Three circumstances combine to render the study of the human nasal vertebra more difficult than that of the preceding. Firstly, it is retracted under the projecting frontal bone, or expanded spine of the frontal vertebra; secondly, the neurapophyses, hollowed out to receive the ethmoidal cells, are united in the mesial line to form the perpendicular lamella' of the ethmoid bone, by which means their typical character is obscured; and, thirdly, the pleurapo-physial and hsemapophysial appendages, by their articulation with the rest of the skull, render this segment of the cranium rather complicated.
The centrum, or vomer, if examined in the fcetal skull, is a long compressed bone, grooved upon its upper surface, to receive the flat triangular plate of bone, which subsequently coalesces with it, and unites it with the two nasal bones, or the divided nasal spine, forming a septum between the two lateral masses of the ethmoid bone. The neurapophyses (prefrontals) are represented by this flat triangular plate, known as the perpendicular lamella of the ethmoid bone. It is composed of two lateral pieces, pressed side by side, so as to obliterate the neural arch: anteriorly they are prolonged by the nasal cartilage; posteriorly they send outwards, on either side, a thin plate of bone, which turns forwards, and assists, under the name of os planum of the ethmoid, to form a bony case, in which the loose spongy olfactory sense-capsule is lodged. In the crocodile the true characters of the perpendicular lamella, and of the ossa plana, may be recognized more easily than in the human skull: the olfactory sense-capsule, composed of cartilage, easily comes away from the two lateral concavities, which are still separated by the compressed neurapophyses, or ethmoidal septum.