Containing a molluscous tumor,

.

Containing a molluscous tumor, the osseous walls being absorbed in parts, Containing a molluscous tumor which projects through the bone into the cerebral cavity, .

Containing a molluscous tumor which projects into the mastoid cells,
Containing a collection of pus,

Containing a collection of pus mixed with epidermis,

Having polypi growing from its walls,

.

Having polypi growing from its walls, the bone being carious,

The dermoid layer so much atrophied as to leave the bone denuded,

The dermoid layer hypertrophied,

The dermoid layer congested,

The dermoid layer soft,

The dermoid layer soft and red,

The dermoid layer soft and detached from the bone,

The dermoid layer soft and thick, the bone being carious,
The dermoid layer ulcerated, the bone being carious,

Osseous walls rough,

Osseous walls carious,

5

3

1

1

10

1

1

1

2

2

7

1

2

2

1

1

1

Osseous walls absorbed in parts,

Osseous walls presenting an orifice superiorly, .

Osseous walls presenting an orifice inferiorly,.

Osseous canal much contracted,

Having bony growths from the osseous walls; canal much contracted,

CHAPTER VIII.

THE MEMBRANA TYMPANI.

STRUCTURE AND FUNCTIONS.

THE EPIDERMOID LAYER-THE DERMOID LAYER-THE FIBROUS LAYERS-THE RADIATE FIBROUS LAYER-THE CIRCULAR FIBROUS LAYER-THE MUCOUS LAYER-TENSOR LIGA

MENT-FUNCTIONS OF THE MEMBRANA TYMPANI.

Anatomical Observations.-There are so many points of interest in Pathology connected with the structure of the membrana tympani, which bear upon its morbid conditions, but which have not hitherto been sufficiently recognized, that the subject deserves a full examination. Looked at from without inwards, the membrana tympani may be described as consisting of the following layers:1. The epidermis.

2. The dermis.

3. The fibrous layer, composed of—

a. The lamina of radiating fibres.

b. The lamina of circular fibres.

4. The mucous membrane.

The epidermis is a thin layer, covering the outer surface of the dermoid lamina; it is continuous with the epidermis of the external meatus, and, when subjected to the process of maceration, can be removed in the form of a small blind pouch, which presents, as it were, a cast of the meatus and of the external surface of the membrana tympani. When floating in water, the pouch reassumes the form it had when in contact with other tissues, and its internal extremity is convex, corresponding with the external concavity of the membrana tympani. The layer of epidermis forming the outer coat of the membrana tympani is thin, and in the living subject so transparent that the dermoid layer can be distinctly seen through its substance; its outer surface is very smooth, and capable of re

flecting light; and there is usually seen at its anterior and inferior part, a triangular shining spot.

In the course of dissection, I have more than once found this delicate epidermis to be the only layer remaining, over portions vary

ing from a line to a line and half in breadth, and yet appearing to have been sufficient to close the cavity of the tympanum and to preserve the power of hearing nearly perfect. The knowledge of this condition of the membrana tympani ought to lead to a cautious use of the syringe when no cerumen is present, since its application may cause a rupture of the epidermis.

The dermoid layer, as its name implies, is continuous with the dermis lining the external meatus, and is situated between the epidermis and the radiate fibrous layer. It is extremely thin, and secretes the epidermis. Previous to the publication of a paper by me on the structure of the membrana tympani, in the "Philosophical Transactions," for 1851, it had been supposed that the epidermis was secreted by the radiate fibrous layer. The presence of the dermis is best demonstrated by carefully dissecting, under water, the membranous meatus from the upper surface of the osseous tube, as far as the attachment of the membrana tympani; at which point the peri

osteum of the meatus is seen to become continuous with the radiate fibrous lamina; this being cut through, the dermoid layer is detected. passing down over the outer surface of the radiate fibres, and separating them from the epidermis. If the upper portion of this layer be drawn gently downwards by one hand, by means of a fine needle in the other, the delicate cellular tissue which connects it with the fibrous lamina can be broken up, and the dermis removed entire. The presence of this lamina may also be shown by introducing between it and the radiating fibrous layer, at the superior part of the membrana tympani, a fine bristle, by passing which down the cellular tissue is lacerated.

In a healthy state, when uninjected by blood or by artificial means, the dermoid lamina is thin and transparent, and its structure, under the microscope, looks like areolar tissue. When injected, however, this membrane is seen to have numerous blood vessels ramifying through it, so as to form an elaborate plexus: these vessels, when enlarged, impart the red appearance to the surface of the mem

FIG. 48.

The Dermoid Layer of the Membrana Tympani continuous with the Dermis lining the upper wall of the Meatus Externus.

brana tympani so frequently met with during life. It is upon the supply of nerves to this lamina that the exquisite sensibility of the membrana tympani depends.

A knowledge of the existence of the membrane just described is of interest to the anatomist, who recognizes in it the secreting organ of the epidermoid layer of the membrana tympani; and to the surgeon, who through its presence is enabled to understand phenomena occurring in certain diseases of the ear. There are peculiar diseased conditions in which the dermoid layer of the membrana tympani becomes much hypertrophied.

The proper fibrous layer of the membrana tympani can be easily separated into two lamina, which are named from the direction of their component fibres. Previous to describing these structures, it

is desirable to cite the opinions of eminent anatomists concerning them.

In the Croonian Lecture, published in the 19th volume of the "Philosophical Transactions," Sir Everard Home advanced the opinion that the membrana tympani in the human subject was muscular. His words are: "When viewed in a microscope, magnified twenty-three times, the muscular fibres are beautifully conspicuous, and appear uniformly the same throughout the whole surface. There being no central tendon as in the diaphragm, the muscular fibres appear only to form the internal layer of the membrane, and are most distinctly seen when viewed on that side." The use of this radiated muscle, Sir Everard states, is "to give those different degrees of tension to the membrane which empower it to correspond with the variety of external tremors." Since the first publication of this opinion as to the muscularity of the membrana tympani, anatomists, though generally conceding that it is fibrous, have widely differed as to its composition. According to Mr. Quain and Dr. Sharpey, "It is made up of fine closely-arranged fibres, the greater number of which radiate from near the centre to the circumference; but within these are circular fibres, which are more scattered and indistinct, except close to the margin of the membrane, where they form a dense, almost cartilaginous ring." Mr. Wharton Jones writes: "The proper membrane can be divided into two layers-an outer thin one, consisting of radiating fibres, and an inner thicker layer, which is less distinctly fibrous, though when torn it does indicate a fibrous disposition, and that in a direction opposite to the former. The fibres which cross the radiating ones are more aggregated at the centre; they run parallel with the handle of the malleus, and turn round its extremity. At the circumference of the proper membrane, there is a thick, firm, ligamentous or cartilaginous ring, which is fixed in the groove of the bone. The ligamentous ring appears to be formed by an aggregation of the circular fibres interwoven with the peripheral extremities of the radiating ones."4

By careful dissection the fibrous layers of the membrana tympani may be separated into two distinct laminæ, the fibres of which have no intercommunication. The external layer may be called the radi ate fibrous lamina, since its fibres radiate from the malleus to be at

1 Loc. cit. p. 5.

2 Loc. cit. p. 11.

a Elements of Anatomy, 5th Edition, 1848, vol. ii, p. 932.

4

♦ Cyclopædia of Anatomy and Physiology, vol. ii, p. 545.