Verrucae are the mind control devices Dr. Ichigaki uses to control his fighters. They are called Blood Slaver Nodes in the Viz adaptation.
Synopsis
Dr. Ichigaki created the Verrucae and surgically attached them to the backs of the three students of Mitamura he decieved into volunteering for the procedure. Yen, Ryo, and Kai were unaware of the controlling powers of the Verruca until it was too late and had their free will sapped as a result. The Verrucae functioned as second hearts and brains to those on whom they were implanted. Removing them would kill the wearer.
Dark Tournament Saga
During their fight with Team Urameshi, it is revealed that the Verrucae infiltrate the vascular system, using the hosts blood to regulate the spirit energy and actions of that person. Genkai utilizes her Spirit Wave: Fist of Cleansing technique to purify the bodies of the three afflicted members of the Dr. Ichigaki Team, with the verrucae being detached as a result. Yen, Ryo, and Kai survived the attack because of their uncorrupted souls, while the Verrucae were destroyed because of their impure nature.