The haemal arch is formed by the scapulas (pleur-apophyses) and by the coracoid processes (hasmapo-physes) : there is no connecting hsemal spine, the shoulders with the upper extremities being thrown outwards, and maintained far apart by the strong cylindrical clavicles, which extend from the first bone of the sternum to the coracoid bone, and the acromion process. It may be here necessary to explain why the term "arch" is used in speaking of the homologies of the bones of the shoulder.

The displacement of bones from their typical position is often observed in the vertebrate skeleton, and is especially common in the disposition of the bones of the head. This displacement is the more frequently needed, because corresponding parts, limbs, or extremities, wherever they may be situated, are always developed from homologous bony elements. The pectoral fin, in fish, is most advantageously placed in immediate connection with the occipital bone at the base of the skull. In man it is removed to the upper part of the thorax ; but both limbs are, strictly speaking, homotypes ; and the connections of the pectoral fin are likewise the connections of the highly-developed arm, forearm, and hand. If we examine the vertebral column of the crocodile, we shall find not only lumbar, and dorsal ribs, but also cervical ribs, connected by suture only to the rest of the bone. The parietal, frontal, and nasal vertebras possess distinct haemal arches. The occipital vertebra is then only one to which an haemal arch is needed. Reference to the skeleton of any fish will render this matter clear; for there the scapulo-coracoid bones retain their typical position, and form a strong, well-marked arch, composed of the supra-scapular, the scapular, and the coracoid bones. The supra-scapula is composed of two short, cylindrical pieces, of which one is attached to the paroccipital, the other to the petrosal sense-capsule ; they coalesce and form an expanded disc, from which the scapula, an elongated styliform bone, is suspended; from the extremity of the scapula proceeds the coracoid bone, which completes the arch by uniting, by ligament, with the corresponding bone of the opposite side. Arguing, then, from the class of fish, the earliest vertebrate inhabitants of this planet, we must refer the scapulae and the coracoid bones to the occipital segment of the skull. The development of the former into a triangular plate of bone for the attachment of muscles, the confluence of the latter to form the coracoid process, and the separation of the two halves of the arch, that in man the movements of the right and left upper extremities, their free appendages, may not be impeded, must not blind us to the typical character of these bones. They are as much parts of the haemal arch of the occipital bone as are the ribs, the costal cartilages, and the segments of the sternum parts of the haemal arches of the dorsal vertebrae, to which they correspond.