(3) Bones in relation to the Olfactory capsules.
In connection with the olfactory capsules, five pairs of bones are developed, two pairs being membrane bones, and three pairs cartilage bones.
Of membrane bones, the nasals (fig. 73, 4) are a pair of long narrow bones, lying closely side by side, and forming the main part of the roof of the olfactory chamber. Their posterior ends overlap the frontals, and the outer margin of each is in contact with the nasal process of the frontal, and with the maxillae and premaxillae.
Lying immediately ventral to the nasals, and on each side of the perpendicular mesethmoid, are the ethmoid or turbinal bones, which have a curious character, being formed of a number of delicate plates intimately folded on one another. The posterior pair of these bones, the ethmo-turbinals (fig. 72, 10), are the larger, and form a mass of intricately folded lamellae attached behind to the cribriform plate, and passing laterally into two thin plates of bone, which abut on the maxillae. The uppermost lamella of each ethmo-turbinal is larger than the others and more distinct. It is sometimes distinguished as the naso-turbinal, and forms an imperfect lower boundary to a canal, which is bounded above by the nasals. In front of and somewhat below the ethmo-turbinals, lie another pair of bones of similar character, the maxillo-turbinals (fig. 72, 11).
The last bone to be mentioned in connection with the olfactory capsules is a membrane bone, the vomer (fig. 72, 9). This is a slender vertically-placed bone, whose anterior part lies between the maxillo-turbinals, while behind it extends beyond the mesethmoid, so as to underlie the anterior part of the presphenoid. The anterior part of the vomer forms a kind of trough, while further back in the region of the ethmo-turbinals it sends out a pair of strong lateral plates, each of which, passing below the ethmo-turbinal, joins the side wall of the nasal cavity, and forms a partition dividing the nasal cavity into a lower narial passage and an upper olfactory chamber.
The Jaws.
In the embryo both upper and lower jaws are formed of cartilaginous bars, but in the adult not only has the cartilage entirely disappeared, but even cartilage bone is absent, the jaws being formed of membrane bone.
Fig. 75. Ventral view of the cranium of a Dog