PART II. THE MIDDLE EAR.
CHAPTER IX. ANATOMY OF THE MIDDLE EAR. By far the greater number of aural diseases affect what is known as the middle ear. Of one thousand cases occurring in the private practice of the author, eight hundred and nine were diseases that involved these parts chiefly. The anatomy of this region, therefore, demands a careful and exact study.
The membrana tympani, or drum-head, forms the boundary between the external and middle ear. It partakes of the characteristics of these two parts, in being composed of integument and mucous membrane, while it has one structure — the middle or fibrous layer — that is peculiar to itself. The upper border of this membrane lies 7mm. nearer to the entrance of the external auditory canal than the lower. The posterior border is about 5mm. nearer this entrance, or meatus, than the anterior.
The angle that the membrana tympani makes with the axis of the auditory canal, is one of about 55°. The inclination of the two membranes to an angle opening upwards is one varying from 130° to 135°. In the newly born, the membrana tympani is more in a horizontal position than in the adult, and lies almost in the same line with the upper wall of the external auditory canal.
<Callout type="important" title="Important">The membrana tympani's unique structure protects it from sudden loud sounds by yielding slightly.</Callout>
In the right temporal bone of a newly born child, without the bony portion, in connection with the ossicula auditus. 4, is above the incus, whose short process is directed nearly horizontally backward. 5. The long arm of the incus, which extends freely into the cavity of the tympanum. 6. The malleus, in articulation with the incus. 7. Long process of the malleus, which runs under the crista tympanica, in a furrow, to the fissura petroso-tympanica. 8. The stapes, in articulation with the incus.
The peculiar manner in which the membrana tympani is placed in the canal causes it to form an acute angle with the lower and anterior wall of the auditory canal, but an obtuse one with the upper and posterior wall. The general shape of the membrane is elliptical; but the regularity of the ellipse is broken upon by the incompleteness of the bony ring surrounding the membrane.
<Callout type="risk" title="Risk">The Bivinian segment, filled by cutis and mucous membrane, can sometimes fall in like a pouch towards the tympanic cavity.</Callout>
This causes an irregular triangular space to be formed, bounded above by the Bivinian segment, and on each side by two bands, which attach the apex of the small process of the malleus to the anterior and posterior corners of the osseous groove. This space, and the tissue filling it, was first described by Mr. Henry Jones Shrapnell* and named by him the membrana flaccida.
<Callout type="tip" title="Tip">The existence of a minute opening in the membrane — the so-called Rivinian foramen — has been hotly debated since its discovery.</Callout>
Professor Patruban found such an opening in 300 membranes, part healthy, part diseased. He allowed a fine stream of quicksilver to pass into the so-called canal, and it always appeared on the other side of the membrana flaccida. Professor Joseph Gruberg has also found the foramen in many specimens.
The ability to blow tobacco-smoke from the ears is the result, he thinks, of a want of development in the upper part of the membrane. Professor Bochdalek, of Prague, rediscovered the opening at the upper margin of the membrana tympani, one-third to three-fourth lines from the edge, and reopened the discussion which Hyrtl seemed to have closed.
<Callout type="warning" title="Warning">The existence of the foramen is hotly debated. Only a fine bristle or hair will pass through it.</Callout>
Bochdalek describes his discovery as follows: 'To my great astonishment I saw, by means of a magnifying-glass, on the posterior portion of a small depression on the membrana tympani, and a little behind the malleus, a very small canal, in which was perceived, although very indistinctly, a punctiform opening. By means of a very fine bristle I succeeded in entering a narrow groove, not more than one-third of a line long, which ran in an oblique direction from above downward, and somewhat anteriorly, into the cavity of the tympanum, so that the bristle passed immediately beneath the handle of the malleus, and just as closely beneath the chorda tympani. On pushing the bristle still farther, it passed under the tendon of the inner muscle of the malleus, and struck on the inner wall of the cavity of the tympanum.'
Dr. Bochdalek also found the foramen in the opposite membrane of the same subject, as well as in sixty-three other preparations of the membrana tympani. Forty of them were from fresh subjects, the remainder had been preserved in alcohol.
Kessel believes that the foramen is the result of inflammation. He says that he has convinced himself of the correctness of this view, by dissections and by examination of the living subject at Gruber's clinic.
<Callout type="important" title="Important">The light spot on the membrana tympani is an important diagnostic feature for certain middle ear diseases.</Callout>
View of Membrana Tympani, showing Handle of Malleus and Triangular Spot of Light. The ordinary breadth of the light spot, at its base, is from one and a half to two millimetres.
The light spot depends upon three factors: I. — The inclination of the membrana tympani to the auditory canal; II. — The traction of the malleus, which renders it concave at the center; III. Its polish or brilliancy.
<Callout type="tip" title="Tip">The light spot is a critical diagnostic tool for certain middle ear diseases.</Callout>
The membrana tympani has three layers: 1. A thin layer of integument; 2. A fibrous layer, which forms the principal thickness of the membrane; 3. A mucous layer continuous with that of the tympanic cavity.
The first or integumentary layer of the membrana tympani has none of the hairs or glands of the lining of the canal, of which it is a direct continuation. The papillae are found as far as the short process of the malleus. The epidermal cells, the cuticle and corium diminish gradually in thickness from the periphery towards the handle of the malleus; they then increase and are thickest on the outer edge of this bone.
The fibrous layer consists of lamellae, each one of which forms a mesh-work of smooth
Key Takeaways
- The membrana tympani is crucial for understanding middle ear anatomy and diagnosing aural diseases.
- The light spot on the membrana tympani is an important diagnostic feature.
- The existence of the Rivinian foramen remains hotly debated.
Practical Tips
- Understand the unique structure of the membrana tympani to better diagnose and treat middle ear issues.
- Use the light spot as a critical diagnostic tool when examining patients with suspected middle ear diseases.
- Be aware that the existence of the Rivinian foramen is still debated, so approach this knowledge with caution.
Warnings & Risks
- The membrana tympani's structure can be complex and varies between individuals, making diagnosis challenging.
- The light spot may not always be visible or present in all cases, requiring additional diagnostic methods.
- The existence of the Rivinian foramen is still debated, so its clinical significance remains uncertain.
Modern Application
While the specific techniques described in this chapter are historical, understanding the anatomy and function of the membrana tympani remains crucial for modern medical practitioners. The principles of diagnosing middle ear diseases through the light spot and recognizing the complex structure of the drum-head still apply today, even as diagnostic tools have advanced.
Frequently Asked Questions
Q: What is the significance of the light spot on the membrana tympani?
The light spot on the membrana tympani is a critical diagnostic feature for certain middle ear diseases. It helps in understanding the condition of the membrane and can indicate issues such as inflammation or infection.
Q: What causes the existence of the Rivinian foramen?
The existence of the Rivinian foramen remains hotly debated, with some experts finding it in many specimens while others deny its existence. It is suggested that it may be a result of inflammation or a normal anatomical variation.
Q: How does the membrana tympani's structure affect diagnosis?
The membrana tympani's unique structure, including its layers and the light spot, plays a crucial role in diagnosing middle ear diseases. Its complex anatomy can provide valuable insights into the condition of the ear.