The mediastinum testis is a thick yet incomplete septum at the posterior part of the testis formed by the tunica albuginea of testis projecting into the testis at its posterior aspect where the testis is not lined by the serous membrane to allow for the attachment of the epididymis. It extends posteriorly between the testis' superior pole and inferior pole. It is wider superiorly than inferiorly. It supports the rete testis and blood and lymphatic vessels of the testis in their passage into and out of the substance of the gland.
The septa testis - extensions of the tunica albuginea into the substance of the testis that form fibrous partitions - converge towards the mediastinum testis.