The Vertebrate Skeleton

by Sidney H. Reynolds

The ribs are a series of segmentally arranged cartilaginous or bony rods, attached to the vertebrae; they tend to surround the body cavity, and to protect the organs contained within it. Ribs are very frequently found attached to the transverse processes of the vertebrae, but a study of their origin in fish shows that they are really the cut off terminations of the ventral arch, not of the transverse processes which are outgrowths from the dorsal arch. In the tail their function is to surround and protect structures like the ventral blood-vessels which do not vary much in size, consequently they meet one another, and form a series of complete ventral or haemal arches. But the trunk contains organs like the lungs and stomach which are liable to vary much in size at different times, consequently the halves of the haemal arch do not meet ventrally, and then the ribs become detached from the rest of the haemal arch. Having once become detached, they are able to shift about and unite themselves to various points of the vertebra. They frequently, as has been already mentioned, become entirely attached to the transverse process, or they may be attached to the transverse process by a dorsal or tubercular portion and to the centrum or to the ventral arch by a ventral or capitular portion.

In all animals above fishes the distal ends of the thoracic ribs unite with a median breast bone or sternum which generally has the form of a segmented rod. The sternum is really formed by the fusion of the distal ends of a series of ribs. In many animals elements of the shoulder girdle enter into close relation with the rib elements of the sternum.

II. The Appendicular Skeleton.

This consists of the skeleton of the anterior or pectoral, and the posterior or pelvic limbs, and their girdles. In every case (except in Chelonia) the parts of the appendicular skeleton lie external to the ribs.

1. The Limb girdles.

The Pectoral girdle[11]. In the simplest case the pectoral or shoulder girdle consists of a hoop of cartilage incomplete dorsally. It is attached by muscle to the vertebral column, and is divided on either side into dorsal and ventral portions by a cavity, the glenoid cavity, at the point where the anterior limb articulates. In higher fishes this hoop is distinctly divided into right and left halves; it becomes more or less ossified, and a pair of important bones, the clavicles, are developed in connection with its ventral portion.

In higher vertebrates ossification sets up in the cartilage and gives rise on each side to a dorsal bone, the scapula, and frequently to an anterior ventral bone, the precoracoid, and a posterior ventral bone, the coracoid. The precoracoid is often not ossified, and upon it is developed the clavicle which more or less replaces it. In some forms a T shaped interclavicle occurs, in others epicoracoids are found in front of the coracoids. In all vertebrata above fish, except the great majority of mammals, the coracoids are large and articulate with the sternum. But in mammals the coracoids are nearly always quite vestigial, and the pectoral girdle is attached to the axial skeleton by the clavicle or sometimes by muscles and ligaments only.

The Pelvic girdle[12] like the pectoral consists primitively of a simple rod or hoop of cartilage, which in vertebrata above fishes is divided into dorsal and ventral portions, by a cavity, the acetabulum, with which the posterior limb articulates. In the pelvic girdle as in the pectoral one dorsal, and (commonly) two ventral ossifications take place. The dorsal bone is the ilium and corresponds to the scapula. The posterior ventral bone is the ischium corresponding to the coracoid. The anterior ventral bone is the pubis and is generally compared to the precoracoid, but in some cases a fourth pelvic element, the acetabular or cotyloid bone is found, and this may correspond to the precoracoid.

The pelvic girdle differs from the pectoral in the fact that the dorsal bones—the ilia—are nearly always firmly united to transverse processes of the sacral vertebrae, by means of rudimentary ribs. The pubes and ischia generally meet in ventral symphyses.

2. The Limbs.