The neural arch of the parietal vertebra is formed by the alisphenoids (abe majores, sph. bone), (neurapophyses) and parietals (divided neural spine). The former are developed from points of ossification distinct from the basi-sphenoid and from the pterygoid processes, which are typical appendages of the nasal or the last cranial vertebra. At the time of birth these different elements are united by cartilage only. The divided spine is represented by the broad and concave parietals.
There is no part of a vertebra which undergoes ereater varieties in shape and size than the neural spine; it readily adapts itself to the highly-developed encephalon, and forms by far the greater part of the arch attached to the cranial vertebras. The permanent separation of the two halves of the parietal spine is seen in fish, where the supra-occipital is prolonged forwards to articulate with the frontal bone. In the crocodile, the neural spines of the atlas, occipital, and parietal vertebras are single flat bones, succeeding one another in linear series. In birds, and in the lower mammalia, the division between the two parietals, or the sagittal suture, becomes soon obliterated, and there results a single convex bone, which, however, always articulates by its extremities with the alisphenoids by a prolonged portion called the anterior inferior angle. The parietal parapo-physis, or transverse process, is the mastoid bone, which in man forms part of that highly-complicated piece known as the temporal bone. It gives attachment by its extremity to the cranial prolongation of a muscle, connected to the diapophyses of all the trunk vertebras, and known in its several regions by the names of longissimus dorsi, trans-versalis colli, and trachelo-mastoid. In the sheep it forms a strong outstanding process, wedged in between the paroccipitals and the tympanic bones. The presphenoid sends outwards the two thin and flat neurapophyses, called in human anatomy the processes of Ingrassias (lesser wings of the sphenoid bone). The neural arch is completed by the frontal bone (neural spine), which in many lower animals, and occasionally in man, is formed of two separate halves united by a suture, continued forwards from the sagittal suture.
The neural arch of the nasal vertebra in man is obliterated, not from want of development of the neurapophyses, but from their being pressed together to form part of the septum nasi. They send outwards and forwards, however, two thin shells (ossa plana), which inclose the olfactory sense-capsule (or the cells of the ethmoid bone), and are prolonged upwards to articulate with the nasal bones, their proper neural spine.
The temporal bone, in man, is inserted between the occipital and parietal vertebras, and forms an important ingredient in the construction of the cranial walls. It is formed by the confluence of five distinct bones. 1. Squamosal; 2. The mastoid ; 3. The stylo-hyal; 4. The tympanic, or external auditory process; and, 5. the auditory sense capsule, or the petrosal bone. The special homologies of these different pieces have been already explained. All that it is necessary to mention here, consists in the fact that with the enlargement of the cranial neural arches, and their separation one from another, one or more of the abovenamed bones are introduced to fill up the vacuum. In the sheep no part of the temporal bone, except the petrosal sense capsule, is seen in the interior of the cranium. In many of the monkey tribe the squamosal likewise is intercalated, and extends so far forwards as nearly to separate the alisphenoids from their spine (parietal bone). In man the petrosal and squamosal bones are united to the mastoid, which is hollowed out to receive the lateral sinus, and the three form a large and irregular surface, extending from the expanded parietals to the occipital and the post-sphenoidal centra.