When well developed, the frontal sinuses may reach 5 cm. (2 in.) upward and 3.7 cm. (1.5 in.) laterally, occupying the greater part of the vertical portion of the frontal bone. When very small, they scarcely extend above the nasal process.

In any case, they are rarely symmetrical. The average dimensions of an adult frontal sinus are 3.7 cm. (1.25 in.) in height, 2.5 cm. (1 in.) in breadth, and 1.8 cm. (0.75 in.) in depth. (Logan Turner.) The sinuses are separated by a septum. The posterior wall is very thin. Each sinus narrows downward into the infundibulum. This is 'deeply placed, at the back of the cavity, behind the frontal (nasal) process of the maxilla and near the medial wall of the orbit. Its termination in the middle meatus is about on a level with the palpebral fissure.' (Thane and Godlee.) Its direction is backward.

The communication of these sinuses with the nose accounts for the frontal headache, the persistence of polypi and ozæna, and the fact that a patient with a compound fracture opening up the sinuses can blow out a flame held close by.

To open the frontal sinus, while the incision which leaves the least scar is one along the shaved eyebrow, superficial laterally so to avoid the supraorbital nerve and vessels, running a little downward at the medial end, it is always to be remembered that, where the sinuses are little developed, this or a median incision may open the cranial cavity. To avoid this complication the sinus should always be opened at a spot vertically above the medial angle.

The development of these by the twentieth or twenty-fifth year may render a fracture here much less grave in the adult than would otherwise be the case, the inner layer (table), if now separated from the outer, protecting the brain. Mr. Hilton showed that the absence of any external prominence here does not necessarily imply the absence of a sinus, as this may be formed by retrocession of the internal layer. In old people these sinuses may enlarge by the inner layer following the shrinking brain. Again, prominence of the supracOiary and frontal eminences does not necessarily point to the existence of a sinus at all, being due merely to a heaping up of bone.

The mastoid cells are arranged in two groups, of the utmost importance in that frequent and fatal disease, inflammation of the middle ear:

(A) The upper, or 'antrum,' present both in early and late life, horizontal in direction, closely adjacent to and communicating with the tympanum.

(B) The lower, or vertical. This group is not developed in early life.

A. Tympanic antrum

This is a small chamber lying behind the tympanum, into the upper and back part of which (epitympanic recess) it opens. Its size varies, especially with age. Almost as large at birth, it reaches its maximum (that of a pea) about the third or fourth year. After this its size usually diminishes somewhat, owing to the development of the encroaching bone around

Tempobal Bone, showing Suprambatal Triangle. (Barr.) The lower part of the transverse sinus is here placed too far back to be relied upon with constant accuracy.

it. Its roof, or tegmen, is merely the backward continuation of the tegmen tympani. The level of this is indicated by the posterior root of the zygoma. 'The level of the floor of the adult skull at the tegmen antri is, on an average, less than one-fourth of an inch above the roof of the external osseous meatus; in children and adolescents, from one-sixteenth to one-eighth of an inch.' (Macewen.) In early life, when the bony landmarks, e. g. the suprameatal crest, are little marked, the level of the upper margin of the bony meatus will be the safest guide to avoid opening the middle fossa.

The lateral wall of the antrum is formed by a plate descending from the squamous bone. This is very thin in early life, but as it develops by deposit under the periosteum, the depth of the antrum from the surface increases. Macewen gives the average of the depth as varying from one-eighth to three-fourths of an inch. The thinness of the outer wall in early life is of practical importance. It allows of suppuration making its way externally - subperiosteal mastoid abscess. This will be facihtated by any delay in the closure of the petro- and masto-squamosal sutures, by which this thin plate blends with the rest of the temporal bone. Further, by the path of veins running through these sutures or their remnants, infection may reach such sinuses as the inferior petrosal. The sutures normally close in the second year after birth. Through the floor, the antrum communicates with the lower or vertical cells of the mastoid. This floor is on a lower level than the opening into the tympanum, and thus drainage of an infected antrum is difficult, fluid finding its way more readily into the lower cells. Behind the mastoid antrum and cells is the bend of the sigmoid part of the transverse (lateral) sinus, with its short descending portion. The average distance of the sinus from the superior meatal triangle is 1 cm. (2/5 in.). It may be further back; on the other hand, it may come within 2 mm. (1/12 in.) from the meatus, and even overlap the outer wall of the antrum.

The Mastoid Antrum and Cells. (Jacobson and Steward.)

1. Posterior root of zygoma forming the supramastoid or suprameatal crest and upper part of Macewen's triangle. 2. Antrum, and in front of it, the epitympanic recess. 3. Vertical cells of the mastoid. 4. Ridge on the inner wall of the tympanum, caused by the facial canal. 5. Fenestrse on inner wall of tympanum, indicated in shadow. 6. A deficiency present in the tegmen tympani, enlarged with a small osteotrite to emphasise the thinness of the roof of the antrum and tympanum. 7. Cells extending, in this case, even into the root of the zygoma.

The exact position of the antrum, a little above and behind the external auditory meatus is represented by Macewen's 'suprameatal triangle.' This is a triangle bounded by the posterior root of the zygoma above, the upper and posterior segment of the bony external meatus below, and an imaginary line joining the above boundaries. "Roughly speaking, if the orifice of the external osseous meatus be bisected horizontally, the upper half would be on the level of the mastoid antrum. If this segment be again bisected vertically, its posterior half would again correspond to the junction of the antrum and middle ear, and immediately behind this lies the suprameatal fossa.' (Macewen.) When opening the antrum through this triangle, the operator should work forward and medially, so as to avoid the transverse sinus ; while, to avoid the facial nerve, he should hug the root of the zygoma and the upper part of the bony meatus as closely as possible. The level of the base of the brain will be a few lines above the posterior root of the zygoma and about 6 mm. (j in.) above the roof of the bony meatus. (Macewen.)

B. The lower or vertical cells of the mastoid are developed later than is the antrum, and vary much in their contents. The condition of the mastoid cells varies very widely. They may be numerous or few. In the latter case they are replaced by diploe, or by bone which is unusually dense, without necessarily any pathological change. Hence mastoids have been classified as pneumatic, diploetic, or sclerosed.

As part of the surgical anatomy of this most important region, the different paths by which infection of the tympanum and antrum may travel should be glanced at.

The most important are:

(1) Upward: either by advancing caries or by infection of veins going to the superior petrosal sinus, or through the tegmina to the membranes; an abscess in the overlying temporal lobe, usually the middle and back part.

(2) Backward : the transverse (lateral) sinus and cerebellum (abscess of the front and outer part of the lateral lobe) are reached in the same ways as those given above, the mastoid vein being the one chiefly affected here. Macewen has shown that the bony wall of the sinus, like those of the tegmina and the aqueduct of Fallopius, may be naturally imperfect.

(3) Downward : where the vertical cells are well developed mischief may reach the mastoid notch and cause deep-seated inflammation beneath the sterno-mastoid. (v. Bezold's abscess.)

(4) Lateralward: the explanation of this, in early life, has been given above.

(5) Medialward : the facial nerve, or by the fenestra ovahs; the labyrinth is now in danger. When the internal ear and auditory nerve are affected, infection finds another path to the cerebellar fossa.

The sphenoidal sinuses are less important surgically, but these points should be remembered:

(1) Fracture through them may lead to bleeding from the nose, which is thus brought into communication with the middle fossa;

(2) the communication of their mucous membrane with that of the nose may explain the inveteracy of certain cases of polypi and ozæna;

(3) here and in the frontal sinuses very dense exostoses are sometimes formed. Before any operative attack on these sinuses is undertaken, their most important relations should be remembered. Thus above are the olfactory and optic nerves, the pituitary body, and front of the pons. Externally lie the cavernous sinus and superior orbital (sphenoidal) fissure. Below is the roof of the nose.

The ethmoidal and maxillary sinuses are considered later in connection with the Nose. See also the sections on Osteology and Respiratory System.