The skull consists of the following three parts: (a) the cranium, with which are included the skeletal supports of the various special sense organs, and the bones of the face and upper jaw; (b) the lower jaw or mandible, which is movably articulated to the cranium, and (c) the hyoid.
(a) The Cranium.
The cranium is a compact bony box, forming the anterior expanded portion of the axial skeleton. It has a longitudinal axis, the craniofacial axis around which the various parts are arranged, and this axis is a direct continuation of that of the vertebral column. Similarly the cavity of the cranium is a direct continuation of the spinal canal. The posterior part of the craniofacial axis, which has relations only with the cranium, is called the basicranial axis.
In the dog as in the other types previously described, the skull in its earliest stages is cartilaginous, containing no bone. In the adult, however, the cartilage is to a great extent replaced by bone, and in addition to this cartilage bone, membrane bone is largely developed, and intimately united with the cartilage bone to form one complete whole.
In the description of the dog's skeleton, as in those of the previous types, the names of the membrane bones are printed in italics, while those of the cartilage bones are printed in thick type.
Most of the numerous foramina perforating the skull walls will be described after the bones have been dealt with.
For purposes of description the cranium may be further subdivided into:—
1. The cranium proper or brain case.
2. The sense capsules.
3. The upper jaw.