The Vertebrate Skeleton

by Sidney H. Reynolds

The Skull includes

a. the cranium,

b. the jaws and visceral skeleton.

The cranium can be further subdivided into

(1) an axial portion, the cranium proper or brain case;

(2) the sense capsules. The capsules of the auditory and olfactory sense organs are always present, and as has been already mentioned, in many animals the eye likewise is included in a cartilaginous capsule.

(1) The cranium proper or brain case.

The cranium varies much in form and structure. In lower vertebrates, such as Sharks and Lampreys, it remains entirely cartilaginous and membranous, retaining throughout life much of the character of the embryonic rudiment of the cranium of higher forms. The dogfish's cranium, described on pp. 73 to 76, is a good instance of a cranium of this type. But in the majority of vertebrates the cartilage becomes more or less replaced by cartilage bone, while membrane bones are also largely developed and supplant the cartilage.

The cranium of most vertebrates includes a very large number of bones whose arrangement varies much, but one can distinguish a definite basicranial axis formed of the basi-occipital, basisphenoid, and presphenoid bones, which is a continuation forwards of the axis of the vertebral column. From the basicranial axis a wide arch arises, composed of a number of bones, which form the sides and roof of the brain-case These bones are arranged in such a manner that if both cartilage and membrane bones are included they can be divided into three rings or segments. The hinder one of these segments is the occipital, the middle the parietal, and the anterior one the frontal.

The occipital segment is formed of four cartilage bones, the basi-occipital below, two exoccipitals at the sides, and the supra-occipital above. The parietal segment is formed of the basisphenoid below, two alisphenoids at the sides and two membrane bones, the parietals above, and the frontal segment in like manner consists of the presphenoid below, the two orbitosphenoids at the sides, and two membrane bones, the frontals, above. The parietals and frontals, being membrane bones, are not comparable to the supra-occipital, in the way that the presphenoid and basisphenoid are to the basi-occipital.