THE BONES OF THE THORACIC LIMB 29 
Most of the bones of the skull are flat bones, developed in membrane; those 
of the cranial base may be classed as irregular, and are developed in cartilage. Only 
two form permanent movable joints with other parts of the skull. The mandible 
(or lower jaw-bone) forms diarthrodial joints with the temporal bones, and the 
hyoid bone is attached to the latter by bars of cartilage. The other bones form 
immovable joints, most of which disappear with age. 
In order to study the separate bones, skulls of young subjects are necessary, since later most 
of the lines of demarcation become effaced. The relations of each bone to its surroundings should 
be specially noted, since the final object is to understand the skull as a whole. In the descriptions 
which follow the skull is considered with its long axis horizontal, and that of the horse will serve 
as a type. 
THE BONES OF THE THORACIC LIMB 
The thoracic limb (Extremitas thoracalis) consists of four chief segments, viz., 
the shoulder girdle, the arm, the forearm, and the manus. 
The shoulder girdle (Cingulum extremitatis thoracalis), when fully developed, 
consists of three bones—the scapula (or shoulder-blade), the coracoid, and the 
clavicle (or collar-bone). In the domesticated mammals only the scapula, a large, 
flat bone, is well developed, and the small coracoid element has fused with it, while 
the clavicle is either absent or is a small rudiment embedded in the brachiocephali- 
cus muscle. There is therefore no articulation of the shoulder with the axial 
skeleton. 
The shoulder girdle is fully developed in birds and the lower mammals (monotremata). In 
the higher mammals the coracoid is reduced to the coracoid process of the scapula, and the develop- 
ment of the clavicle is in conformity with the function of the imb. Thus in typical quadrupeds, 
such as the horse and ox, in which the forelimbs are used only for support and locomotion, the 
clavicle is absent. Other animals which use these limbs for grasping, burrowing, climbing, etc. 
(e. g., Man, apes, moles), have well-developed clavicles which connect the scapula with the sternum. 
The arm (Brachium) contains a single long bone, the humerus (or arm bone). 
In the forearm (Antibrachium) are two bones, the radius and ulna. These 
vary in relative size and mobility. In the horse and ox the two bones are fused, 
and the distal part of the limb is fixed in the position of pronation. The radius is 
placed in front and supports the weight. The ulna is well developed only in its 
proximal part, which forms a lever for the extensor muscles of the elbow. In the 
pig the ulna is the larger and longer of the two bones, but is closely attached to 
the back of the radius. In the dog the ulna is also well developed and a small 
amount of movement is possible between the two bones. 
The manus,! the homologue of the hand in man, consists of three subdivi- 
sions, viz., the carpus, metacarpus, and digit or digits. 
The carpus, popularly termed the ‘‘knee” in animals, and homologous with 
the wrist of man, contains a group of short bones, the ossa carpi. These are typic- 
ally eight in number and are arranged in two transverse rows—a proximal or anti- 
brachial, and a distal or metacarpal. The bones of the proximal row, named from 
the radial to the ulnar side (7. e., from within outward), are the radial, intermediate, 
ulnar, and accessory carpal bones. The bones of the distal row are designated 
numerically, in the same direction, as first, second, third, and fourth carpal bones. 
This nomenclature, introduced by Gegenbaur, and now used largely by comparative anat- 
omists, seems decidedly preferable to the variety of terms borrowed from human anatomy and 
based on the form of the bones in man. The following table of synonyms in common use is ap- 
pended for comparison. The Latin terms and abbreviated notations are given in parentheses. 
_,\It is unfortunate that there is no popular name for this part of the limb. The term “fore- 
foot” is sometimes applied to it, but this leads to confusion, since the word “foot” has long been 
used in a different sense. 
: *The term “knee” as applied to this region is unfortunate, but the usage is very firmly 
established and there is no other popular name. 
