Each becomes horizontal near its caudal end and at the same time expands. The expanded portion lies ventrad of the labyrinth of the ethmoid, closing in some of its cells: its lateral angles are united with the labyrinths.
At its caudal end the bone articulates with the body of the presphenoid, and each half of it is produced caudad near the middle line into a triangular spine which lies ventrad of the body of the presphenoid. The horizontal portion of the bone helps to separate the olfactory and respiratory passages of the nasal chamber, while its vertical portion contributes to the formation of the nasal septum.
The ventral margin formed by the junction of the two halves of the bone is smooth and free caudad, but at its cranial end is broad and rough for articulation with the palatal processes of the maxillæ.
Palatine Bone. Os palatinum
([Fig. 33]).—The palate bone or palatine bone consists of two portions, a horizontal or palatal portion (a) and a perpendicular or nasal portion (b), uniting at an angle of about forty-five degrees.
[Fig. 33].—Palatine Bone, Dorsal View.
![[Fig. 33]](#3418956513767812439_illo045lg.jpg.id-2549579370764667095.wrap-0.html.html){kind=link}
a, horizontal portion; b, perpendicular portion; c, maxillary spine; d, posterior nasal spine; e, sphenopalatine foramen; f, caudal opening of posterior palatine canal.
The horizontal portions (a) of the two bones are received between the maxillary bones and form the caudal and medial part of the roof of the mouth. Each is irregularly quadrilateral in form, with the caudolateral angle produced caudad into a long process which is continuous with the perpendicular portion of the bone. The lateral margin of the horizontal portion articulates over its cranial half with the maxillary bone. At about its middle a short thick maxillary spine (c) projects caudolaterad. The remainder of the lateral margin is directly continuous with the perpendicular plate of the bone. The medial margin is rough for articulation with the corresponding margin of the opposite palatine; the caudal angle of this margin projects caudad as the short posterior nasal spine (d). The caudal margin forms a free edge which bounds the choanæ; it passes laterally into the perpendicular portion.
The ventral surface ([Fig. 41], 8) looks into the mouth. Near the middle of its craniolateral margin are two or more small foramina ([Fig. 41], q) which form the cranial termination of the posterior palatine canal. The dorsal surface is smooth and looks into the nasal cavity.