From the under surface of the cribriform plate and at right angles with it, there descend, on the median line, the nasal lamella, and, on each side, a cellular mass which partly forms the outer wall of the nostril and the inner wall of the orbit.
The nasal lamella, or vertical plate, forms the upper portion of the septum narium ; it is immediately 'underneath the crista galli, and becomes gradually thinner as it descends; its anterior border is rough, thicker above than below, and articulates, first, with the nasal process of the frontal bone, and, secondly, with the nasal bones-themselves; its posterior border is also rough and is articulated to the crest on the fore part of the sphenoid bone; its inferior border is, in its posterior half, thin and inclined downwards and forwards to be articulated to the vomer, and, in its anterior half, somewhat thicker and rougher, and in clined downwards and backwards to be arti culated with the triangular cartilage of the nose; its sides are plain, and exhibit sulci which are continuous with the foramina that open on its root.
On each side of this lamella and between it and the lateral masses there is a space which is encroached upon in the, middle more than it is above or below, and a portion of the cribri form plate forms its roof.
The lateral masses are delicate in their struc ture and complicated in their arrangement. Each consists of a number of cells (cellula ethmoidales), which are divided *by a partition Into an anterior and a posterior set, with the former of which the frontal sinus communi cates, and with the latter the sphenoidal. The outer surface of each lateral mass is compact and smooth, and constitutes the greater portion of the inner wall of the orbit. This is the orbitar process or os planurn, which articulates above with the frontal bone, below with the superior maxillary and palate bones, behind with the sphenoid, and in front with the la chrymal. On its upper border are seen the two notches which assist the frontal in forming the anterior and posterior orbital foramina. The inner surface of this cellular mass, that which looks towards the nasal lamella, is ren dered irregular by two curved processes (the superior and nziddle turbinatcd processes), of which the upper one is smaller, delicate, re gular in its curve, and is seen only on the posterior half of the wall ; the other is larger, more spongy, and extends the entire length of the wall. Both of them are convex on the
side next the cavity of the nostril, and concave on that which looks towards the cells ; but the inferior is also at its lower edge again curled in such a manner as to offer a convexity on both of its surfaces. Between the two turbinated pro cesses there is a triangular space (the superior meatus) the apex of which is directed forwards, and in which there is an opening commu nicating with the posterior ethmoidal cells. Underneath the middle turbinated process, and bounded by. its concavity on the one hand and the cells on the other, is the middle meatus ; into which open the anterior ethmoidal cells, and the tubular communication with the frontal sinus, called infundibullim.
The connexions of this bone are, behind to the sphenoid ; in front to the frontal and nasal bones; laterally by its upper borders to the orbitar processes of the frontal, by its under borders to the same-named processes of the superior maxillary and palate bones,- and by its anterior border to the lachrymal ; by the under edge of its middle vertical plate to the vomer and triangular cartilage ; and by the anterior extremity of the outer surface of the middle turbinated process to the inferior tur binated bone.
The ethmoid is the most tardy in its deve lopment of all the bones of the cranium. The lateral masses exhibit each of them an ossific deposit about the middle period of intra uterine life, but neither the cells nor turbinated processes are much developed at birth, at which time also the central portion is carti laginous. The ossification of this part pro ceeds from above downwards, so that the crista galli is completely formed while the lower part of the nasal lamella is yet cartila ginous. During infancy the cribriform plate becomes narrower, curved, and as it were compressed ; the nasal lamella advances for wards; and the spaces between the septum and outer walls are considerably increased.