46 GENERAL ANATOMICAL CHARACTERS 



instance of the chevron bones, that the intercentral capitular articu- 

 lation of the ribs of mammals is a feature directly inherited 

 from those extinct types by the gradual disappearance of the 

 intercentra. 



Appendicular Skeleton. — The appendicular portion of the frame- 

 work consists, when completely developed, of two pairs of limbs, 

 anterior and posterior (Fig. 15). 



Anterior Limb. — The anterior limb is present and fully developed 

 in all mammals, being composed of a shoulder girdle and three seg- 

 ments belonging to the limb proper, viz. the upper arm or brachium, 

 the fore-ai'm or antebrachium, and the hand or manus. 



Shoulder-girdle. — The shoulder or pectoral girdle'm the large majority 

 of mammals is in a rudimentary or rather modified condition, com- 

 pared with that in which it exists in other vertebrates. In the Mono- 

 tremata (Ornithorhynchus and Echidna) alone is the ventral portion, or 

 coracoid, complete and articulated with the sternum below, as in the 

 Sauropsida ; and in this group alone do we find an anterior ventral 

 element, apparently corresponding with the precoracoid of the Anom- 

 odont reptiles, although generally known as the epicoracoid. In all 

 other mammals the coracoid, though ossified from a distinct centre, 

 forms only a process, sometimes a scarcely distinct tubercle, projecting 

 from the anterior border of the glenoid cavity of the scapula. The 

 last-named cavity, which in the Monotremes is formed jointly by 

 the scapula and coracoid, receives the head of the humerus, or 

 arm-bone. The scapula is always well developed, and generally 

 broad and flat (whence its vernacular name " blade bone "), with a 

 ridge called the " spine " on its outer surface, which usually ends in 

 a free curved process, the "acromion." As the scapula affords 

 attachment to many of the muscles which act upon the anterior 

 limb, its form and the development of its processes are greatly 

 modified according to the uses to which the member is put. Thus it 

 is most reduced and simple in character in those animals whose limbs 

 are mere organs of support, as the Ungulates ; and most complex 

 when the limbs are also used for grasping, climbing, or digging. 

 The development or absence of the clavicle or " collar-bone," an 

 accessory bar which connects the sternum with the scapula and 

 steadies the shoulder-joint, has a somewhat similar relation, though 

 its complete absence in the Bears shows that this is not an invariable 

 rule. A complete clavicle is found in Man and all the Primates, in 

 Chiroptera, all Insectivora (except Potaniogale), in many Rodents, in 

 most Edentates, and in all Marsupials, except Perameles. More or 

 less rudimentary clavicles (generally suspended freely in the muscles) 

 are found in the Cat, Dog, and most Carnivora, Myrmecophaga, and 

 some Rodents. Clavicles are altogether absent in most of the I Wsidce, 

 all the Pinnipedia, Manis among Edentates, the Cetacea, Sirenia, 

 Ungulates, and some Rodents. 



