Edward Hallarran Bennett: Bennett's fracture of the base of the thumb.

Bennett's fracture involves part of the base of the metacarpal bone of the thumb. The fracture line runs obliquely and involves the joint between the first metacarpal and the adjacent carpal bone--the trapezium. The shaft of the metacarpal is pulled proximally by the action of the long muscles attached to the thumb, so a fracture-dislocation of the thumb carpo-metacarpal joint results. The injury occurs typically in young men as a result of a blow on the clenched fist, either in a fall or, very often, in landing a blow in boxing. Now this joint happens to be of extreme importance in fine movements of the thumb, because it is here that opposition of the thumb takes place. Look at your own hand; now put the tip of your thumb onto the tip of your index finger; this is opposition and only man and monkeys can carry out this action. It is this important movement that you use when writing, when holding a scissors or a scalpel. When you look at the two bones articulating together they resemble someone sitting on a saddle and the joint, the only one of its kind in your limbs, is appropriately called a 'saddle joint'.