Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 VERTEBRAL COLUMN. Ill (lured l>y alterations in tlie notochordal sheath as well as in the surrounding skeletogenous sheath (rig. 570, a). The latter gives rise to cartilaginous or bony rings, which • represent the first rudiments of the vertebral bodies. These rings constrict the notochord till they assume the form of biconcave car- tilaginous or bony discs, and become connected with cartilaginous or bony arches which are developed round the spinal cord and the perivisceral cavity (ti
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 VERTEBRAL COLUMN. Ill (lured l>y alterations in tlie notochordal sheath as well as in the surrounding skeletogenous sheath (rig. 570, a). The latter gives rise to cartilaginous or bony rings, which • represent the first rudiments of the vertebral bodies. These rings constrict the notochord till they assume the form of biconcave car- tilaginous or bony discs, and become connected with cartilaginous or bony arches which are developed round the spinal cord and the perivisceral cavity (tig. 570 a, f>). Each vertebra therefbre consists of a principal median portion, the body of the vertebra or centrum, which frequently retains the re- mains of the notochord in its axis; of a dorsal or neural arch, and a ventral or ha?mal arch. The two limbs of the dorsal arch are called neurapophyses, those of the ventral arch hsemapophyses, and the unpaired median prolongation of each arch is known as the spinous process (tig 570, J), D'}. The trans- verse processes (pleurapophyses) which arise from different parts of the vertebra*, either from the neural arches or from the centra, are not independent structures but merely processes. The ribs, on the other hand, are independent lateral bony or cartilaginous rods which are attached either to the ha-mapophyses (fishes) or to the pleurapophyses, and embrace the part of the body cavity containing the viscera. Regions of the vertebral column.—In the higher Vertebrates the primitive homonomous si'-mentation of the skeleton gives place to a heteronomous segmentation which leads to the origin of a number of regions. In this point as in others there is a parallel between the segmented Invertebrates and Vertebrates. In the first place an anterior region or head can always be distinguished from the posterior uniformly segmented region or trunk (fig. 571); and this division corresponds with the enlargement of the anterior part of th
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