Of these three vertebrae, the first is much the largest, and is firmly united to the ilium on each side by a structure formed by the transverse processes and expanded ribs. In the adult this structure forms one continuous mass, but in the young animal a ventral portion formed by the rib is clearly distinguishable from a dorsal portion formed by the transverse process. All three have low neural spines. The anterior sacral vertebra bears a large pair of prezygapophyses, while the posterior one bears a small pair of postzygapophyses. THE CAUDAL VERTEBRAE. The =caudal vertebrae= are about nineteen in number. The earlier ones have well-developed neural arches, transverse processes, and zygapophyses, but as the vertebrae are followed back they gradually lose all their processes, and the neural arch as well, becoming at about the thirteenth from the end reduced to simple cylindrical centra. B. THE SKULL. 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. 1. THE CRANIUM PROPER OR BRAIN CASE. Taking the membrane and cartilage bones together, they are seen to be more or less arranged in three segments, which however must not be regarded as homologous with the segments forming the vertebral column. [Illustration FIG. 71. DIAGRAM OF THE RELATIONS OF THE PRINCIPAL BONES IN THE MAMMALIAN SKULL (modified after FLOWER). Cartilage is dotted. Cartilage bones are marked by dots and dashes, membrane bones are left white.