is comparatively well developed.
3. The Ribs and Sternum.
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.
It will be most convenient to defer a discussion of the limbs of fishes to chap. VIII.
All vertebrates above fishes have the limbs divisible into three main segments:—
| Anterior or Fore limb. | Posterior or Hind limb. | |
| Proximal segment. | upper arm or brachium. | thigh. |
| Middle segment. | fore-arm or antibrachium. | shin or crus. |
| Distal segment. | manus. | pes. |
The proximal segments each contain one bone, the humerus in the case of the upper arm, and the femur in the case of the thigh. The middle segments each contain two bones, the radius and ulna in the case of the fore-arm, and the tibia and fibula in the case of the shin.
The manus and pes are further subdivided into
(a) two or three proximal rows of bones forming the wrist or carpus in the case of the manus, and the ankle or tarsus in the case of the pes.
(b) a middle row called respectively the metacarpus and metatarsus.
(c) a number of distal bones called the phalanges which form the skeleton of the fingers and toes, or digits.
Typically the manus and pes both have five digits (pentedactylate). The first digit of the manus is commonly called the pollex, and the first digit of the pes the hallux.
In a very simple carpus such as that of Chelydra, there are nine bones. They are arranged in a proximal row of three, the radiale, intermedium, and ulnare—the first being on the radial side of the limb, and a distal row of five called respectively carpale 1, 2, 3, 4, 5, beginning on the radial side. Between these two rows is a single bone the centrale, or there may be two.
Similarly there are nine bones in a simple tarsus such as that of Salamandra. They form a proximal row of three, the tibiale, intermedium and fibulare, and a distal row of five, called respectively tarsale 1, 2, 3, 4, 5, beginning on the tibial side. Between the two rows there is a centrale as in the carpus, or there may be two.
The following names derived from human anatomy are commonly applied to the various carpal and tarsal bones:
| Carpus. | Tarsus. |
| radiale = scaphoid | tibiale} |
| intermedium = lunar | intermedium } astragalus |
| ulnare = cuneiform | fibulare = calcaneum |
| centrale = central | centrale = navicular |
| carpale 1 = trapezium | tarsale 1 = internal cuneiform |
|
"2 = trapezoid
|