The simplest instance of a vertebral appendage is seen in that flat process in the skeleton of birds, which, coalesced with the posterior border of the ribs, extends backwards and upwards, and overlaps the exterior of the rib immediately preceding it. From its costal attachments, it is called ' pleurapophysial,' to distinguish it from a second appendage, attached in certain regions of the body to the hae-mapophysis, and hence called 'haemapophysial.' The pleurapophysial appendage of the coraco-scapu-lar arch in fish is called the pectoral fin: it is composed of bones homologous to those we see in the anterior extremities of man and mammalia, and the study of the different steps or stages of development observed in the pleurapophysial appendages, throughout the vertebrate series, affords some of the most striking illustrations of the preservation of uniformity of type in adaptation to special purposes. One of the most astonishing facts brought to light by the study of homologies is, the occasional mutation of position, or of connexions of the vertebral elements. For special purposes, a particular part or segment of a vertebra is removed from its typical seat to some other region of the trunk, where it is maintained in situ by muscular or bony attachments. Its degree of development depends not upon the degree of development of the parent vertebra, but upon its own teleological importance, upon the number of muscles to which it affords attachment, or upon its office as an organ of locomotion or prehension. The haemal arch of the occipital vertebra, which supports the pectoral extremity, maintains its typical position in fish, at the junction of the head with the trunk, where the organs of respiration and of circulation are collected. The pectoral fin is placed in that position where it can act most favourably as an organ of locomotion. But in birds, who, from the immobility of the dorsal and sacral vertebrae, require a long and flexible neck, it is obvious that the maintenance of the typical attachments of the haemal arch of the occiput, or the scapulae and coracoid bones, with the highly-developed appendages forming the wings, would be incompatible with the powers of flight. The haemal arch, then, is removed to the upper dorsal region, where the weight of the trunk is greatest; the bones become flattened and broad, to afford insertion to muscles; and the cylindrical pleurapophysis, or rib, becomes the scapula, which, sabre-shaped in birds, is developed into a triangular plate in man and mammalia.
The clavicles belong to the atlas: the haemal spine, by which they should be united, is represented by the small tubercle which projects from the anterior border of its arch. In many animals, e.g. the cat, the clavicles are reduced to mere fines of bone, which are loosely imbedded in the subcutaneous tissues of the neck; but in man they are strong bony cylinders, which throw out the shoulders to some distance from the trunk, that the sphere of movement of the upper extremities may be the more free and extensive. No other animal enjoys, in a like degree, such perfect liberation of the pectoral extremity from the office of supporting the weight of the body in progression. Man alone, by reason of his upright posture, is enabled to employ his hands in the perfection of those various arts by which this once wild and rugged earth has been cultivated; and both the fruits which it bears upon the soil and the treasures which lie concealed under its crust are made subservient to his wants and his enjoyment.