The ethmoid bone is cubical in shape, and exceedingly light ; it is situated at the anterior part of the base of the cranium, and assists in forming the medial walls of the orbital cavities, the septum of the nose, and the roof and lateral walls of the nasal cavity. It consists of four parts : a horizontal, perforated plate named the cribriform plate, a perpendicular plate and two labyrinths or lateral masses.
The cribriform plate of the ethmoid (fig. 366) occupies the ethmoidal notch of the frontal bone and forms a part of the roof of the nasal cavity. A thick, smooth, triangular process, called the crista galli from its resemblance to a cock’s comb, projects upwards from this lamina in the median plane. Its posterior border, long, thin, and curved, gives attachment to the falx cerebri. Its anterior border, short and thick, articulates with the frontal bone by two small projecting aloe, which complete the foramen cecum. Its sides are smooth, and sometimes bulging owing to the presence of a small air-sinus in the interior. On each side of the crista galli the cribriform plate is narrow and depressed ; it supports the gyrus rectus and is perforated by foramina for the passage of the olfactory nerves. The foramina in its middle part are small and transmit the nerves from the mucous membrane of the roof of the nasal cavity ; those in the medial and lateral parts are larger- the former transmit the nerves from the mucous membrane on the upper part of the nasal septum, the latter the nerves from the mucous membrane on the superior nasal concha. At the front part of the cribriform plate, on each side of the crista galli, there is a small slit-like fissure, which is occupied by a process of dura mater. The foramen which transmits the anterior ethmoidal nerve to the nasal cavity is placed lateral to the anterior end of the fissure, and from it a groove runs backwards to the orifice of the anterior ethmoidal canal.
The perpendicular plate of the ethmoid (figs. 367, 368), thin, flat and somewhat quadrilateral in form, descends from the under surface of the cribriform plate and forms the upper part of the nasal septum; it is generally deflected a little to one or other side. The anterior border articulates with the nasal spine of the frontal bone and the crest of the nasal bones. The posterior border articulates with the sphenoidal crest above and with the vomer below. The superior border is attached to the cribriform plate. The inferior border is thick, and serves for the attachment of the septal cartilage of the nose. The surfaces of the lamina are smooth, except above, where numerous grooves and canals are seen ; these lead from the medial foramina in the cribriform plate and lodge filaments of the olfactory nerves.
Each labyrinth of the ethmoid bone consists of a number of thin-walled ethmoidal sinuses, arranged in three groups: anterior, middle, and posterior, and interposed between two vertical plates of bone; the lateral or orbital plate (lamina papyracea) forms part of the medial wall of the orbit, the medial plate, part of the lateral wall of the nasal cavity. In the disarticulated bone many of these ethmoidal sinuses are opened into, but in the articulated skull they are everywhere closed, except at their apertures of communication with the nasal cavity. The upper surface of the labyrinth .(fig. 366) presents a number of air-sinuses, the walls of which are completed, in the articulated skull, by the edges of the ethmoidal notch of the frontal bone (fig. 363). This surface is crossed by two grooves which are converted into the anterior and posterior ethmoidal canals by articulation with the frontal bone. On the posterior surface of each labyrinth (fig. 368) are large air-sinuses, the walls of which are completed by the sphenoidal concha and the orbital process of the palatine bone. The lateral surface (fig. 369) consists of a thin, smooth, oblong plate, named the orbital plate (lamina papyracea), which covers the middle and posterior ethmoidal sinuses and forms a large part of the medial wall of the orbit; it articulates above with the orbital plate of the frontal bone, below with the maxilla and the orbital process of the palatine bone, in front, with the lacrimal bone, and behind with the sphenoid bone (fig. 317).
A few sinuses lie in front of the orbital plate and their walls are completed by the lacrimal bone and the frontal process of the maxilla. A curved lamina, termed the uncinate process; which is subject to considerable variation in size, projects downwards and backwards from this part of the labyrinth ; it forms a small part of the medial wall of the maxillary sinus (fig. 317), and articulates with the ethmoidal process of the inferior nasal concha. The upper edge of this process is free and forms the medial boundary of the hiatus semilunaris.
The medial surface of the labyrinth (fig. 370) forms part of the lateral wall of the corresponding nasal cavity ; it consists of a thin lamella, which descends from the under surface of the cribriform plate and ends in a free, convoluted portion, named the middle nasal concha. The upper part of the medial surface is marked by numerous grooves, directed nearly vertically downwards ; they lodge branches of the olfactory nerves, which are distributed to the mucous membrane covering the superior nasal concha. The posterior part of the medial surface is subdivided by a narrow, oblique fissure, termed the superior meatus of the nose, which is bounded above by a, thin, curved plate, named the superior nasal concha; the posterior ethmoidal sinuses open into this meatus. Below and in front of the superior meatus is the convex surface of the middle nasal concha ; it extends along the whole length of the medial surface of the labyrinth. Its lower margin is free and thick, while its lateral surface is concave and assists in forming the middle meatus of the nose. The middle ethmoidal sinuses produce a rounded swelling, named the bulla ethmoidalis, on the lateral wall of the middle meatus (fig. 371) ; on the bulla, or immediately above it, these sinuses open into the meatus. A curved passage, named the infundibulum, extends upwards and forwards from the middle meatus ; it communicates with the anterior ethmoidal sinuses, and in rather more than 50 per cent. of skulls is continued upwards as the frontonasal duct into the frontal sinus.
Ossification.-The ethmoid bone is ossified in the cartilaginous nasal capsule from three centres ; one for the perpendicular plate, and one for each labyrinth. The centre for each labyrinth appears in the region of the orbital plate between the fourth and fifth months of fetal life, and extends into the concha. At birth, the two labyrinths, which are small and ill-developed, are partially ossified, but the rest of the bone is cartilaginous. During the first year after birth, the perpendicular plate and crista galli begin: to ossify from a single centre, and are joined to the labyrinths about the beginning of the second year. The cribriform plate is ossified partly from the perpendicular plate and partly from the labyrinths. The ethmoidal sinuses begin to develop during fatal life, and in the new-born infant have the form of narrow pouches.