EXTREMITY, (in human anatomy), mem brum, artus ; Gr. 'Amos, xotor; Fr. ex trenzia, menibre ; Germ. Gliedmasscn ; Ital. membro. This term is used to denote certain appendages most manifest in the vertebrated classes of animals, employed as instruments of prehen sion, or support, or motion, also occasionally employed for other purposes sufficiently in dicated by the habits of the animal. In fa miliar language we apply the word, limb, synonymously, and the superior and inferior limbs of man, or the anterior and posterior ones of the Mammiferous Quadrupeds, are the best examples by which we can illustrate our de.. finitioe. When these appendages exist in their complete number, i. e. four, they are distin guished either by the appellatives already mentioned, anterior and posterior, or superior and inferior, or more precisely pectoral, and pelvic or ventral, or again atlantal and sacral.
In Fishes we find that in most instances the anterior limbs (pectoral fins) are larger than the posterior (ventral fins): and sometimes the posterior are absent altogether, as in the com mon eel. In Fishes we look for the simplest form of the skeleton of the more highly de veloped limbs in Man and Mammalia : and here we find, more or less obviously in dtflbr cut instances, the same elements which sub sequently appear in a more distinct and com plete form. Thus, in the case of the Lophius piseatorius, we find very distinctly the scapula and clavicle forming the bond of connection of the other bones of the limb to the trunk. We can also recognize the radius and ulna, what seems to be a very rudimentary humerus, and the bones of the carpus, as well as the phalanges, which generally greatly exceed in number any arrangement that is to be found in the higher classes. The ventral fins, how ever, the analogues of the posterior extremities, are not so developed: while bones analogous to the phalanges of the feet are found in it, we meet no trace of the femur, tibia, or fibula.
In all the other Vertebrata we find the an terior and posterior extremities developed on a plan similar to that in man, with such vari ations as the manner of life of the animal requires. We must, however, notice an excep tion in the case of serpents and Cetacea. In the former there are no limbs, or at least the merest trace of them; in the latter the pos terior are absent, although the anterior exhibit very perfectly all the elements of the human upper extremity.
We propose to devote the present article to the detail of the descriptive anatomy of the osseous system of the extremities in Man, in whom, by reason of his erect attitude, the terms superior and inferior are substituted for anterior and posterior, as applied to the ex tremities of the lower animals.
Superior extremity.—The superior extremity is connected to the trunk through the medium of two bones, which, as being intimately con nected with the motions of the limb, first de mand attention. These bones are the clavicle and scapula, and are commonly called the bones of the shoulder.
Clavicle (from clavis, a key;) collarbone; syn. ligula, jugulam, os furcale; Germ. Schlus selbein. This bone is situated at the upper and anterior part of the thorax, and forms the anterior part of the shoulder : its direction is from within outwards, so that its external end, which is articulated with the scapula, is pos terior, and on a plane superior to its internal end, which is articulated with the sternum. It thus constitutes the key to the bony arch formed at the shoulder, and hence its integrity is especially necessary to the integrity of the motions of the shoulder.
The clavicle is a long bone, cylindrical, and so curved as to resemble the italic placed horizontally. Its internal extremity is thick and rounded, while its external one is flat tened; of its two curves one is internal, with its convexity directed forwards; the other ex ternal, with its convexity directed backwards.