Page:The New International Encyclopædia 1st ed. v. 09.djvu/575

* HAND. 521 HAND. the leg. Then come the carpal bones, the meta- carpal bones, and the plialangcn, as we have tarsal bones, mrtalarsal lioncs. nml pliiilniuics in the foot. We have a diagram showing tlic way i" which the bones of the iiand are arranged. The carpal bones are eight in ininil)i'r. and are ar- ranged in the wrist in Iwo rows. The first or up- i-EPT HAND (dorsal surfare). ra., radius; iij., ulna; sea., Bcaphoid ; semi., semilunar; pisi., pisiform: cunei., cuneiform; trpzm., trapezium; /rpzd., trapezoid ; maf^., inagDWm ; WDri., unciform : mete, metacarpal; f.ptia., first phalange: s.ph.i., second pha- lange; t.pba., third phalange. per row consists practically of three bones, the scaphoid, semilunar, and unciform, the fourth, the pisiform, being regarded as belonging to the class of sesamoid bones (q.v.), and the second row of four bones, trapezium, trapezoid, os magnum, and unciform : so that, excluding the pisiform hone, the carpal and the tarsal bones correspond in number. As Ave eonunonly term the palm the front of the hand, the tluunb becomes convention- ally the outer, and the little linger the inner, digit; but according lo the rules of comparative anatomy, and in order to compare the hand and foot, we ought to reverse these terms. Tlic outer of the carpal bones of the first row supports the bones of the thumb and forefinger, and constitutes with them the outer division of the hand. The inner of the carpal bones bears the little and the next (the ring) finger, and con.stitutes with them the inner division of the hand: while the middle one bears the middle finger, and belongs to the middle division of the hand, llie two outer bones are connected vitli tlic radius, while the inner bone is connected (indirectly by a thick ligament) with the ulna. Collectively, the carpal bones are so arranged that the carpus presents a dorsal convex surface, upon which the tendons of the extensor muscles of the fingers play, and a palmar concave surface, on which the tendons of the fiexor muscles lie. The several bones; are joined to one another — each bone lieina united to three or more others — by a lai'ge extent of surface, and are girded to- gether by strong ligamentous bands. The wrist is thus as strong as if it had l)cen constructed of one solid piece of bone, while the sliglit gliding movements whieh occur between the several bones give it an elasticity which serves to break the shocks that result from falls upon the hand. The upiiermost surface of the first row of carpal bones is convex, and this convex surface is re- ceived into a wide eup or socket, formed by the lower articular surface of the radius, and by a ligaineiit passing from that bone to the ulna. The metacarpal bones and the phalanges require no special description. J. ike the great toe. the tinniib has only two phalanges, while each of the other digits has three. We shall now notice the various movements of which the hand is capable. They may be di- vided into ( 1 ) the difTerent directions in which the hand collectively can be moved, and (2) the movements of which the hand itself, without ref- erence to the arm, is capable. The scapula or shoulder-blade, with which the principal arm bone articulates, is itself movable to a very consider- able extent on the surface of the ribs on which it rests. Again, the socket in which the nearly .spherical head of the humerus or arm-bone lies is very shallow — not unlike the eup in the well- known toy cup-and-hall — and the arrangements of the shoulder-joint generally are such as to Iiermit so great a variety and so extensive a range of movements that we are able to apply the hand to every part of the body. This freedom of motion is due in a great degree to the clavicles or collar-bones, which, by steadying the shoulder- blades and ki'Cjiing the shoulders apart, ad'ord a fixed point for the various mu.seles which we em- the chest, in the act of hugging, etc. The move- ment at the ne.xt junction of bones, the elbow- joint, is very dift'erent from that at the shoulder. The latter is termed^ from its construction, a ball-and-socket joint, and admits of motion in all directions within definite limits; while the elbow is a hinge joint, and merely admits of bending and straightening, or, in other words, of motion in one plane. We have next to con- sider a class of movements of the forearm and hand, to which there is nothing analogous (at least to any material extent) in the leg. The movements in question are called 'pronation and supination.' In pronation (from Latin pronus, with the face downward), we turn the palm of the hand downward, as in picking up any sub- stance from the table; in supinalion (from Latin snpinus. with the face upward), we turn the palm upward, as for the (nirpose of receiving anything that may be |)lae*d in it. These movements of pronation and supinalion are so important to the usefulness of the hand (hat we' must notice the three muscles by whieli they are chiefly effected. One of the three muscles (lasses from a projecting pi'ocess on the inner side of the ann-bone, at its lower end, to the outer edge of the middle of the radius. Its contraction causes the radius to roll over, or in front of, the ulna. It thus pronates the hand, and is called a pronator muscle,. other muscle jiasses from a projecting process on the other side of the arm-bone to the inner edge of the radius near its upper part. It runs, therefore, in an opposite direction to the former muscle, and
 * iloy in raising the arms, in folding them over