In order to study the osseous structure and trabecular architecture of the carpometacarpal, metacarpal and interphalangeal joints of the thumb, seventeen thumbs were obtained from nine cadvers. The specimens were fixed with 10 per cent formalin washed in water for 24 hours dehydrated in a graded series of 70, 90 and 99 per cent ethyl alcohol for 24 hours at each concentration dried at room temperature for 4 hours, then embeded in polyester resin. The embeded specimens were cut into serial sections with a high speed cutter. The planes of cutting were five in the carpometacarpal joint, and two in the metacarpal and interphalangeal joints. The results were as follows: 1. At the maximum rectangular abduction of the carpometacarpal joint, the prominence of the metacarpal bone was locked with a hollow of the dorsal aspect of the trapezium. Thus, the joint was most stable in this position. 2. Great freedom of motion was obseved during abduction of the carpometacarpal joint, because there was no locking mechanism of the articular surface in the plane of abduction. However, flexion of the carpometacarpal joint was limited by the tubercle of the trapezium. 3. The metacarpal bone rotated in opposition with the thumb, because the carpometacarpal joint was not a complete saddle joint. The shape of the articular surface was not a concentric circle in the five cutting planes of the present study. 4. The metacarpophalageal joint was ellipsoid which showed three freedoms of motion. 5. The interphalageal joint was a hinge joint. 6. The trabecular orientation of the trapezium, metacarpal and phalageal bones was simpler than the weight bearing joints of the lower extremity. The trabeculae perpendicular to the joint surgface were crossed with the trabeculae oriented parallel to the articular surfaces. They crossed each other at right angles. These findings were considered to support the classical trajectorial theory.