Building blocks

Abstract

One of the major requirements in the evolution of the spine has been flexibility without weakness. One method of imparting flexibility is by segmentation. In fish the spine is segmented thus enabling side to side movements to take place. These range from tail movements alone in some species to lateral undulations which progress along the whole animal from front to rear in others. The vertebral column of the fish can be regarded as a series of rigid units hinged to each other by surfaces that allow the body to bend only sideways. The centra of the vertebrae of most fish are concave at both ends (this arrangement also exists in less-specialized urodeles and in primitive living reptiles). In bony fish a neural arch and neural spine are associated with each vertebral centrum (Figure 7.1). In the tail region there is also a haemal arch and haemal spine. Centra occupy the position immediately beneath the neural tube. During embryonic life the notochord occupied this position before it developed into a chain of ossified elements connected by intervening deformable discs.