The region occupied by the glenoid cavity is separated from the body of the bone by a constriction--the neck of the scapula. In birds the scapula is elongated in a direction parallel to the vertebral column, and very narrow in the opposite (Fig. 18): it is also flat, and has no spine. Its coracoid process is represented by a peculiar bone--the coracoidean or coracoid bone--which we shall describe later on when we come to the study of the clavicle and of the anterior region of the shoulder (see p. 26). =The Clavicle.=--The clavicle is found only in the human being, and in animals whose anterior limbs, possessing great freedom of movement in all directions, require that the scapula should possess a point of support which, while affording this, can be displaced with it, or draw it in certain directions. Now, this point of support is furnished by the clavicle. In animals possessed of hoofs (ungulates), such as the sheep, ox, and horse, the clavicle does not exist. Indeed, in them the freedom of movement of the anterior limbs is limited; they move by projection in the forward and backward directions only; they merely fulfil the functions of giving support to and carrying about the body. The clavicle is rudimentary in the cat and the dog; in the cat it is a small, elongated bone (Fig. 16), 2 centimetres in length, thin and curved, connected with the sternum and the scapula by ligamentous bundles. In the dog it is represented by a small osseous plate only (Fig. 17), which is not connected with any of the neighbouring bones. It is on the deep surface of a muscle which passes from the head and neck to the humerus (mastoido-humeral, a muscle common to the arm, neck, and head) in which this rudimentary bone is found to be developed. The clavicle exists in perfect state in mammals which use their limbs for digging, grasping, or flying; the insectivora (hedgehog, mole) and some rodents (squirrel, woodchuck) are provided with it. The cheiroptera (bats) possess an extremely well-developed clavicle, on account of the varied movements which their thoracic limbs execute. [Illustration: FIG. 16.--LEFT CLAVICLE OF THE CAT: SUPERIOR SURFACE (NATURAL SIZE). 1, Internal extremity; 2, external extremity.] [Illustration: FIG. 17.--CLAVICLE OF THE DOG (NATURAL SIZE).] This formation of the shoulder which favours flight in the bat is even more remarkable in birds. In these latter (Fig. 18) the clavicles, fused together by their lower extremities, form one bone, having the shape of the letter V or U, which is known as the _fourchette_; this bone, acting as a true spring, keeps the shoulders apart, and prevents their approximation during the energetic movements which flight necessitates.