The lateral portions of the body of the fornix are joined by a thin triangular lamina, named the psalterium (lyra). This lamina contains some transverse fibers which connect the two hippocampi across the middle line and constitute the commissure of fornix (hippocampal commissure).
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The terminal lamina creates the commisure plate. This structure gives existence to the corpus callosum, the septum pellucidum and the fornix. The latter will split into two columnae fornicis (anterior), and will split into two crus fornicis (posterior). These two crus are joined together through the commissura hippocampalis. The beginning of the splitting is called the psalterium or Lyra Davidis. The latter name is used, because the structure resembles a lyra (or triangular harp): The two crus are the "chassis" of the lyra, and the commisure connections are the fibers.