He describes the wind as the "dark vapors of the earth exhaling," thus indicating that the sound of the wind somehow brings him the essence of nature: at the time, this sound may have been his only indication that the world outside his cell continued. The sound of the wind rustling through the leaves likewise puts him in touch with fellow countrymen who have died. He hears the "voices of our dead" in the rustling noise because the dead have fertilized the soil and thus made possible the life and growth of trees. This wind also brushes the hairs on Soyinka's skin. Since he has had so little physical or mental stimulation for such a long time, this wind on his skin revives all the "spaces where dead memories are laid." In other words, all the memories that darkness and isolation have allowed him to forget, this simple gust of wind and touch of cobweb stimulate into existence.
Soyinka seems to emphasize the lightness of the cobweb by placing "A thread" on a line of its own; this lightness is contrassted by the heavy effect it has upon him. Indeed, the web finds its way to another web -- it touches the "heart" of Soyinka's being and in so doing affects the whole web of emotions connected to his heart. This "ancestral web" probably branches out to include the dead people whose voices Soyinka can head through the wind. Thus, the cobnweb branches out from its starting point -- Soyinka -- to include all of those who have died for Nigeria. The adjective "ancestral" hence serves to reinfoce the web imagery.