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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 auricula. 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 concha; 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. 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 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.
FIG. 703.-DIAGRAMMATIC VIEW OF THE ORGAN OF HEARING.
The medial or cranial surface also is irregular, and presents elevations corresponding to the depressions on its lateral surface, e.g. eminentia conchæ, eminentia triangularis, etc.
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.
Crus antihelicis superior
Crus antihelicis inferior
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 × 100 X length of auricula
- Auricular index.
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.
Structure of the Auricula.-The greater part of the auricula consists of a lamella of yellow fibro-cartilage, the cartilago auricula; the cartilage is, however, absent from the lobule, which is composed of fat and connective tissue. When laid bare, the cartilage (Figs. 705,
Incisura terminalis auris
tilage of the helix projects anteriorly
FIG. 705.-LATERAL SURFACE OF CARTILAGE OF THE
to those already described, are usually present, one in the tragus and another immediately behind the spina helicis.
On the cranial surface of the cartilage (Fig. 706) the eminences produced by the concha and fossa triangularis are separated by a transverse furrow, the sulcus
antihelicis transversus, corresponding with the crus antihelicis inferior; further, the eminentia concha is crossed horizontally by a groove, the sulcus cruris helicis, and almost vertically by a slight ridge, the ponticulus : the latter indicates the attachment of the m. auricularis posterior.
Ligaments of the Auricula.-The cartilage of the auricle is attached to the temporal bone by two fibrous bands which form its extrinsic ligaments, viz. an anterior, stretching from the zygomatic process to the spina helicis and tragus; and a posterior, passing from the eminentia concha and upper wall of the meatus to the mastoid portion of the temporal bone. Small ligamentous bands pass between individual parts of the auricle, and form what are termed its intrinsic ligaments.
FIG. 706.-MEDIAL SURFACE OF THE CARTILAGE OF
Muscles of the Auricula (Figs. 705, 706). The muscles of the auricle are divided into two groups, extrinsic and intrinsic. The extrinsic muscles pass to the auricula from the skull or the scalp, and are described in the section on Myology. The intrinsic muscles, on the other hand, are confined to the auricula and are six in number, four on its lateral and two on its cranial or medial surface.
(a) On the lateral surface (Fig. 705)
1. M. helicis major passes upwards from the spina helicis along the ascending part of the helix. 2. M. helicis minor covers the crus helicis. 3. M. tragicus, quadrangular in shape, consists of fibres running vertically over the greater part of the tragus. Some of its fibres are prolonged upwards to the spina helicis and constitute the m. pyramidalis. 4. M. antitragicus covers the antitragus and runs obliquely upwards and backwards as far as the antihelix and cauda helicis.
(b) On the medial surface (Fig. 706)
1. M. transversus auriculæ consists of scattered fibres, which stretch from the eminentia concha to the convexity of the helix. 2. M. obliquus auriculæ (Tod) comprises a few fasciculi, which run obliquely or vertically across the furrow corresponding with the crus antihelicis inferior. A small muscle, the m. stylo-auricularis, sometimes extends from the root of the styloid process to the cartilage of the meatus.
Skin of the Auricula.-The skin covering the auricle is thin and smooth, and is prolonged, in the form of a tube, as a lining to the external acoustic meatus. On the lateral surface of the auricula, it adheres firmly to the subjacent perichondrium. Strong hairs are present on the tragus and antitragus, and also in the incisura intertragica, forming the barbula hirci, which guard the entrance to the concha; soft downy hairs are found over the greater part of the auricula and point towards the tuberculum auriculæ. Sebaceous glands, present on both surfaces of the auricle, are most numerous in the concha and fossa triangularis. Sudoriferous glands are found on the medial surface; few or none on the lateral surface.
Vessels of the Auricula. The arteries of the auricle are derived (a) from the superficial temporal, which sends two or three branches to the lateral surface; and (b) from the posterior auricular, which gives three or four branches to the medial surface. From the posterior auricular artery two sets of twigs are prolonged to the lateral surface, one turning round the free margin of the helix, and the other passing through small fissures in the cartilage. The veins from the lateral surface open into the superficial temporal vein; those from the medial surface chiefly join the posterior auricular vein, but some communicate with the mastoid emissary vein. lymph vessels take three directions, viz.: (a) forwards to the parotid lymph glands, and especially to the anterior auricular gland in front of the tragus; (b) downwards to the lymph glands which accompany the external jugular vein, and to the lymph glands under the sternocleidomastoideus; and (c) backwards to the posterior auricular lymph glands.
Nerves of the Auricula. The muscles of the auricle are supplied by the facial nerve. The skin receives its sensory nerves from-(a) the great auricular, which supplies nearly the whole of the medial surface, and sends filaments in company with the branches of the posterior auricular artery to the lateral surface; (b) the auriculo-temporal, which supplies the tragus and ascending part of the helix; (c) the lesser occipital, which sends a branch to the upper part of the medial surface.
MEATUS ACUSTICUS EXTERNUS.
The external acoustic meatus (Figs. 707, 708) is the passage leading from the concha to the membrana tympani. Its average length, measured from the bottom of the concha, is about 24 mm., but, if measured from the level of the tragus, about 35 mm. On account of the obliquity of the membrana tympani the anterior and inferior walls of the meatus are longer than the posterior and superior. The meatus consists of two parts, viz.: (a) an external portion, the pars cartilaginea, about 8 mm. in length; and (b) an internal portion, the pars ossea, about 16 mm. in length. The entire meatus forms a somewhat S-shaped bend (Fig. 708), and may be divided into three portions-external, intermediate. and internal; each is directed medialwards, but, in addition, the external part is inclined forwards and slightly upwards; the intermediate, backwards; and the internal, the longest, forwards and slightly downwards. On transverse section the canal is seen to be elliptical, its greatest diameter having an inclination downwards and backwards. Widest at its lateral extremity, it becomes somewhat narrower at the medial end of the pars cartilaginea; once more expanding in the lateral portion of the pars ossea, it is again constricted near the medial end of the latter, where its narrowest part, or isthmus, is found at a distance of about 19 mm. from the bottom of the concha. The medial extremity of the meatus is nearly circular and is closed by the membrana tympani.
Bezold gives the diameters of the meatus as follows:
At the commencement of the pars cartilaginea
Least. 6.54 mm.
The lumen of the pars cartilaginea is influenced by the movements of the mandible, being increased when that bone is depressed. This can be easily verified by inserting a
finger into the meatus, and then alternately opening and shutting the mouth.
The condyle of the mandible lies
in front of the pars Ossea, while between the condyle and the pars cartilaginea
a portion of the parotid gland is sometimes present. Behind the pars ossea, and separated from it by a thin plate of bone, are the mastoid air-cells.
FIG. 707.-FRONTAL SECTION OF RIGHT EAR; ANTERIOR HALF OF SECTION, viewed from behind (natural size).
Structure of the Meatus.-The cartilage of the meatus, directly continuous with that of the auricula, is folded on itself to form a groove, opening upwards and backwards, the margins of which are connected by fibrous tissue. The medial end of the cartilaginous tube is firmly fixed to the lateral margin of the bony meatus, whilst its lateral extremity is continuous with the cartilage of the tragus
(p. 829). Two fissures exist in the anterior portion of the pars cartilaginea, and are filled by fibrous tissue.
Membrana tympani In the lateral part of
First turn of
SECTION, seen from below (natural size).
the meatus the cartilage forms about threefourths of the circumference of the tube; but, near the medial end of the pars cartilaginea the cartilage forms merely a part of the anterior and lower boundaries of the canal.
The pars ossea of the meatus is described on p.
127; but it may be well to state here that in the new-born child it is represented only by an incomplete ring of bone, the annulus tympanicus, together with a small portion of the squama temporalis, which articulates with, and bridges over the interval between, the extremities of the ring superiorly. In the concavity of the annulus is a groove, the sulcus tympanicus, in which the circumference of the membrana tympani is fixed. On the medial surface of the anterior part of the annulus, a little below its free extremity, a groove, the sulcus malleolaris, is directed downwards and forwards. It transmits the anterior process and the anterior ligament of the malleus. the tympanic artery, and the chorda tympani nerve. It is limited above by a ridge, the crista spinarum (Henle), which ends in front and behind in a spinous process (spina tympanica major and minor). Below the sulcus malleolaris there is a second, less prominent ridge, the crista tympanica (Gruber), which subsequently unites with a process of the tegmen tympani, and so shuts off the canalis musculotubarius from the petrotympanic fissure. A fibrous tympanic plate (Symington) intervenes between the annulus tympanicus and the cartilage of the meatus, and into this plate the bony ring extends. The bony outgrowth does not, however, proceed uniformly from the whole of circumference of the annulus, but occurs most rapidly in its anterior and posterior parts. These outgrowths fuse about the end of the second