THE BONES OF THE THORACIC LIMB 29 



Most of the bones of the skull are flat bones, developed in membrane; those 

 ot the cramal 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. 



of the Ws'lTHP^«w^i^^'''P'''''*®«°''''1' '^'r^^"'^ of young subjects are necessary, since later most 

 hP i!.ni^llt .V;!^ • °.? become effaced. The relations of each bone to its surroundings should 

 wS fotth^skTf, .o ^""f object IS to understand the skull as a whole. In the des^criptions 

 ^ a type considered with its long axis horizontal, and that of the horse will serve 



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 manomals 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 Umb. Thus in typical quadrupeds, 

 such as the horse and ox, in which the forelimbs are used only for support and locomotion, the 

 cla^acle is absent. Other animals wliich use these Umbs for grasping, burrowing, chmbing, etc. 

 (e. g., man, apes, moles), have w^ell-developed cla\acles wliich 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 (i. 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. 



1 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. 



2 The term "knee" as applied to this region is unfortunate, but the usage is very firmly 

 established and there is no other popular name. 



