Tenia of fornix

The tenia of fornix (taenia fornicis) is simply a line of attachment on the lateral edge of the fornix.

In the body of the lateral ventricle, the tela choroidea—the thin vascular membrane that carries the choroid plexus—attaches along this line. The taenia fornicis therefore forms the medial edge of the choroidal fissure.

A useful way to picture the arrangement from medial → lateral is: Fornix → taenia fornicis → choroidal fissure containing the tela/choroid plexus → taenia choroidea → thalamus

So, there are two attachment lines around the choroidal fissure: taenia fornicis on the fornix side and taenia choroidea on the thalamic side.

As the lateral ventricle curves into the temporal horn, the fornix continues as the fimbria of the hippocampus. Here, the corresponding attachment line is called the taenia fimbriae.

In one sentence: The taenia fornicis is the line along the fornix where the tela choroidea attaches, forming the medial boundary of the choroidal fissure.