Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/381

Rh SKELETON.] MAMMALIA 359 rachi- bone% the radius and the ulna, placed side by side, articu- n and lating with the humerus at their proximal, and with the itibra- car p us a fc their distal extremity. In their primitive and unmodified condition these bones may be considered as placed one on each border of the limb, the radius being preaxial or anterior, and the ulna postaxial or posterior, when the distal or free end of the limb is directed outwards or away from the trunk. This is their position in the earliest embryonic condition, and is best illustrated in adult mammals in the Getacea, where the two bones are fixed side by side and parallel to each other. In the greater number of mammals the bones assume a very modified and adaptive position, usually crossing each other in the fore arm, the radius in front of the ulna, so that the preaxial bone (radius), though external (in the ordinary position of the limb) at the upper end, is internal at the lower end ; and the hand, being mainly fixed to the radius, also has its preaxial border internal. In the large majority of mammals the bones are fixed in this position, but in some few, as in Man, a free movement of crossing and uncrossing or pro- nation and supination, as it is termed is allowed between them, so that they can be placed in their primitive parallel condition, when the hand (which moves with the radius) is said to be supine, or they may be crossed, when the hand is said to be prone. In most mammals which walk on four limbs, and in which the hand is permanently prone, the ulna is much reduced in size, and the radius increased, especially at the upper end ; and the articular surface of the latter, instead of being confined to the external side of the trochlea of the humerus, extends all across its anterior surface, and the two bones, instead of being external and internal, are anterior and posterior. In many hoofed or ungulated mammals, and in Bats, the ulna is reduced to little more than its upper articular extremity, and firmly ankylosed to the radius, stability of these parts being more essential than mobility. iuus. The terminal segment of the anterior limb is the hand or manus. Its skeleton consists of three divisions : (1) the &quot; carpus,&quot; a group of small, more or less rounded or angular bones with flattened surfaces applied to one an other, and, though articulating by sy no vial joints, having scarcely any motion between them ; (2) the &quot; metacarpus,&quot; a series of elongated bones placed side by side, with their proximal ends articulating by almost im movable joints with the carpus ; (3) the &quot; phalanges &quot; or bones of the digits, usually three in num ber to each, articulating to one another by freely movable hinge- joints, the first being connected in like manner to the distal end of the corresponding metacarpal bone. rpus. To understand thoroughly the arrangement of the bones of the carpus in mammals, it is neces sary to study their condition FIG. in some of the lower vertebrates. Fig. 11 represents the manus in one of its most complete and at the same time most generalized forms, as seen in one of the Water Tortoises (Chelydra serpentina). The carpus consists of two principal rows of bones. The upper or proximal row contains three bones, to which Gegenbaur has applied the terms radiale (r), intermedium (i), and ulnare, the first being on the radial or preaxial side of the limb. The 11. Dorsal Surface erf the Kight Manus of a Water Tortoise (Chelydra serpentina). After Gegenbaur. U, ulna; R, radius; , ulnare ; t, intermedium ; r, radiale ; c, centrale ; 1-5, the five bones of the distal row of the carpus ; m -m 5, the fire metacarpais. mists Eadiale = Scaphoid Intermedium Lunar Ulnare = Cuneiform Centrale = Central Carpale 1 = Trapezium Carpale 2 = Trapezoid Carpale 3 *= Magnum Carpale it TT .,. Carpale 5 = Uncifo lower or distal row contains five bones, called carpale 1, 2, 3, 4, and 5 respectively, commencing on the radial side! Between these two rows, in the middle of the carpus, is a single bone, the centrale (c). In this very symmetrical carpus it will be observed that the radiale supports on its distal side two bones, carpale 1 and 2 ; the intermedium is in a lino with the centrale and carpale 3, which together form a median axis of the hand, while the idnare has also two bones articulated with its distal end, viz., carpale 4 and 5. Each of the carpals of the distal row supports a metacarpal. In the carpus of the Mammalia there are usually two additional bones developed in the tendons of the flexor muscles, one on each side of the carpus, which may be called the radial and ulnar sesamoid bones ; the latter is most constant and generally largest, and is commonly known as the pisiform bone. The fourth and fifth carpals of the distal rows are always united into a single bone, and the centrale is very often absent. As a general rule all the other bones are present and distinct, though it not unfre- quently happens that one or more may have coalesced to form a single bone, or may be altogether suppressed. The following table shows the principal names in use for the various carpal bones, those in the second column being the terms most generally employed by English anato- = Naviculare. Semilunare, Lunatum. = Triquctrum, Pyramidale. = Intermedium (Cuvier). = Multanguhim majus. &amp;lt;= Multangulum minus. Capitatum. =Hamatum, Uncinatum. The metacarpal bones, with the digits which they Meta- support, are never more than five in number, and are carpus described numerically first, second, &c., counting from a &quot; d the radial towards the ulnar side. The digits are also f an t&quot; es sometimes named (1) the pollex, (2) index, (3) medius, (4) annularis, (5) minimus. One or more may be in a rudi mentary condition, or altogether suppressed. If one is absent, it is most commonly the first. Excepting the Getacea, no mammals have more than three phalanges to each digit, but they may occasionally have fewer by suppression or ankylosis. The first or radial digit is an exception to the usual rule, one of its parts being con stantly absent, for, while each of the other digits has commonly a metacarpal and three phalanges, it has only three bones altogether ; whether the missing one is a metacarpal or one of the phalanges is a subject which has occasioned much discussion, and has not yet been satis factorily decided. The terminal phalanges of the digits are usually specially modified to support the nail, claw, or hoof, and are called &quot;ungual phalanges.&quot; In walking, some mammals (as the Bears) apply the whole of the lower surface of the carpus, metacarpus, and phalanges to the ground; to these the term &quot; plantigrade&quot; is applied. Many others (as nearly all the Ungidata) only rest on the last one or two phalanges of the toes, the first phalanx and the metacarpais being vertical and in a line with the fore-arm. These are called &quot;digitigrade.&quot; Intermediate conditions exist between these two forms, to which the terms &quot; phalangigrade &quot; (as the Camel) and &quot; subplantigrade,&quot; (as in most Carnivora) are applied. When the weight is borne entirely on the distal surface of the ungual phalanx, and the horny structures growing around it, as in the Horse, the mode of progression is called &quot; unguligrade. &quot; In the Chiroptera the digits are enormously elongated, and support a cutaneous expansion constituting the organ of flight. In the Cetacea the manus is formed into a