Fig. 73. Dorsal view of the cranium of a Dog

(Canis familiaris) × 2/3.

1. supra-occipital.10. postorbital process of frontal.
2. parietal.11. infra-orbital foramen.
3. frontal.12. anterior palatine foramen.
4. nasal.13. lachrymal foramen.
5. maxillae (facial portion).i 1. first incisor.
6. premaxillae.c. canine.
7. squamosal.pm 4. fourth premolar.
8. jugal.

The cranial cavity is continuous in front with the nasal or olfactory cavities, but the passage is partially closed by a screen of bone, the cribriform plate (fig. 72, 5), which is placed obliquely across the anterior end of the cranial cavity, and is perforated by a number of holes through which the olfactory nerves pass. The plane of the cribriform plate is called the ethmoidal plane, and as was the case also with the occipital plane, the angle that it makes with the basicranial axis varies much in different mammals, and is of importance. The olfactory fossa in which lie the olfactory lobes of the brain, is partially separated from the cerebral fossa, or cavity occupied by the cerebral hemispheres, by ridges on the orbitosphenoids and frontals. The presphenoid is connected in front with a vertical plate formed partly of bone, partly of unossified cartilage; this plate, the mesethmoid (fig. 72, 7), separates the two olfactory cavities which lodge the olfactory organs. Its anterior end always remains unossified, and forms the septal cartilage of the nose.

The brain case may then, to use the words of Sir W.H. Flower, be described as a tube dilated in the middle and composed of three bony rings or segments, with an aperture at each end, and a fissure or space at the sides between each of them.

2. The Sense capsules.

Each of the three special sense organs, of hearing, of sight, and of smell, is in the embryo provided with a cartilaginous or membranous protecting capsule; and two of these, the auditory and olfactory capsules, become afterwards more or less ossified, and intimately related to the cranium proper.

(1) Bones in relation to the Auditory capsules.

These bones lie on each side wedged into the vacuity between the lateral parts of the occipital and parietal segments; they are three in number, the periotic, the tympanic and the squamosal.