Caries and necrosis of the mastoid bone are resultants of the above-described conditions, and are especially found in early childhood, and generally caused by retention of pus in the mastoid cells and breaking down of their walls. This process can be limited to the cell portion of the bone or can also involve the cortex, with formation of an external fistulous opening.
TREATMENT.—Use of heat and moisture, either by hot-water fomentations or warm poultices, like flaxseed, over the entire temporal region of the head on which the diseased mastoid is located. The flaxseed poultice is to be covered with oil silk and changed as often as needful to keep it warm. The use of leeches to the mastoid is indicated by tenderness of the part to the touch, with heat and swelling of the tissue covering the bone. Two or three foreign leeches can be used, and if the abstraction of more blood is desired the after-bleeding is to be encouraged by warm moist applications. If the disease advances notwithstanding this treatment, an opening down to the bone is indicated. The incision is usually described as the Wilde incision. The length of the cut is to be from a half to one inch, down to the bone, the point of the knife entering the skin on a level with the upper wall of the auditory canal, about half an inch behind the auricle. Occasionally the posterior auricular artery is cut, but hemorrhage is readily controlled by pressure over the artery. During the entire treatment the external auditory canal is to be cleansed from time to time of the purulent secretions, so as to facilitate the discharge of pent-up fluids from the middle ear and antrum. Also, the condition of the pharynx is to be noted, and treated if needful. Finally, if all these measures fail to relieve, and the patient shows signs of meningeal or brain involvement, together with marked redness, tenderness, and swelling over the mastoid bone, showing that pus is being retained in the mastoid cells, there only remains the making of an opening into the mastoid process and antrum by means of a bone-drill or gouge. This is best done by a free vertical incision through the skin and periosteum covering the mastoid process. Examine then the bone, and a fistulous opening may be found which can be enlarged by a probe, and so allow the free escape of pus. If such does not exist, apply a drill to the bone at a point a quarter of an inch posterior to the external canal and just below a horizontal line drawn tangent to its upper wall. The instrument is to have a direction inward, upward, and slightly forward. The depth to which it should penetrate varies: usually cell-structure is reached at a slight depth, when the drill should be withdrawn. If sclerosis of bone exists, it will be necessary to go deeper, but never more than three-quarters of an inch, or about 20 millimeters. This is Buck's rule. Schwarze says, never go deeper than 25 millimeters, otherwise there is risk of plunging the drill into the labyrinth. Also, during the drilling process Buck recommends keeping the fore finger of the operating hand constantly pressed against the neighboring bone, so as by counter-pressure to reduce to a minimum the risk of wounding the lateral sinus if it should lie in an abnormal position in the path of the drill. After-treatment consists in keeping the canal open by gentle washing. The use of a bone-gouge is preferred by some to the drill, as being a less dangerous instrument.
Diseases of the Internal Ear.
ANATOMY.—The internal ear consists of a central cavity, from one end of which arise the semicircular canals, and from the other the cochlea. The interior of these contains the membranous portion and fluids of the internal ear. The cochlea contains the most important part—namely, the terminal endings of the auditory nerve. Sound-vibrations pass through the external canal and strike against the tympanic membrane, throwing it into vibration. The vibrations of this membrane are carried across the middle ear by the chain of small bones to the membrane closing the foramen ovale of the internal ear, throwing this and the labyrinthine fluid also into vibration, and these latter vibrations, impinging on the terminal endings of the auditory nerve in a way as yet unknown, produce sound.
Vessels of the Labyrinth.—The labyrinth obtains its blood partly from the arteria auditiva interna, a branch from the basilar artery which comes from the vertebral, and partly through vessels communicating with the middle ear viâ the round and oval windows, and through others passing through the long walls themselves. The arteria auditiva interna divides in the internal meatus into a vestibular and cochlear branch. The former is distributed to the soft structures of the vestibule and semicircular canals. The cochlear branch is distributed to the modiolus and layers of the lamina spiralis. The venæ auditivæ internæ empty into the inferior petrosal sinus or the lateral sinus; other branches empty into the superior petrosal sinus.
The auditory nerve or portio mollis of the seventh nerve arises by two roots in the medulla oblongata. One ganglionic nucleus of origin is in the floor of the fourth ventricle, the other is in the crus cerebelli ad medullam (Stieda). The nerve winds around the restiform body, and passes into the meatus auditorius internus, and finally divides into a vestibular and cochlear branch. The vestibular branch divides into three branches: the superior is distributed to the utricle and ampullæ of the superior vertical and horizontal semicircular canals; the middle to the sacculis, and the inferior to the ampulla of the inferior vertical semicircular canal. The cochlear branch enters the modiolus and breaks up into smaller branches, which radiate fan-shaped into the lamina spiralis, and are then distributed between the two plates of the lamina spiralis through all its turns.
TINNITUS AURIUM.—It may be assumed that the normal ear is filled with continuous sound. The blood flowing through the large arteries and veins in close proximity to it (such as the carotid arteries and jugular vein), as well as the blood flowing through the vessels of the internal ear, will give rise to sound by throwing into vibration the soft tissues surrounding them, including also the walls of the vessels themselves. This motion is sufficient to excite the auditory nerve-elements by causing vibrations of the intra-labyrinthine fluids, and so produce sound; which, being a normal condition, and one to which the ear is accustomed, will remain unnoticed.11
11 To Theobald we are indebted for the vascular theory of sound.
The arterial system of the body throws the neighboring tissue into vibration, but this is not recognized unless our attention is particularly directed to it; or, in other words, the entire body is filled with movement as a normal condition, and therefore attracts no attention. But let this movement be increased—for instance, by violent muscular exertion, increasing the arterial action—or lessened, as in syncope, and at once an abnormal condition draws our attention to it.
In the same way the ear is filled with continuous sound as a normal condition, and therefore it is not perceived, these sound-vibrations escaping out through the middle ear and external canal. This can be readily proved. Let the external auditory canal be obstructed artificially, either by the finger or by a cork. At once a tidal tinnitus, so called, is produced, this being caused by the normal sound-vibrations being impeded in their outward passage and being thrown back again to impress the nerve-elements for a second time. This, being an abnormal condition, is at once recognized.