This is a dense membrane not unlike the sclerotic coat or white of the eye. It is made up of a number of closely - interwoven strands of white fibres, and encloses within it the glandular or secreting structure of the organ (d, fig. 223).

At the superior border of the testicle an offshoot from this tunic dips down into its substance for a short distance, forming an incomplete vertical partition, which Sir Astley Cooper named the "mediastinum testis". In this ramify the small blood-vessels of the gland, and a net-work of seminal ducts termed the " rete testis" (f, fig. 223). There are also given off from it numerous fibrous cords, which, after dividing the gland up into a number of incompletely-separated compartments, become united with the interior of the tunic from which they spring. These strands of fibrous tissue preserve the shape of the gland, and serve as a matrix by which the blood-vessels traverse its structure.

The blood supply to the testis is derived from the spermatic artery (J, fig. 223), whose branches on entering the organ form a close net-work over the inner surface of the tunica albuginea, which is known as the "tunica vasculosa".