Histological septa are seen throughout most tissues of the body, particularly where they are needed to stiffen soft cellular tissue, and they also provide planes of ingress for small blood vessels. Because the dense collagen fibres of a septum usually extend out into the softer adjacent tissues, microscopic fibrous septa are less clearly defined than the macroscopic types of septa listed above. In rare instances, a septum is a cross-wall. Thus it divides a structure into smaller parts.
One of the radial calcareous plates of a coral. b. One of the transverse partitions dividing the shell of a mollusk, or of a rhizopod, into several chambers. c. One of the transverse partitions dividing the body cavity of an annelid.