Part 4

=The Frontal.= Location.—The frontal bones, or ossa frontales, two in number, form the forehead, a portion of the nose, and a portion of the roof of the orbital cavities. They are related posteriorly with the parietal, laterally with the temporal and zygomatic, and anteriorly with the nasal and premaxillary. They touch inferiorly the ethmoid.

Description.—Each of these bones has a processus orbitalis which is seen at the outer margin of the posterior and upper orbital wall and just in front of the zygomatic process of the squamous portion of the temporal. The frontal bone forms the anterior portion of the superior wall of the cranial cavity. The two bones are thin, flat, and meet at the median line of the forehead. The external surface is convex. The inner surface has a ridge located longitudinally and in the center the bone becomes narrow anteriorly.

=The Parietal.= Location.—The parietal bones, or ossa parietalia, two in number, form the posterior part of the roof of the cranial cavity. They meet in the median line and are related posteriorly with the occipital, anteriorly with the frontal, and laterally with the temporal bones.

Description.—The parietal bones are short and very broad (Fig. 5, Part II, No. 8). Each bone is quadrilateral in outline and has two surfaces, four borders and four angles. The external parietal surface is convex and smooth and the internal, cerebral surface is concave.

=The Temporal.= Location.—The temporal bones, or ossa temporales, two in number, are located at the lateral portion of the cranium, and aid in the formation of the cranial wall. The temporal bone is related posteriorly with the occipital, superiorly with the parietal and frontal, externally with the quadratus, anteriorly with the ethmoid, and inferiorly with the sphenoid.

Description.—The temporal bones consist of the fused squamous and petrous temporals; they contain the essential organs of hearing.

FIG. 8.—Inferior view of the skull of a hen. 1, Occipital. 2, Foramen magnum. 3, Occipital condyle and just below the basi-occipital. 4, Foramen for the passage of the hypoglossal nerve. 5, Foramen for the passage of the vagus. 6, Canalis caroticus and jugulare. 7, Sphenoid. 8, Temporal wings of the sphenoid. 9, Foramen auditiva. 10, Os articulare. 11, Os angulare. 12, Vomer. 13, Os dentale. 14, Pterygoid. ]

The squamous portion of the temporal bone possesses the long thin zygomatic process sometimes called the posterior orbitalis. It forms a small flattened tongue, directed forward, sometimes free, and at other times united by its superior border to the summit of the orbital process. This is especially true in the turkey. This process is seen near the lower outer portion of the posterior orbital wall. The squamous portion is also provided antero-laterally with an articular facet which articulates with the quadrate bone. The large portion of the temporal bone lies on the side of the cranium superior to the ala sphenoida temporale. It extends outward and anteriorly over the rims of the petrosum and ala sphenoida orbitale. The squamous part forms the upper three-fourths of the cochlea, the inner auditory canal, the upper part of the fenestra ovalis, the anterior vertical and the outer semicircular canal, and the lower part of the posterior vertical semicircular canal.

The petrous portion forms the posterior wall of the foramen ovale and the fenestra ovalis in which lies the columella. The fenestra ovalis and the fenestra rotunda are the only two entrances into the labyrinth. In this region may be seen the fusion line between the os petrosum and the os occipitale. Superiorly and posteriorly the petrous portion touches the external parietalia and occipitalis; infero-laterally it unites with the basi-sphenoid. The outer rim of the foramen ovale is broadened by the ala sphenoida and mesially by the basi-sphenoid. This foramen gives exit to the second and third divisions of the fifth pair of cranial nerves.

The lateral surface of the temporal bone presents a short tube, the external acoustic process, or processus acusticus externus, to which is attached the concha of the ear. The process is directed outward. Its lumen, the external acoustic meatus, or meatus acusticus externus, conducts to the cavity of the middle ear in the bare skull, but is separated from it by the tympanic membrane in the natural state.

BONES OF THE FACE

The bones of the face, or ossa faciei, are the premaxillary, or upper mandible, the maxillæ, the nasal, the lacrimal, the palatine, the pterygoid, the zygomatic, the vomeral, the jugal and the quadrato-jugal. The vomer is single, the others are paired. In the inferior maxillary group there are the quadrati, and the inferior maxillary, or lower mandible. The turbinated and hyoid bones are also discussed with the bones of the face.

The beak consists of the anterior portion of the upper and lower mandibles which are covered with a horny structure.

=The Premaxilla.= Location.—The premaxillæ, or ossa incisiva, or mandibular structures, are located in the extreme anterior facial region.

Description.—The premaxillæ are long and the anterior end is pointed. Each consist of two lateral halves which become fused before the chick is hatched. They partly circumscribe the openings into the nose. This bone is the base of the upper portion of the beak and determines its form. It forms the anterior walls of the nasal cavity. It has between the nasal bones, two processes which extend back to the anterior point of the cerebral cavity. The posterior part of the incisivum and nasale are flat, thin, and elastic. The extensions then are the processus maxillaris, the processus palatinus, and the processus frontalis, the first of which forms part of the jaw rim, the second, which aids in forming the gum plate, and the last which reaches to the anterior portion of the cranial wall.

=The Maxilla.= Location.—The superior maxillaries, or ossa maxillares, two in number, form the floor of the upper beak, a part of the palatine roof and nasal walls.

Each maxilla borders laterally the premaxilla and the nasal; inferiorly, the anterior point of the palatine; and posteriorly, the jugale.

Description.—The maxillæ are thin, flat, bones. They have three borders and three angles. The palatine processes of the two bones do not meet in the median line, which results in a cleft in the median palatine region. The bone-like gums are formed partly by the palatine processes but more largely by lateral wings of the os incisivum. Each maxilla has posteriorly a yoke-like extension, superiorly a small extension, and also a palatine process. The latter articulates by a facet with the vomer.

=The Nasal.= Location.—The nasal bones, or ossa nasalia, two in number, are located in the lateral facial region. The nasal bone articulates posteriorly with the frontal; laterally, on the inner border, with the processus frontalis of the premaxilla, and on the outer border with the lacrimal; and inferiorly with the premaxilla, maxilla, jugale, and vomer.

Description.—The nasal bone, or os nasale, is broad, flat, and elastic and forms the posterior wall of the nasal opening. Under this bone is located the infraorbital sinus.

There are three extensions: first, the processus intermaxillaris, which forms the upper wall of the nasal cavity; second, the processus maxillaris anterior, directed downward and forward, which fuses with the maxillary bone and forms the posterior rim of the nasal cavity; and third, a posterior processus frontalis, which lies parallel with the ethmoid.

=The Lacrimal.= Location.—The lacrimal bones, or ossa lacrimalia, two in number, are located at the outer border and at the junction of the processus frontalis of the nasal, with the frontal bone.

Description.—The lacrimal bone is small, and rather filiform. They become fused with the nasal and the frontal bones, forming part of the margin of each.

=The Palatine.=—Location. The palatine bones, or ossa palatina, two in number, enter into the formation of the inner part of the bony gum and hard palate, or roof of the mouth. They form the support for the hard palate. Each palatine articulates posteriorly with the rostrum, or nasal portion of the sphenoid, and with the pterygoid; and anteriorly with the maxilla.

Description.—The palatine bone is curled posteriorly and is thin at the upper portion and thick at the lower border. Anteriorly it has a long rather filiform projection.

=The Pterygoid.= Location.—The pterygoid bones, or ossa pterygoidea, two in number, are located back of the region of the posterior nares. They extend diagonally outward and backward from the median region of the sphenoid rostrum to the quadrate bone. They articulate anteriorly with the sphenoid rostrum and the palatine, and posteriorly with the quadrate.

Description.—The pterygoid bones are slender and cylindrical, and are expanded at the ends into an articular facet.

=The Zygomatic.= Location.—The zygomatic, or ossa zygomatica, two in number are situated below the orbital cavity and extend from the maxilla to the quadrate bone.