Clavicle.—None of the Pachyderrnata have the slightest rudiment of a clavicle, an arrange ment which permits the anterior shoulders to be closely approximated beneath the thomx, and thus brought nearer to the centre of gravity.
Humerus.—The humerus is in all cases short, massive, and remarkable for the size and strength of the ridges and prominences for the origin and insertion of the muscles connected with it. The head of the bone which articulates with the scapula is very flat, and although large, forms but a very small proportion of its scapular extremity, the rest being made up of enormous protuberances, to which are affixed the muscles of the shoulder. ( Figs. 474 and 475.) The lower articulating surface is a simple pulley, articulating with the conjoined heads of the radius and ulna, so as to admit of flexion and extension only, no movements of pronation or supination being here admissible.
The humerus of the Elephant (kg. 464) is distinguishable from that of all other quadrupeds by the prodigious extent of the external condyle, which extends upwards nearly one-third of the length of the bone, where it terminates abruptly so as to give a square form to this part of the bone.
_Radius and ulna.—As the position of the fore-arm in the Pachydermata Is permanently that of pronation, no arrangement has been made in any instance to articulate the radius with the ulna by means of a moveable joint, a certain degree of elasticity (the result of liga mentous connection) being all the motion allowed even where the separation between the two bones is most complete. Sometimes, in deed, as in the case of the Hippopotamus and some of the hog tribe, the two bones of the fore-arm are completely consolidated into one mass, the only vestiges of their having been originally distinct being the indication of a suture near the distal extremity of the fore-arm and a deep groove running along the middle third of the bone for the lodgement of the inter-osseous artery. In the Rhinoceros and Tapir, (figs. 475 and 476,) however, these bones remain perma nently distinct, the elbow-joint beingformed by the radius in front, which articulates with both condyles of the humerus and the ulna pos teriorly, which completes the articulation. At their distal extremity the ratlius lies in front and to the inner side of the ulna, with which it is either anchylosed or immoveably connected by ligaments, both assisting to form the radio carpal articulation. In the Elephant, the
arrangement of these bones is very curious and perhaps unique : the upper head of the radius is firmly fixed between two projections in front of the head of the ulna, and assists in forming the elbow-joint articulating with the outer con dyle of the humerus only. It then passes obliquely downwards across the anterior face of the ulna to its distal extremity, where it expands into a broad articulating surface, and assists almost coequally with the ulna in forming the carpal joint.
Carpus.—The bones of the carpus are chiefly remarkable for their large dimensions ; they are, however, always distinct and generally the same in number as in Man, although from heir altered shape they little conform to the names bestowed upon them in the human sub •ect. The first row, consisting of the analogues of the scaphoid, the lunar, the cuneiform, and the pisiform bones, is firmly connected by liga ments with the distal extremities of the ulna d radius to form the wrist-joint, which, how ver, is here only capable of the movements of lexion and extension. The second row consists f the representatives of the trapezium ; the rapezoid, the os magnum, and the unciform vones support the metacarpus and are generally june distinct, although occasionally two or ore of them are consolidated into one mass.
n the Rhinoceros, which has but three toes, trapezoid, the os magnum, and the uneatorm bones each support a single metacarpal bone. The trapezium is totally wanting, but there are two supernumerary pieces in connection with the scaphoid and unciforme.
Metacarpus.—The metacarpal bones are ge nerally short and excessively robust, their num ber of course corresponding with that of the toes. Thus in the Elephant there are five, and in the Hippopotamus, Hog, and Tapir only four, which are small and extremely massive in proportion to the weight they have to sus tain. In the genus Sus, where the whole bur den. of progression is throvvn upon the two middle toes, and a considerable degree of ac tivity is permitted, the corresponding metacarpal bones are much elongated, and far surpass in size and strength those which support the ex ternal and internal fingers, which have rather the appearance of appendages to the outer and inner sides of the metacarpus, than bones ar ticulated with the carpal series.