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Paries Mastoidea. The mastoid or posterior wall presents, from above downwards: (1) a rounded or triangular opening, extending backwards from the recessus epitympanicus and leading into the tympanic antrum (Fig. 709); (2) a depression, the fossa incudis, situated in the postero-inferior part of the recessus epitympanicus (Fig. 710), for the reception of the end of the short crus of the incus; (3) a minute conical bony projection, the eminentia pyramidalis (Fig. 709), the summit of which is perforated by a round aperture for the passage of the tendon of the stapedius muscle. This aperture is continued downwards and backwards as a canal in front of the facial canal, and frequently opens, by a minute orifice, on the base of the skull in front of the stylo-mastoid foramen; it communicates with the facial canal by one or two small foramina, which transmit the vessels and nerve to the stapedius muscle; a minute spicule of bone often extends from the eminentia pyramidalis to the promontory on the labyrinthic wall of the tympanum; (4) a small aperture, the apertura tympanica canaliculi chorda (Fig. 710), which is situated close to the posterior edge of the membrana tympani, nearly on a level with the upper end of the manubrium mallei; (5) a rounded eminence, the prominentia styloidea, is sometimes seen below the last, and is caused by the upward and forward prolongation of the styloid process.
Paries Carotica. The carotid or anterior wall is narrowed in its transverse diameter by the approximation of the lateral and medial boundaries of the tympanic cavity, and in its vertical diameter by the descent of the roof and the ascent of the carotid canal. It presents (Fig. 709) two parallel semicanals, one above the other separated by a thin lamella of bone, the septum canalis musculotubarii (O.T. processus cochleariformis). These run forwards on the lateral wall of the carotid canal and open in the angle between the squama and the petrous part of the temporal bone. The higher and smaller of the two is termed the semicanalis m. tensoris tympani, and lies immediately below the tegmen tympani. It has a diameter of about 2 mm., and extends on to the medial wall of the tympanic cavity above the anterior part of the fenestra vestibuli. The lower and larger semicanal gradually increases in size from before backwards, and is named the semicanalis tuba auditiva. It forms the bony part of the auditory tube and opens on the carotid wall of the tympanic cavity opposite the orifice leading into the tympanic antrum. Below the orifice of the auditory tube the anterior part of the tympanic cavity is separated from the ascending portion of the carotid canal by a thin plate of bone in which there are sometimes gaps or deficiencies. It is perforated by the carotico- tympanic canal, which transmits the carotico-tympanic nerve from the sympathetic plexus of the carotid artery to the tympanic plexus. The auditory tube is described on p. 837.
Paries Membranacea. The membranous or lateral wall is formed almost entirely by the membrana tympani (Fig. 710), which closes the medial end of the external acoustic meatus, and is fixed throughout the greater part of its circumference in a groove, the sulcus tympanicus. The bony ring containing this sulcus is deficient superiorly, where it exhibits a distinct notch, the notch of Rivinus. On a level with the upper edge of the membrane, and in front of the ring of bone in which it is fixed, is the medial end of the petrotympanic fissure. This transmits the tympanic branch of the internal maxillary artery, and lodges the anterior process and anterior ligament of the malleus. Close to the medial end of the fissure is the iter chorda anterius through which the chorda tympani nerve leaves the tympanic cavity.
Membrana Tympani.-The tympanic membrane is an elliptical disc, its greatest diameter, 9 to 10 mm., being directed from above, downwards and forwards, whilst its least diameter is from 8 to 9 mm. It is placed very obliquely, forming an angle of about 55° with the lower and anterior walls of the external acoustic meatus; it is said to be more oblique in cretins and deaf mutes, and more perpendicular in musicians.
The circumference of that portion of the membrane which is fixed in the sulcus tympanicus is considerably thickened, and is named the annulus fibrocartilagineus. It is prolonged from the anterior and posterior extremities of the notch of Rivinus to the processus lateralis of the malleus, in the form of two ligamentous bands, the
anterior and posterior malleolar plicæ. The small triangular portion of the membrane (Fig. 710) situated above these folds is thin and lax, and constitutes the pars flaccida (O.T. membrane of Shrapnell); the main portion of the membrane is, on the other hand, tightly stretched and termed the pars tensa. A small orifice, sometimes seen in the pars flaccida, is probably either a pathological condition or has been produced artificially during manipulation. The manubrium mallei is firmly fixed to the medial surface of the membrana tympani, the central portion of which is drawn towards the tympanic cavity so that its lateral surface is concave. The deepest part of this concavity corresponds with the lower end of the manubrium of the malleus, and is named the umbo membranæ tympani.
The membrana tympani consists of three layers: (1) a lateral, the stratum cutaneum; (2) an intermediate, the membrana propria; (3) a medial, the stratum
The stratum cutaneum is continuous with the skin of the meatus, and consists of a thin layer of cutis covered with epidermis. The cutis is thickest near the circumference; the epidermis, on the other hand, is thickest near the centre of the membrane.
The membrana propria consists of two sets of fibres: (a) a lateral, the stratum radiatum, situated immediately under the stratum cutaneum, and radiating from the manubrium of the malleus to the annulus fibrocartilagineus; (b) a medial, the stratum circulare, the fibres of which are numerous near the circumference, but scattered and few in number near the centre of the membrane (Fig. 710). Both radial and circular fibres are absent from the pars flaccida, which consists only of the cutaneous and mucous strata. Gruber pointed out that, in addition to the radial and circular fibres, there exists, next the stratum mucosum, a series of dendritic or branched fibres, which are best developed in the posterior part of the membrane.
The stratum mucosum is continuous with the mucous lining of the tympanic cavity. It is thicker over the upper part of the membrane than near its centre, and is covered with pavement epithelium.
Otoscopic Examination of the Membrana Tympani (Fig. 711). The membrana tympani, in the living, is of a "pearl-gray" colour, but may present a reddish or yellowish tinge, depending upon the condition of its mucous lining and on the condition of the cutaneous lining of the meatus; the posterior segment is usually clearer than the anterior. At the antero-superior part, close to its periphery, a whitish point appears
as if projecting towards the meatus; Membrana flaccida this is the processus lateralis of the Anterior malleolar malleus. Passing downwards and
backwards from this point to the umbo is a ridge caused by the Manubrium mallei manubrium mallei, the lower extremity of which appears rounded. Two ridges, corresponding with the malleolar plicæ, extend from the processus lateralis of the malleus, one forwards and upwards, the other backwards and upwards. Behind, FIG. 711.-LEFT TYMPANIC MEMBRANE (as viewed from the external and near the lower extremity of the acoustic meatus). x 3. manubrium mallei, is a reddish or yellowish spot, due to the promontory of the medial wall of the tympanic cavity shining through. If the membrane is very transparent, the long crus of the incus may be visible behind the upper part of the manubrium mallei, and reaching downwards as far as its middle. From the lower end of the manubrium mallei, the "cone of light" or "luminous triangle" extends downwards and forwards, its apex being directed towards the handle; this triangle varies in size in different people. A line prolonging the manubrium downwards divides the membrane into two parts, while another, drawn at right angles to this through the umbo, will subdivide it into quadrants, viz., postero-superior, postero-inferior, antero-superior, and antero-inferior; this subdivision is useful in enabling the otologist to localise and describe accurately the seat of lesions in the membrane.
Lateral process of
Vascular and Nervous Supply of the Membrana Tympani. The arteries are arranged in two sets, one on the cutaneous and another on the mucous surface; they anastomose by means of small branches which pierce the membrane, especially near its periphery. The first set is derived chiefly from the deep auricular branch of the internal maxillary, whilst those on the
pars ciliaris and pars iridica retina. The indentation of the optic cup extends as a groove for some distance along the postero-inferior aspect of the optic stalk, forming what is termed the chorioidal fissure (Fig. 701). Through this fissure mesoderm passes inwards between the lens and the retina to form a part of the vitreous body, while the arteria centralis retina also becomes enclosed in it and so gains its future position in the centre of the optic
Cavity of fore-brain
Ectoderm forming lens rudiment
Optic vesicle becoming
FIG. 701.-SECTIONS THROUGH PORTIONS OF THE HEADS OF FETAL RABBITS, to illustrate the connexion of the optic cup with the fore-brain, and the invagination of the ectoderm to form the lens.
Outer layer of optic cup
The arteria centralis is prolonged forwards from the porus opticus through the vitreous body, as a cone of branches, as far as the back of the lens. By the fifth or sixth month all these branches have disappeared except one, the arteria hyaloidea, which persists until the last month of foetal life, when it also atrophies, leaving only the canalis hyaloideus to indicate its position.
The vitreous body is developed between the optic cup and the lens, and is derived partly from ectoderm and partly from mesoderm. It consists primarily of a series of fine protoplasmic fibres which project from the cells. of the retinal layer of the cup and form a delicate reticular tissue. At first these fibres are seen in relation to the whole of the optic cup, but later they are limited to the ciliary region, where by a process of condensation Chorioidal they appear to form the zonula ciliaris. When the mesoderm reaches the cup through the chorioidal fissure it unites with this reticular tissue to form the vitreous body.
FIG. 702.-OPTIC CUP AND LENS VIEWED
The lens, at first in contact with the ectoderm
FROM BEHIND AND BELOW, to show from which it is derived, is soon separated from it by
formation of chorioidal fissure and enclosure of the arteria centralis retinæ (from model by Ziegler).
mesoderm, and then consists of a rounded vesicle with epithelial walls. The anterior wall remains as a single layer of cells-the anterior lens epithelium of the adult; the cells of the posterior wall become elongated into lens fibres, and by the forward growth of these the cavity of the vesicle is obliterated. This elongation into lens fibres is greatest at the centre of the lens, while near the equator the fibres are shorter, and here the gradual transition between the anterior epithelium and the lens fibres is seen (Fig. 696). The lens becomes enveloped in a vascular tunic, which receives its vessels from the arteria centralis retinæ and from the vessels of the iris. The front part of this tunic forms the membrana pupillaris, and this, like the rest of the tunic, disappears before birth.
The hollow optic stalk becomes solid by the thickening of its walls and, acquiring nerve-fibres, is transformed into the optic nerve. These nerve-fibres are mostly centripetal, and are derived from the nerve-cells of the retina; but a few are centrifugal and have their origin in the brain. The further development of the retina resembles, in certain respects, that of the spinal medulla.
Cameron states (Journ. Anat. and Physiol., vol. xxxix.) that in the early stages of the development of the inner or retinal layer of the optic cup all the structures, described by His as being present in the spinal medulla of the human embryo, are to be found, viz., (a) spongioblasts, (b) germinal cells, and (c) neuroblasts.
The spongioblasts undergo ramification and form a network or myelospongium, and also give rise to the inner and outer limiting membranes; the latter is next the original cavity of the optic vesicle, and therefore corresponds to the inner limiting membrane of the spinal medulla. The spongioblasts also form the groundwork of the inner and outer molecular layers into which the processes of the neuroblasts grow and arborise.
The germinal cells are always situated beneath the external limiting membrane, and by their division give rise to the neuroblasts. The first-formed neuroblasts are larger than those of succeeding generations, and are found in the site of the future ganglionic layer. The germinal cells in the middle of the convexity of the retinal cup cease to divide at an early stage of development, and become directly transformed into the rod and cone cells from which the rods and cones develop as processes; hence these structures appear first over the middle of the convexity of the retina, and gradually extend towards the margin of the retinal cup.
The molecular layers make their appearance as plexuses of myelospongium. The internal molecular layer is first developed at the centre of the retinal cup, and gradually extends towards the cup margin, and into it the processes from the nuclei on either side grow and ramify. The rod and cone fibres, and the outer processes of the internal nuclear layer, grow into and arborise within the external molecular layer.
The condensed mesoderm surrounding the optic cup becomes the sclera and chorioid. In the portion of the mesoderm which lies in front of the lens a cleft-like fissure appears, and divides it into a thick anterior and a thin posterior layer. The former becomes the substantia propria of the cornea; the latter, the stroma of the iris and anterior part of the vascular tunic of the lens. The fissure represents the future camera oculi anterior, and its lining cells form the endothelium of this chamber.
The eyelids arise as two integumentary folds above and below the cornea, each being covered on both its surfaces by the ectoderm. By the third month the folds meet and unite with each other at their edges, the eyelids being only permanently opened shortly before birth; in many animals they are not opened until after birth. The ectoderm forms the epithelium of the conjunctiva and the epithelium corneæ. It is also invaginated at the lid margins to form the hair follicles and the lining cells of the tarsal glands and glands of Moll, and, at the fornix conjunctivæ superior, to form the lining of the alveoli and ducts of the lacrimal gland.
The naso-lacrimal duct, lacrimal sac, and ducts represent the remains of the furrow between the maxillary and lateral nasal processes (p. 49). It is at first filled with a solid rod of cells, which becomes hollowed out to form the sac and ducts.
The ear or auditory organ (Fig. 703) consists of three portions-external, middle, and internal-the last constituting its essential part, since the peripheral terminations of the acoustic nerve are distributed within it.
The external ear1 includes--(a) the auricula, attached to and projecting from the side of the head; and (b) the external acoustic meatus leading inwards from the most depressed part of the auricula to the tympanic membrane.
The auricle (O.T. pinna) (Fig. 704) presents two surfaces, lateral and medial, the latter forming an angle (cephalo-auricular angle) of about 30° with the side of the head. The lateral surface is irregularly concave, but presents several wellmarked elevations and depressions. The deepest of the depressions is situated near its middle, and is named the concha auriculæ. It is divided by an almost transverse ridge, the crus helicis, into an upper, smaller, and a lower, larger portion: the former is termed the cymba conchæ; the latter, which leads into the meatus, the cavum concha. Anteriorly, the crus helicis is continuous with the margin of the auricula or helix, which is incurved in the greater part of its extent, and is directed at first upwards, and then backwards and downwards, to become gradually
1 Although it is usual to speak of the external, middle, and internal ear, it would be more correct to use the terms external, middle, and internal portions of the ear.
lost in the upper part of the lobule. Near the point where the helix begins to descend a small tubercle, the tuberculum auriculæ (Darwini), is often seen.
In front of the descending part of the helix is a second elevation, the antihelix Single below, it divides superiorly into two limbs, termed the crura antihelicis, between which is a triangular depression, the fossa triangularis. The elongated furrow between the helix and antihelix is named the scapha. The concavity of the concha is overlapped in front by a tongue-like process, the tragus, and below and behind by a triangular projection, the antitragus; the
Tympanic cavity, with chain of ossicles
FIG. 703.-DIAGRAMMATIC VIEW OF THE ORGAN OF HEARING.
notch, directed downwards and forwards between these two processes, is named the incisura intertragica. The tragus consists really of two tubercles, the upper of which constitutes the tuberculum supratragicum of His, and is separated from the helix by a groove, the sulcus auris anterior. The lobule is situated below the incisura intertragica, and is the most dependent part of the auricle.
The auricula is usually smaller and more finely modelled in the female than in the male, but presents great variations in size and shape in different individuals. In the newly born child its length is about one-third of that of the adult, while it increases slightly in length and breadth in old age.
FIG. 704.-VIEW OF LATERAL SURFACE OF LEFT AURICULA (half natural size). The relation of the width to the height is termed the auricular index, and is expressed as follows:width of auricula x 100
length of auricula
This index is less in white than in dark races.
The cephalo-auricular angle may be practically absent, as in those cases where the skin of the head passes directly on to the lateral surface of the auricula, or it may be increased to nearly a right angle, so that the lateral surface of the auricula looks directly forwards. The tuberculum auriculæ, the significance of which was recognised by Darwin, is a somewhat triangular prominence which projects forwards in cases where the helix is well rolled over, but backwards and upwards when the incurving of the helix has been arrested. More frequently present in men than in women, it is of developmental interest since it has been shown to be well marked at the sixth month of foetal life, the entire auricula, at this stage, resembling in appearance that of the adult macaque monkey.
The lobule may be small and sessile or considerably elongated; it may adhere to the skin of the cheek (ie. webbed), or may tend to bifurcate at its lower extremity.