This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The membrana tympani is inclined downward and inward at an angle of about 1400 to the upper wall (Troltsch) and 270 to the lower wall (Bezold) of the meatus; it does not lie directly transverse, therefore in introducing instruments into the ear the upper posterior part will be first encountered. The membrane is located 2.5 cm. (1 in.) from the surface; this is to be borne in mind in puncturing the membrane or other operations. The membrane has three coats: an outer, continuous with the skin of the meatus; a fibrous or middle layer; and an internal or mucous layer, continuous with the lining of the tympanic cavity. The membrana tympani at birth is fastened at its circumference to the tympanic bone, which unites with the other portions of the temporal bone soon after birth. This ring of bone is incomplete at its upper portion for a distance equaling one-eighth of its circumference. This is called the notch of Rivinus. The fibrous layer does not extend across this notch, which is closed by the mucous membrane on the inside and by the skin layer of the membrane on its outer side. The part closing the notch
Fig. 102. - Outer surface of the tympanic membrane of the left ear, is called Shrapnell' s membrane or membrana flaccida. As it possesses no fibrous • layer it is weaker than the membrane elsewhere and consequently is a favorite spot for pus to perforate in order to find exit from the middle ear.
In examining the membrane by means of light thrown into the meatus through a speculum by the head mirror, one sees extending downward from its centre a small cone of light; any depression or bulging of the membrane will cause this cone of light to be altered in its position, or even cause it to disappear entirely. From the centre of the membrane upward extends a line which indicates the attachment of the long handle of the malleus, one of the bones of the middle ear. Stretching across the upper portion is the membrane of Shrapnell or membrana flaccida, so called on account of its not being so tense as the remaining portion. It is better supplied with blood-vessels than the other portion.
The membrana tympani is of surgical interest on account of its being often distended or perforated. A purulent discharge from the ear usually indicates disease of the middle ear or tympanum. If pus is coming from a furuncle of the meatus, the latter will be swollen and its source can readily be recognized. If it comes from outside of the meatus, as in cases of suppuration of the parotid gland, it will be recognized by an examination of the gland. There is no other source of pus but the middle ear and for it to gain exit it must perforate the membrane; this perforation can usually be seen with the speculum and head mirror, as can also bulging.
In inflammation of the middle ear the effused serum or pus bulges the membrane outward. When this condition is accompanied, as it often is, by intense pain, paracentesis or puncture is resorted to. The preferable spot is the posterior lower quadrant. Paracentesis of the membrane should be done by beginning the incision a little above and behind the centre of the tympanic membrane, which slopes downward and forward at an angle of 1400 to the upper wall, and cutting downward to its lower edge. One must avoid the long handle of the malleus, which extends directly upward from the centre of the membrane. In the upper posterior part are the incus and stapes, therefore this portion should be avoided; and running across the upper edge beneath the mucous membrane is the chorda tympani nerve. Division of this nerve is said to be a matter of not much account. Incision through the anterior part is not considered suitable for drainage.
Perforations frequently occur through Shrapnell's membrane on account of its not having any fibrous layer; thus the pus does not go through the tympanic membrane proper. If perforation with a purulent discharge has existed for a long time granulations come through the opening, forming an aural polyp. To remove these a snare is used or caustic is applied.