A thermal pack (10) for therapeutic treatment of a mammal that includes a bladder (12, 4) and a separator element insert (22). The bladder (12, 14) is formed of a pair of superposed sheets (12, 14) of flexible, heat conductive liquid impermeable polyurethane material and includes an inlet (18) and an outlet (20). The sheets (12, 14) are welded together at a central spine (17) and along a series of branches (16) so as to provide a turning, tortuous fluid passageway therein. The separator element insert (22) is received within the fluid passageway and provides a fluid conveying track that extends throughout the fluid passageway from the inlet (18) to the outlet (19) to maintain the sheets (12, 14) in spaced apart relationship to mitigate restrictions in fluid flow when the pack (10) is flexed out of the plane of the sheets (12, 14). The separator element insert (22) includes a series of deflector ribs (36, 38, 40, 42, 44) along the fluid conveying track that deflect fluid flow away from the separator element insert (22) and into the fluid passage way along at each turn of the fluid passageway.