The skeleton of the wing presents the bone of the arm, called the _humerus_ (Fig. 12, _l_), and the two bones of the fore-arm, named respectively the _radius_ (_m_) and the _ulna_, or _cubit_ (_n_). The _wrist_, or _carpus_, consists of two bones (_b' o_), and the metacarpus (_p_) is likewise made up of two pieces; these, with two, or sometimes three, rudimental fingers (_p' q q_), complete the framework of the wing. The largest finger consists of two, or sometimes of three joints; a second offers but a single joint; and the third, when present, is a mere appendage to the carpus, representing a sort of apology for a thumb. The bones of the leg likewise exhibit the same parts as exist in the human skeleton, but modified. The _thigh_, or _femur_ (Fig. 12, _t_), is a short and strong bone, to which succeeds the leg, consisting of two bones (_u_), named the _tibia_ and the _fibula_, but the latter is generally very imperfectly developed. That part which is commonly considered to be the leg consists of the bones of the ankle and a part of the foot (the tarsal and metatarsal bones) consolidated into a single piece, called by anatomists the _tarso-metatarsal_ bone, but known to ornithologists as the _tarsus_ (_x_). At the lower extremity of the tarsus are three joints that support the three front toes (_z, z_), while a fourth toe (technically called the _hallux_) (_y_), which is directed backwards, is attached to it by the intervention of a small accessory piece. In the gallinaceous order there exists a bony spur, considered by some as representing a fifth toe. In order to facilitate the description of a bird, it is usual for the ornithologist to consider its exterior as being mapped out into sundry _regions_ (see Fig. 13), to each of which has been assigned a definite and appropriate name; with the names of these regions, and their precise application, it is requisite that the reader should be intimately acquainted. A bird, like any other vertebrate animal, is divisible into the head, the body, and the limbs; under one or other of which divisions all subordinate parts may be classed. The head consists of the _skull_ and the _bill_, and is joined to the body by the _neck_. Commencing with the bill, we see that it is composed of two pieces, corresponding to the jaws of quadrupeds; that which is above is called the _upper mandible_ (Fig. 13, 1), that which represents the under jaw is called the _lower mandible_ (2). The upper mandible contains the _nostrils_ (3); its highest part is called the _culmen_ or _ridge_ (4), while the corresponding ridge of the lower mandible is called the _gonys_ or _keel_ (9). The lateral edges of the mandibles which meet when the bill is closed are called the _margins_. In some birds the margins of the upper mandible fold over those of the lower, while in others the edges meet; when this is the case, the line of junction between the two is called the _commissure_ (5).