Cassell's book of birds; vol. 1

by Alfred Edmund Brehm

The sternum or breast-bone (Fig. 12, e) is developed in proportion to the size of the three pectoral muscles subservient to flight, and is prolonged beneath into a deep keel-like projection. In the cursorial races, such as the Ostriches and the Apteryx, whose wings are not available for flying, the keel is entirely wanting.

Whoever considers the position of the hip-joint in the skeleton of a bird, and reflects how far it is necessarily removed behind the centre of gravity when the bird walks with its body in a horizontal position, will at once perceive that the hinder portion of the spinal column, having to support the whole weight of the body under the most disadvantageous circumstances, and at the same time to give attachment to the strong and massive muscles that wield the thigh, must be consolidated and strengthened in every possible manner, and that even the slight degree of movement permitted in the region of the back would here be inadmissible. Most of the hinder vertebræ are, therefore, solidly conjoined into a single piece (Fig. 12, r, s), sufficiently strong and massive to bear the great strain to which it is continually subjected, leaving only a few of the hindmost pieces (Fig. 12, d) free, upon which the feathers of the tail are supported.

The fore limb of a bird, although used for the purpose of flight, when stripped of the feathers and quills that, as we have already seen, form the extensive surface of the wing, will be found very much to resemble the human arm in its general arrangement, presenting only such modifications as are required for the performance of its peculiar function.

The framework of the shoulder consists of three bones (Fig. 12, h, i, k), named respectively the scapula or shoulder-blade, the clavicle or collar-bone, and the coracoid bone. The scapula (h) is a long and comparatively slender piece placed upon the ribs, and embedded in the muscles, to which it gives attachment. The coracoid bone (k) is the strongest piece of the shoulder; it supports the wing at one extremity, while at the opposite it is firmly united to the sternum by a broad and massive joint. But the most peculiarly formed part of the shoulder is the furculum, or "merry-thought," as it is usually called (i i), consisting of the two collar-bones united, so as to form but a single fork-shaped apparatus, the presence of which materially enlarges and strengthens the shoulder, without unnecessarily adding to the weight. It is by the union of the three last-mentioned bones that a place is made for the socket of the shoulder, with which the wing is more immediately connected.

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).