Among the Edentata those tribes possess a clavicle whose habits are fossorial, as the Ant eater, the Armadillos, and even the Gigantic Megatherium, in which animal, however, the clavicle presented the peculiarity of being arti culated with the first rib instead of with the sternum. In the Quadrumana the clavicles are strong and curved as in the human subject.
In Birds, the bone which is analogous to the clavicle presents similar variations in its developement, according to the range of motion required in the anterior extremity, or in other words, in proportion to the extent to which the powers of flight are enjoyed. Thus, in some these bones are anchylosed along the mesial line, and constitute the furculum ; in others they are cartilaginous internally; and in others they do not reach the sternum.* In women the clavicle is in general less curved than in men ; the diminution in the incurvation is most manifest in the external portion. Accord ing to Cruveilhier, the clavicles are often une qually developed in the same individual accord ing as one limb is more used than the other, and sometimes the difference is sufficiently obvious to enable one to ascertain from the relative size of the clavicles, whether the individual is right or left-handed.
Structure.—The clavicle contains a conside rable proportion of compact tissue in its shaft, and a cylindrical medullary canal; at the ex tremities the compact tissue greatly diminishes, and is replaced by the reticular, which likewise fills up the bone and obliterates the medullary cavity.
Developement.—A strong argument as to the great importance of this bone to the motions of the shoulder, is derived from its precocious de velopement; for although the cartilaginous nidus of the vertebra as well as that of the ribs appear before that of the clavicle, yet the latter bone begins to ossify sooner and is completed more rapidly than any other bone in the body, ex cepting perhaps the lower jaw, which some times takes the precedence in the process of ossification. It is remarkable too for the diver sity in its proportional size, which it presents at different periods; thus, according to ]Heckel, about the middle of the second month of pregnancy, the clavicle is four times longer than the humerus or femur, and it is not until the fourth month that the humerus exceeds it in length. The clavicle has but one primitive point of ossification : a supplementary point is developed under the form of a very thin lamella at the anterior part of the sternal extremity.t
Scapula, seapulum, omoplata, (wihoc, hume rus, TrXezTvc, latus.) Fr. omoplate; Germ. das Schulterblutt.—This bone forms the posterior and principal portion of the shoulder; it is placed on the posterior and outer part of the thorax, and occupies a space which extends from the second to the seventh rib.
The scapula is very thin in the greatest part of its extent, quite papyraceous in some places. It is triangular in form, and anatomists com monly describe its sides or borders, its angles, and its surfaces.
The borders, or costa, of the scapula are three in number, and are named according to the position they occupy or the. relation they bear: thus there are the superior border or cervical, the posterior or vertebral, and the anterior or arillary. The cervical border (also called the coracoid) is the shortest, being some what less than a fourth of the length of the vertebral border; it is connected posteriorly with the vertebral at an angle the apex of which is rounded off; it is slightly concave, and the bone for some way below it is very thin, and the bor der itself is acute. Anteriorly it terminates in a notch which is bounded in front by one root of the coracoid process, (incisura semilunuris, lunulu, coracoid notch.) This notch is converted into a foramen by a ligament which is often ossi fied, and thus the suprascapular nerve, which is lodged in the notch, is separated from the artery of the same name, which passes over the ligament. The extent, therefore, of the cervical border is from the posterior superior angle to this notch. The levator anguli sca pulae and the omo-hyoid muscles are attached to this border.
The vertebral border, also called the base of the scapula, is the longest, being in an ave rage-sized bone from seven to eight inches in length ; it is sharp in its whole extent, which is limited above by the posterior superior angle, and below by the inferior angle. At the junc tion of the superior fourth with the remaining portion there is an inclined surface, triangular in form, the base confounded with the margin of the bone, the apex continued to the spine. This surface is smooth, and the ascending fibres of the trapezius muscle glide over it. To that part of this edge, which is above the surface, the levator anguli scapula; is attached, and below it the rhomboidei.