The Ethmoid Bone.—In front of and below the sphenoid and extending forward to the frontal bone is the ethmoid, the last of the cranial bones. It consists of a horizontal cribriform or sieve-like plate, from either side of which depend lateral masses of ethmoid cells. To the inner side of these masses are attached the thin curved turbinated bones, superior and middle, while between them is a vertical plate that forms the bony septum of the nose. Rising from the upper surface of the cribriform plate is another vertical plate, the crista galli, with the olfactory grooves on either side for the reception of the olfactory bulbs, filaments of the olfactory nerve passing down through the perforations of the cribriform plate to the nose. For the brain, which fills almost the entire cavity of the cranium, is supported by the sphenoid and ethmoid bones internally, as it is protected externally by the other cranial bones.
Ossification of Sutures.—If premature ossification of all the sutures occurs, idiocy results, while in cephalocele there is a gap in the ossifying of the bones so that the membranes or brain protrude. In rickets the forehead is high and square and the face bones poorly developed, so that the head looks larger than it really is. In Paget’s disease the bones enlarge and soften. This affects the head but not the face and often the first thing noticed is that the hat is too small. Craniotabes is thinning of the bone in places, the bone becoming like parchment and being easily bent. It is generally caused by pressure of the pillow or the nurse’s arm.
Bones of the Face.—The facial bones serve to form the various features of the face, which after all are merely organs of special sense. Many delicate muscles control the facial expression which, consciously or unconsciously, reflects the character of their owner.
Surgically the most important of the facial bones are the two [superior maxillary bones], because of the number of diseases to which they are liable. They meet in front, together forming the upper jaw, and with the malar bone help form the lower part of the orbit of the eye. They are cuboid in shape and are hollowed out into a pyramidal cavity called the antrum of Highmore, which opens by a small orifice into the middle nasal meatus and which sometimes becomes infected and has to be tapped. The nasal process for articulation with the frontal and nasal bones has, at its lower edge, a crest for the inferior turbinated bone, and close beside this on the inside, extending down from the upper edge, is a deep groove which, with the lachrymal and inferior turbinated bones, helps to form the lachrymal canal for the nasal tear duct. The bones give attachment to many small muscles, connected for the most part with the nose and mouth, of which the masseter is the only important one.
The two [malar] or cheek bones are small quadrangular bones, which form the prominences of the cheeks and help form the orbits of the eyes. Projecting backward from each is a zygomatic process for articulation with the zygomatic process of the temporal bone, while a maxillary process extends downward for articulation with the superior maxillary. Here again the most important muscle attached is the masseter. If the malar bone is crushed great deformity results.
The lachrymal bones are two small bones, about the size and shape of a finger-nail, situated at the front of the inner wall of the orbit. At the external edge is a groove which lodges the lachrymal sac above and forms part of the lachrymal canal below.
The two palate bones are at the back of the nasal fossæ and help to form the floor of the nose, the roof of the mouth, and the orbit. Each has a vertical and a horizontal plate, and it is these last that by their juncture form the hard palate. Oftentimes in cases of hare-lip cleft palate also occurs, the result of incomplete development. To remedy the consequent opening in the roof of the mouth, which makes articulation difficult, operation is generally resorted to, though sometimes a plate is fitted over the opening by a dentist.
The [nasal bones] are two small oblong bones which articulate with the frontal and superior maxillary bones and with each other. They form the bridge of the nose, the rest of the nose being wholly of cartilage, except for the vomer, a bone shaped like a plough-share, which forms part of the nasal septum, articulating along its anterior edge with the ethmoid and the triangular cartilage.
The two inferior turbinated bones lie along the outer walls of the nasal fossæ. They are thin scroll-like bones covered with mucous membrane and serve to heat the air as it passes in. Sometimes when one has a cold, the membrane and the bone too swell up and close the nares. Loss of the sense of smell in a bad cold may be due to such swelling and the consequent impeding of the entrance of odoriferous particles—a condition that would likewise interfere with the sense of taste. Part of the bone is sometimes removed, to enlarge the passage, enough being left to warm the air.
Lastly, there is the [inferior maxillary bone] or lower jaw. This has a horseshoe-shaped body and two rami, one at either end. Each ramus has a pointed process in front called the coronoid process, into which is inserted the temporal muscle. At the back, and separated from the coronoid process by the sigmoid notch, is the condyle, which articulates with the glenoid fossa on the temporal bone. The rami also give attachment to the masseter muscle at its point of insertion. In adult age the ramus is almost vertical but in old age the portion of the jaw hollowed out into alveoli for the teeth becomes absorbed and the angle of the jaw becomes very obtuse. On the inner side of the jaw near the middle on either side is the fossa for the sublingual gland, while the submaxillary gland lies in a fossa farther back on either side.