Movements of the Thumb
In its adaptation to the purposes of bearing the weight and ministering to the locomotion of the body the human foot excels that of any other animal; and, unquestionably, the human hand is not less preeminently distinguished by the nicety, the variety, and the freedom of its movements. This is shown by the manner in which it can be twisted about, by the exquisite play of the fingers; and it is shown, above all, by the mode in which the thumb can be moved to and fro, can be opposed to the other fingers, and to any part of them individually and collectively, and can be folded beneath them or clasped upon them as occasion may require.
The power which the thumb possesses, and gives to the hand, is signified by its name—“pollex,”—derived from the Latin word pollere, which means to have power. Some have supposed that the word “poltroon” is derived from pollice truncato, and signifies one so cowardly that he has submitted to have his thumb cut off in order that he may be incapacitated for fighting.
The faculty which we possess of moving the thumb in the way I have mentioned, athwart the other fingers, and of touching any part of the palmar surface of either of them depends, partly, upon its being set, not quite in the same plane with them, but, obliquely, so that when it is moved towards the palm it faces or opposes the other fingers; and, partly, upon the nature of the joint between its metacarpal bone and the bone of the wrist with which it is connected.
This joint is so constructed as to admit of three different movements. First, the thumb can be moved forwards or backwards, that is, towards, or, away from, the palm. Secondly, it can be “adducted” or “abducted,” that is, approximated to the forefinger or inclined away from it. Thirdly, it can be “circumducted,” that is, its extremity can be made to describe a circle, as in “twiddling the thumbs.” These several movements are effected with great power and rapidity by means of the bundle of muscles which forms the “ball of the thumb” (fig. 65. D), as well as by the long muscles and tendons which descend, from the forearm, to the thumb.