The mouse "ribcage" was one of the key innovations in the Hovey-Kelley mouse: it held the precision parts together securely, could be manufactured inexpensively, and its design facilitated assembly. One of the optical encoder shafts fits in the section to the right of the large ball (a housing for the ball); the second encoder shaft sits behind the housing. The design of the ribcage is discussed by its creator, Jim Yurchenco, in an interview.
A more formal drawing was prepared for the mouse patent. The view in the drawing below is turned 90 degrees clockwise from the drawing above.