Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/739

(co.), fitted by its proximal end in the groove in theantero- lateral edge of the sternum. The inner ends of the coracoids often overlap, as in Lacertilia ; otherwise the shoulder-girdle is unlike that of any of the Rcptilla, except the Pterosauria. The coracoid is usually completely ossified, and presents no fontanelle. There is no distinct epicoracoid. The two bones take nearly equal shares in the formation of the glenoidal cavity, and usually remain

iinankylosed and distinct in this region.

In the Ratitw the long axis of that part of the scapula which lies near the glenoidal cavity is parallel or coincident with that of the coracoid, and the two bones become com pletely ankylosed. But in all the Carinatce the long axis of the scapula forms an acute, or only slightly obtuse angle (Ocydromus, Didus) with that of the coracoid. A small bone, the scapula accessoria, is developed on the outer side of the shoulder-joint in most Coracomorphcs and Celeomorphce.

In the Carinatce the glenoidal end of the scapula is divided into two portions : a glenoidal process, which expands to form the upper part of the glenoidal cavity. and to units with the coracoid ; and an acromial process, which gives attachment to the outer end of the clavicle. The glenoidal end of the coracoid is in like manner divided into two portions : a glenoidal process, which unites with the scapula, and a clavicular process, which articulates with the outer surface of the clavicle near its outer end. The clavicular process of the coracoid does not represent the procoracoid of Lacertilia; rudiments of that bone unite vith the clavicle. In the Ratitce there is no distinct clavicular process ; but the anterior part of the coracoid, near the glenoid cavity, may be produced and separated by a notch or fontanelle from the rest, or developed down to the sternum (in Struthio) as a lacertilian procoracoid. There is no trace of clavicles in Apttryx, Rhea, Struthio, and some Parrots ; but in the latter there is generally an ossified, distinct, short procoraoid. In the Emeu and in some Carinatce (Didus, Rliamphastos toco, Corytliaix buffoni, Buceros albirostris), the clavicles remain distinct from one another, or connected only by fibrous tissue : but in the majority of Birds they are very early ankylosed together, and with the representative of the inter-clavicle, in the middle line, into a single bone, the furculum, the strength of which bears a pretty close relation to the exertion required of the wings in flight or in natation. In the Passerine Birds the scapular end of the clavicle is enlarged by a procoracoid rudiment of cartilage, which ossifies separately, producing the expansion above and in front called epideidium.

A median process (hypodeidiuni) is frequently developed from the inter-clavicular part of the furculuin, and this may be united with the carina of the sternum by strong fibrous tissue, or even by continuous ossification. In Opisthocomus^[1] the furculuin is ankylosed with the manubrial part of the sternum on the one hand, and with the coracoids on the other. Ankylosis of the furculuin with the coracoids has also been observed in Didus,^[2] and with both keel and coracoid in Frcgata aquila (Shoulder- girdle and Sternum, p 15, 1 ; see also the actual specimen in the museum of Col. Surg. Eng.)

The fore-limb of a Bird, when in a state of rest (fig. 33), exhibits a great change of position, if it be compared with that of an ordinary Reptile; and the change is of a character similar to, but in some respects greater, than that which the arm of a man presents when compared with the fore-limb of a quadrupedal Mammal. The humerus lies parallel with the axis of the body, its proper ventral surface looking outwards. The fore-arm is in a position midway between pronation and supiuation, and the manus is bent back upon the ulnar side of the fore-arm in a position not of flexion but of abduction.

FIG. 33.- -Bones of Fowl s right wing, adult, nat. size, h., hum eras; r., radius;

/., ulnar ; r 1 ., u ., radial and uhiar carpal bones ; with the three digits I.,II.,IU.

In ordinary Birds the proximal end of the humerus is expanded, and its articular head transversely elongated. Its ventral face is convex, and provided with a strong preaxial ridge, which gives attachment to the pectoral muscle. The proper dorsal face is concave from side to side, especially towards the postaxial margin, where the pneumatic aperture occurs in those birds which have the humerus hollow. The distal end is expanded, and the articular surface for the radius is a convex facet, directed obliquely inwards on its ventral face. In this respect the Bird s humerus exaggerates a feature of that of the Lizard s.

In the Ratitce these peculiarities are very feebly, or not

at all, marked, the humerus being a slender, cylindrical, slightly-curved bone. In the Casuariidce, Dinornitliidce, and Apterygidce, the fore-limb is extraordinarily reduced,

and may become rudimentary. In the Penguins and, to

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1. Huxley " On the Alectorcmorphcp," p. 306, fig?. 8 and 9.
2. Owen "On the Dodo." Trans. Zool. Soc., vol. vi. part 2. p 63 plate 20, fig. 4.