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  Section: Anatomy of Vertebrate Animals » Organisation of the Vertebrata Skeleton
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The Osseous Brain-case


When the skull undergoes complete ossification, osseous matter is thrown down at not fewer than three points in the middle of its cartilaginous floor. The ossific deposit, nearest the occipital foramen, becomes the basioccipital bone; that which takes place in the floor of the pituitary fossa becomes the hasisphenoid; that which appears in the reunited trabeculae, in front of the fossa, gives rise to the presphenoid. Again, in front of, and outside, the cranial cavity, the ethmoid may be represented by one or more distinct ossifications.

An ossific centre may appear in the cartilage on each side of the occipital foramen, and give rise to the ex-occipital; and above it, to form the supra-occipital. The four occipital elements, uniting together more or less closely, compose the occipital segment of the skull.

In front of the auditory capsules and of the exit of the third division of the fifth nerve, a centre of ossification may appear on each side and give rise to the alisphenoid; which, normally, becomes united below with the basisphenoid.

In front of, or above, the exits of the optic nerves, the arbitosphenoidal ossifications may appear and unite below with the presphenoid.

In front of the occipital segment, the roof of the skull is formed by membrane; and the bones which complete the two segments of which the basisphenoid and presphenoid form the basal parts, are membrane bones, and are disposed in two pairs. The posterior are the parietals, the anterior the frontals; and the segments which they complete are respectively called parietal and frontal. Thus the walls of the cranial cavity in the typical ossified skull are divisible into three segments.
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I. Occipital, II. Parietal, III. Frontal - the parts of which are arranged with reference to one another, the sensory organs and the exits of the first, second, fifth, and tenth pairs of cranial nerves (I., II., V., and X.), in the manner shown in the diagram (The names of the purely membrane bones in this diagram are in large capitals, as PARIETAL; while those of the bones which are preformed in cartilage are in smaller type, as BASISPHENOID.) on the preceding page.

The cartilaginous cases of the organs of hearing, or the periotic capsules, are, as has been said, incorporated with the skull between the ex-occipitals and the alisphenoids - or, in other words, between the opcipital and the parietal segments of the skull. Each of them may have three principal ossifications of its own. The one in front is the prootic; the one behind and below, the opisthotic; and the one which lies above, and externally, the epiotic. The last is in especial relation with the posterior vertical semicircular canal; the first with the anterior vertical semicircular canal, between which, and the exit of the third division of the fifth nerve, it lies. These three ossifications may coalesce into one, as when they constitute the petrosal and mastoid parts of the temporal bone of human anatomy; or the epiotic, or the opisthotio, or both, may coalesce with the adjacent supra-occipital and ex-occipitals, leaving the prootio distinct.
The prootic is, in fact, one of the most constant bones of the skull in the lower Vertebrata, though it is commonly mistaken, on the one hand for the alisphenoid, and on the other for the entire petro-mastoid. Sometimes a fourth, pterotic ossification, is added to the three already mentioned. It lies on the upper and outer part of the ear-capsule between the prootic and the epiotic (see the figure of the cartilaginous cranium of the Pike, infra).

In some Vertebrata the base of the skull exhibits a long and distinct splint-like membrane bone (Bones may bo formed in two ways. They may be preceded by cartilage, and the ossiflo deposit in the place of the future bone may at first be deposited iu the matrix of that cartilage, or the ossific deposit may take place, from the first, in indifferent, or rudimentary connective, tissue. In this case the bone is not prefigured by cartilage. In the skulls of Elasmobraneh fishes, and in the sternum and epicoracoid of Lizards, the bony matter is simply ossified cartilage, or cartilage bone. The parietal or frontal bones, on the other hand are always devoid of cartilaginous rudiments, or, in other words, are membrane bones. In the higher Vertebrata the cartilage bones rarely, if ever, remain as such: but the primitive ossified cartilage bcoomea, in great measure absorbed and replaced by membrane bone, derived from the perichondrium.) - the parasphenoid, which underlies it from the basi-occipital to the pre-sphenoidal region. In ordinary fishes and Amphibia, this bone appears to replace the basisphenoid and presphenoid functionally, while in the higher Vertebrata it becomes confounded with the basisphenoid. The Vomer is a similar, splint-like, single or double, membrane bone, which, in like manner, underlies the ethmoid region of the skull.

In addition to the bones already mentioned, a prefrontal bone may be developed in the prefrontal region of the nasal capsule, and bound the exit of the olfactory nerve externally.

A postfrontal bone may appear behind the orbit above the alisphenoid. Sometimes it seems to be a mere dismemberment of that bone; but, in most cases, the bone so named is a distinct membrane bone.

Furthermore, on the outer and upper surface of the auditory capsule a membrane bone, the squamosal, is very commonly developed; and another pair of splint-bones, the nasals, cover the upper part of the ethmovomerine chambers, in which the olfactory organs are lodged.

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