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(1) In reference to vertebrates, including man, the word “trabecula” has several meanings. The trabeculae cra-nii, in the cranium of the embryo, are two cartilaginous bars anterior to the notochord, under the base of the forebrain. The part of the cranium that develops from the trabeculae is called prechordal, as distinct from the parachordal part of the cranium, which lies behind the end of the notochord and which bears traces of segmentation. The trabeculae occupy the orbital region of the cranium and lie horizontally or in flexed position, depending on the flexure of the rudimentary brain. As the cranium develops, the trabeculae fuse with each other and with the parachordal part of the cranium, forming its base. Anterior to the developing trabeculae are attached the cartilaginous nasal capsules. On the sides are the auditory vesicles.
The word “trabeculae” is also used to denote the septa that depart from the external capsule of connective tissue and project into the lymph nodes and spleen, forming the stroma of those organs; blood vessels pass through the trabeculae.
The word also denotes supporting structures in the spongy substance of the bone.
(2) In reference to invertebrates, the word is also used in several senses. In ascidians, the trabeculae are slender cords that unite the outer wall of the peribranchial cavity with the pharynx and provide support. In gastropods, they are folds that project into the kidneys. In arachnids, they are bands of connective tissue between the leaflike folds of the lung book; they prevent the leaves from collapsing and thus facilitate blood circulation.
(3) In plants, the trabeculae are incomplete transverse septa in the sporangia of certain plants, for example, the genera Pleuromeia, Lepidodendron, and Isoetes. The word is used to denote the elongated cells of the endoderm in the trunks of Selaginella; the trabeculae suspend the stele in the air cavity. In addition, trabeculae are transverse thickenings on the peristome teeth along the margins of the spore-bearing capsule of certain mosses.