zygomatic process is the area called the suprameatal triangle (Macewen triangle), a landmark for mastoid antrum.
The internal surface of the squama 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. Mastoid Portion - The mastoid portion forms the posterior part of the temporal bone Surfaces—its outer surface is rough, and gives attachment to the Occipitalis and Auricularis posterior. It is perforated by numerous foramina; one of these, of large size, situated near the posterior border, is termed the Mastoid Foramen; The mastoid portion is continued below into a conical projection, the mastoid process, This process serves for the attachment of the Sternocliedomastoid, Splenius capitis, and Longissimus capitis. On the medial side of the process is a deep groove, the mastoid notch (digastric fossa), for the attachment of the Digastric. The inner surface of the mastoid portion presents a deep, curved groove, the sigmoid sulcus, which lodges part of the transverse sinus; mastoid process 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, but toward the lower part they diminish in size, 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.
Petrous pyramid -The petrous portion or pyramid is pyramidal and is wedged in at the base of the skull between the sphenoid and occipital. It contains the essential parts of the organ of hearing. Base—the base is fused with the internal surfaces of the squama and mastoid portion Apex — angular interval between the posterior border of the great wing of sphenoid and the basilar part of the occipital bone ; it presents the anterior or internal orifice of the carotid canal, and forms the postero-lateral boundary of the foramen lacerum. Surface—The anterior surface forms the posterior part of the middle fossa of the base of the skull, and is continuous with the inner surface of the squamous portion, to which it is united by the petrosquamous suture, anterior surface presents- (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 tegmen tympani; (3) a shallow groove, the hiatus of the facial canal, for the passage of the greater superficial petrosal nerve; (4) near the apex of the bone, the termination of the carotid canal;. The posterior surface forms the front part of the posterior fossa of the base of the skull, and is continuous with the inner surface of the mastoid portion. Near the center is a large orifice, the internal acoustic meatus, it leads into a short canal, about 1 cm. in length, which runs lateralward. It transmits the facial and acoustic nerves and the internal auditory artery. The lateral end of the canal is closed by a vertical plate, which is divided by a horizontal crest, the crista falciformis, into two unequal portions Each portion is further subdivided by a vertical ridge i.e. bills bar into an anterior and a posterior part. The inferior surface is rough and irregular, and forms part of the exterior of the base of the skull. near the apex is a rough surface behind this is the large circular aperture of the carotid canal, behind these openings is a deep depression, the jugular fossa,; it lodges the bulb of the internal jugular vein; 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; in the lateral part of the jugular fossa is the mastoid canaliculus for the entrance of the auricular branch of the vagus nerve; styloid process, a sharp spine, about 2.5 cm. in length; between the styloid and mastoid process is the stylomastoid foramen.
Tympanic Part -The tympanic part is a curved plate of bone lying below the squama and in front of the mastoid process. Surface—its postero-superior surface is concave, and forms the anterior wall, the floor, and part of the posterior wall of the bony external acoustic meatus. Medially, it presents a narrow furrow, the tympanic sulcus, for the attachment of the tympanic membrane. Borders—its lateral border is free and rough, and gives attachment to the cartilaginous part of the external acoustic meatus. Internally, the tympanic part is fused with the petrous portion, posteriorly; it blends with the squama and mastoid part, and forms the anterior boundary of the tympanomastoid fissure. Styloid Procéss -The styloid process is slender, pointed, and of varying length; it projects downward and forward, from the under surface of the temporal bone. Its distal part gives attachment to the stylohyoid and stylomandibular ligaments, and to the Styloglossus, Stylohyoid, and Stylopharyngeus muscles. Ear Both functionally and anatomically, it can be divided into three parts. A. External Ear - that portion external to the tympanic membrane. It serves chiefly to protect the tympanic membrane, but also collects and directs sound waves and plays a role in sound localization. The skin of the external ear normally migrates laterally from the umbo of the malleus in the tympanic membrane to the external auditory meatus (at a rate of 2-3 mm per day). This is a unique and essential mechanism for maintaining patency of the canal. The Auricle - elastic cartilage covered with closely adherent skin. The configuration is intricate, and extremely difficult to duplicate. External Auditory Canal Lateral Portion - cartilaginous with thick, loosely applied skin containing ceruminous and sebaceous glands. Medial Portion- very thin skin directly over bone, no skin appendages. Curves anteriorly and medially in adults, which may obscure the anterior tympanic membrane. It comprises two-thirds of the total canal in adults, less in infants and children.
B.ANATOMY OF MIDDLE EAR
1. TYMPANIC CAVITY
The tympanic cavity is an irregular air filled space within the temporal bone and contains the auditory ossicles and their attached muscles and tendons. LATERAL WALL Central portion of the lateral wall is formed by tympanic membrane. Above and below this is the bone which forms the outer lateral walls of the epitympanum and hypotympanum.
ANTERIOR WALL In the lower part, there is a thin plate of bone covering the carotid artery and is perforated by superior and inferior caroticotympanic nerves. The upper part shows two tunnelsOne for Eustachian tube. Other for tensor tympani muscle. MEDIAL WALL Promontory is a rounded elevation formed by basal turn of cochlea. Behind and above the promontory is fenestra vestibuli (oval window) that connects the tympanic cavity with the vestibule and is closed by the footplate of stapes and its annular ligament. Above it lies the horizontal part of facial nerve. Fenestra cochlea(round window) lies below and a little behind fenestra vestibuli from which it is separated by promontory. Behind the fenestra vestibuli the facial canal starts to turn inferiorly as it begins its descent in the posterior wall of the tympanic cavity. The dome of the lateral semicircular canal extends a little lateral to facial canal and is the major feature of posterior portion of epitympanum. POSTERIOR WALL The posterior wall in its upper part has aditus which is the opening into the mastoid antrum. Below the aditus is fossa incudis which houses the short process of incus. Below the fossa incudius is the pyramid which contains stapedius muscle. Between the promontory and tympanic annulus is the facial recess. Deep to both promontory and facial nerve is the posterior extension of the mesotympanum- the sinus tympani.
FLOOR Consists of a thin plate of bone which separates the tympanic cavity from the dome of jugular bulb. ROOF
It is formed by tegmen tympani which separates the tympanic cavity from the dura of middle cranial fossa. Contents of the tympanic cavity Ossicles- malleus, incus, stapes: three small bones which are involved in sound conduction. From lateral to medial, these are the malleus, the incus, and the stapes. The handle and lateral process of the malleus is attached to the tympanic membrane and can be easily seen on otoscopic examination. The long process of the incus can often be seen through the posterior superior quadrant of the membrane. The stapes is attached to a foot plate which is in direct contact with the fluid of the inner ear. SHAPE \* MERGEFORMAT
Ligaments of ossicles. Muscles namely tensor tympani and stapedius. Vessels supplying and draining the middle ear. Chorda tympani and tympanic plexus of nerves. 2. MASTOID ANTRUM
Air filled sinus within the petrous part of temporal bone. The antrum is well developed at Birth and has the volume of 1ml. Dimensions -14mm anteroposterior -9mm superoinferior -7mm medial to lateral Corresponds externally to Macewan’s triangle which is a triangle bounded by extention of Supramastoid crest superiorly, a vertical tangent through posterior margin of external auditory meatus and Posterior superior margin of external auditory meatus itself. Relations Superior - dural plate separates mastoid antrum from temporal lobe. Inferior - related to digastric muscle and sigmoid sinus. Anterior - aditus in the upper part and facial nerve as it descends to stylomastoid foramen in anterior part Posterior - sigmoid sinus.
3. EUSTACHIAN TUBE The Eustachian tube is a channel connecting the tympanic cavity with the nasopharynx. In the adult, it is about 36mm long and consists of lateral bony portion (12mm) and a medial fibro cartilaginous part (24mm). The tube is lined with respiratory mucosa containing goblet cells and mucous glands and has a carpet of ciliated epithelium on its floor. In cross section the tube is triangular or rectangular with the horizontal diameter being the greater. In the nasopharynx, the tube opens 1- 1.25cm behind and a little below the posterior end of the inferior turbinate. The opening is almost triangular in shape and is surrounded above and behind by the tubal elevation. Behind the tubal elevation is the pharyngeal recess or fossa of Rosenmuller where lymphoid tissue is present. The muscles attached to the Eustachian tube are tensor palati , levator palati and salpingopharyngeus which help in opening and closing of the tube. Blood supply of middle ear
Anterior tympanic branch of maxillary artery. Stylomastoid branch of posterior auricular artery. Mastoid branch of stylomastoid artery. Petrosal branch of middle meningeal artery. Superior tympanic branch of middle meningeal artery. Inferior tympanic branch of ascending pharyngeal artery. Tympanic branch of internal carotid artery. The vein drains into pterygoid plexus and superior petrosal sinus. Nerve supply of middle earThe mucosa of the middle ear is supplied by the tympanic plexus, which is formed by the tympanic branch of the glossopharyngeal nerve and by caroticotympanic nerves which arise from the sympathetic plexus around the internal carotid artery. C. Inner Ear - consists of a fluid-filled labyrinth which functions to convert mechanical energy into neural impulses. The bony labyrinth is subdivided into smaller compartments by the membranous labyrinth. Fluid surrounding the membranous labyrinth is called perilymph; fluid within is called endolymph. There are three main divisions of the bony labyrinth.
Vestibule - just medial to the oval window, and contains the utricle and the saccule, two organs of balance. The vestibule is an antechamber, leading to both the cochlear and the semicircular canal. The Cochlea - a snail-shaped chamber anterior to the vestibule. It bulges into the middle ear and its bony covering is the promontory. The cochlea also
communicates with the middle ear via the round window. In this organ, sound waves are converted into neural impulses with elaborate coding. The Semicircular Canals - three in number; project posteriorly from the vestibule. These organs detect angular acceleration. They consist of superior, posterior and lateral, or horizontal canal.
FACIAL NERVE-
Facial nerve is composed of 10,000 motor, sensory and parasympathetic fibers. Motor Nucleus: situated in lower pons, below the fourth ventricle. From it arise 7000 Special Visceral Efferent fibers that supply the facial muscles. Sensory And Parasympathetic Fibers (3000): They are carried by the nervus intermedius (nerve of Wrisberg) and consist of General Visceral Efferent fibers from the superior salivatory nucleus that are preganglionic and innervate submandibular, sublingual, minor salivary and lacrimal glands. Special Visceral Afferent that provide taste to the anterior two-thirds of the tongue.
Somatic Afferent fibres that supply innervation to the skin of the external auditory meatus.
Course of the Facial NerveThe course of the facial nerve can be divided into-Intracranial Portion -Intratemporal Portion: It is further divided into. _ Labyrinthine segment _ Tympanic segment _ Mastoid segment* -Extra temporal Portion INTRACRANIAL PORTION From brainstem to fundus of the internal auditory Meatus. Length of this segment is approximately 24 mm.It is covered only by a thin layer of piameter and is not covered by perineurium, so extremely vulnerable to CP angle tumour surgery. The FN enters the foramen of the IAM in its anterosuperior segment and runs a distance of 5-12 mm. The CRISTA FALCIFORMIS is a thin bony horizontal septum that divides the internal
auditory canal in two parts. The facial nerve and superior vestibular nerve lie above this septum, with the cochlear nerve and inferior vestibular nerve below. BILL’S BAR is osseous septum that lies between facial nerve and superior vestibular nerve.
INTRATEMPORAL PORTION From the entrance of the facial canal at the fundus of the IAM to the stylomastoid foramen. Length is 28-30 mm.
Labyrinthine SegmentIt is the shortest (3-5 mm) and thinnest part of facial nerve within the fallopian canal. Most vulnerable to ischaemia due to lack of anastomosing arterial arcades. Owing to its bottleneck like anatomical nature, it is also the part that most probably suffers from ischemia in the event of edema following trauma or inflammation. At the distal end of this segment, the geniculate ganglion forms part of a sharp turn, the FIRST GENU of the facial nerve. The GREATER SUPERFICIAL PETROSAL NERVE, first branch of facial nerve, arises from this ganglion. This segment is at risk while drilling in translabyrinthine approach. The ampullated ends of SSC and LSC are identified and are known as the CAT’S EYE; the nerve lies just anterior to this area. This segment is also most likely to be injured in temporal bone fractures.
Tympanic SegmentLength is 8-11 mm. From geniculate ganglion to pyramidal eminence. It lies beneath the LSC and above the oval window. At its proximal it passes just above and medial to the COCHLEARIFORM PROCESS and the tensor tympani tendon.
The Pyramidal Eminence is useful landmark for the Second Genu where the facial nerve makes a sharp turn downwards, marking the beginning of the mastoid segment. The second genu hugs the inferior aspect of LSC and this relation is extremely constant. The nerve is lateral and posterior to the pyramidal process which creates 2 recesses in the mesotympanum, the Facial Recess laterally and the Sinus Tympani medially. Mastoid Segment From pyramidal process to stylomastoid foramen Length is 10-14 mm.A useful landmark for the course of the mastoid segment is the Digastric Ridge. This segment has 3 branches. The nerve to stapedius muscle, the chorda tympani nerve, and the sensory auricular branch. As soon as the nerve exits from the stylomastoid foramen it gives off the POSTERIOR AURICULAR NERVE to supply the occipital belly of occipitofrontalis.
Extra temporal Portion – From stylomastoid foramen to pes anserinus.Length is 15-20 mm.The first major subdivision of the extracranial facial nerve is usually situated within the parotid gland. The main trunk divides into two major divisions- the upper temporofacial and the lower cervicofacial.Within the substance of the parotid gland 5 branches emerge- Temporal, Zygomatic, Buccal, Marginal Mandibular And Cervical. Landmark for identification of facial nerve belly of digastric and stylohyoid muscle. Tragal Pointer is the anterior medial edge of the canal cartilage; the nerve is usually located 1 cm medial and inferior to the pointer. Tympanomastoid Fissure; the nerve can be identified 6-8 mm below the inferior ‘drop off’ of the fissure.
