Le premier rayon de la main et du pied chez les primates (II). Anatomie fonctionnelle

To study the articular amplitude of the first ray of both hands and feet using passive mobilisation, either on fresh human cadavers or on living primates after anesthesia (namely eight pongidae, 15 cercopithecidae, two platirrhinii, eight lemuroidae, three daubentonidae, two loridae and two galagonidae). Plans slightly parasagittal and parafrontal have been chosen to obtain the maximum dorsal and palmar or plantar inclinations and the maximum lateral and medial inclinations. The pronosupination of the first ray has been evaluated in relation of these plans and alike for the hand of the simian primates on the plan of the concavity and the convexity of trapezial surface. The results have been compared with these obtained in the man. The pronosupination of the first ray, so denominated by analogy with the pronosupination of the fore-arm is the movement which allows the pulp of the thumb or of the hallux to look now forward or now backward in order to be opposed at the other fingers during the pollici- or the hallucidigital pinch. It can be considered like “effective” unless if there is rotation around the longitudinal axis of its metacarpus (or metatarsus). The prosimian primates had great and sensibly similar circular amplitude of their first metacarpus and first metatarsus, which reached more than 90° in all the directions. It allowed a pronosupination of 90°, but this was only “apparent” because it has been obtained by the alone combined action of the two orthogonal axes at the basis of the ray by simple effect of cardan joint. The anthropoids had only limited circular amplitude, not exceeding 45°. It was associated with an “effective” pronosupination according to a longitudinal axis. The rotation reached 90°. It was produced around the point of anchorage made up by the medial ligaments for the trapezometacarpal articulation and the lateral ligaments for the cuneometatarsal. This movement was possible because of the peculiar relief of the articular surfaces: modified cardan for the trapezometacarpal and spiraled trochoid for the cuneometacarpal joint. The man was the sole primate, which had practically no mobility at the level of the first ray of his foot. However, the morphology of his cuneometatarsal articulation was similar to that of the other anthropomorphous primates. The muscles which was inserted on the first metacarpus (or metatarsus) played an active role in the circular amplitude and in the apparent circular amplitude, but very discreet in the effective circumduction. The transversal fascicle of the short abductor of the thumb and of the hallux had a little pronator effect. At the level of the foot, the tendon of the fibular longus had an action of pronation and the abductor longus muscle an action of supination. The metacarpophalangeal and the metatarsophalangeal articulations of all the primates were condylar with laterality movements, which were able to reach 40° and which were then unsteady. The metacarpopalangeal articulation of the man was the single exception. He was the sole one with a really steady thumb.