Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Five The Skull 619
C A LVA R I U M
The calvarium comprises the frontal, temporal, parietal, and occipital bones as well as the
bones of the base of the skull. In dogs, the shape of the calvarium and the appearance of
specific structures vary with the breed. 13 In some breeds (e.g., Pit Bull), the dorsal part of
the skull may be remarkably thick, up to several centimeters. In other breeds (e.g.,
Chihuahua), the dorsal part may be very thin and the entire skull appears dome shaped.
The sutures may be apparent in very young animals. In some toy breeds, these remain open
throughout the animal’s life. Normally there are irregularities in the bony density of the
calvarium. These irregularities are caused by the juga, which are bony protrusions of
the skull into the spaces (sulci) between the gyri of the brain. Vascular channels, appearing
as straight or branching, relatively radiolucent lines with fine, sclerotic, radiodense borders,
may be visible on the lateral view of the calvarium.
The occipital crest is the dorsal-most portion of occipital bone and is seen on the lateral
view as a bony projection extending caudally, dorsal to the first cervical vertebra. The
size of this projection varies among breeds. The occipital condyles are in the midventral
portion of the occipital bone and are seen routinely on both the ventrodorsal and lateral
views as smooth rounded projections extending caudally from the occipital bone. Their
articular surfaces should appear smooth and regular. The foramen magnum is centered
between the occipital condyles and is oval or triangular in shape, with smooth and regular
borders. The foramen magnum is seen best on the modified occipital or open-mouth
frontal views. On the modified occipital view, the impression in the skull for the vermis of
the cerebellum may be seen as a slightly less-dense part of the skull projecting dorsally from
the dorsal aspect of the foramen magnum.
B A S E O F T H E S K U L L , M I D D L E E A R S , T E M P O R O M A N D I B U L A R J O I N T S
With the exceptions of the jugular processes, osseous bullae, and temporomandibular
joints, the remaining structures of the base of the skull are difficult to assess radiographically.
The jugular processes are small, smooth-bordered, triangular bony projections of the
occipital bone extending laterally and caudally from each side of the skull just caudal to the
osseous bullae. On the ventrodorsal view, the osseous bullae are normally round, thinwalled,
air-filled bony structures; the air-filled external ear canals are seen extending laterally
from the osseous bullae. The lateral oblique views of the osseous bullae reveal smooth,
thin-walled, air-containing bony structures. Immediately dorsal, medial, and rostral to the
osseous bullae are the petrous temporal bones. These are identified easily by their extreme
bony density. The temporomandibular joints are rostral to the osseous bullae. They consist
of the condyloid processes of the mandibles and the mandibular fossa of the temporal bone
near the base of the zygomatic arch. The temporomandibular joints are seen on the
dorsoventral view as a thin radiolucent line between the wide-based triangular mandibular
condyloid processes and the matching shape of the articular surface of the zygomatic
bone. The surfaces are smooth and regular. Only a small portion of the mandibular condyle
should extend rostral to the mandibular fossa. On the lateral and lateral oblique views, the
articulation appears as a crescent-shaped radiolucency between the round to oval-shaped
mandibular condyloid processes and the semilunar cup shape of the articular portion of
the mandibular fossa, with the retroarticular process forming its caudal border. These
articular surfaces should be smooth and regular.
The zygomatic arches are laterally directed arches of bone that arise from the base of
the skull at the temporal bone. The zygomatic arches extend cranially to meet the zygomatic
bones, which fuse rostrally with the maxillae. The zygomatic arches are made up of
two bones, the zygomatic bone rostrally and the zygomatic process of the temporal bone
caudally. These two bones join in the middle of the zygomatic arch, with a long, slightly
oblique suture that has the zygomatic bone extending dorsal and rostral to the zygomatic
portion of the temporal bone. This suture usually is apparent in young animals as a radiolucent
line but it rarely is apparent in the adult.
M A N D I B L E
The mandible consists of right and left portions that are united rostrally at the mandibular
symphysis. Each one half of the mandible may be divided into two major portions, the