Girdles


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Girdles

 

in vertebrates, including man, the parts of the skeleton that connect the paired limbs to the trunk. The girdles support the limbs and serve as a point of attachment for a number of muscles that govern limb movement.

The pectoral, or shoulder, girdle is the articulated attachment for the pectoral fins or the anterior extremities (in humans, the arms). In most fishes the primary girdle remains cartilaginous (Selachii, Dipnoi, some ganoids) and consists of a dorsal, or scapular, region and a ventral portion, or coracoid bar. In some bony ganoids and in bony fishes the primary girdle ossifies and is somewhat reduced. In cartilaginous ganoids, the primary girdle is connected to the secondary girdle, which is covered externally and anteriorly by a number of bones cutaneous in origin. The bones usually include paired clavicles, large paired cleithra, and one or two supplementary ossicles that join the secondary girdle to the skull. In bony fishes the cleithra and clavicles are merged.

The primary girdle of terrestrial vertebrates, which is completely free from the skull, is considerably more highly developed than in fishes, whereas the secondary girdle is somewhat reduced and usually consists only of the clavicles and an unpaired inter-clavicle. The primary girdle usually consists of the scapula and the coracoid, whose junction forms the articular socket for the head of the humerus. In terrestrial vertebrates both halves of the pectoral girdle are joined on the ventral side of the body by means of the sternum, which in the Amniota unites the girdle with the spine through the ribs.

In birds the well-developed coracoids are joined to the sternum. The scapula is long and narrow, and the clavicles are fused at the bottom, forming the furcula, or wishbone.

In cloacal mammals the pectoral girdle resembles that of reptiles. It consists of paired scapulae, two ossified coracoids (an anterior and a posterior, as in mammal-like reptiles), clavicles, and an unpaired interclavicle. In higher mammals, beginning with marsupials, only the scapula, which bears the articular socket mainly at the lower part, has been preserved from the primary girdle. The anterior coracoid has disappeared completely, and the posterior one forms a small coracoid process on the scapula. The rod-shaped clavicles extend from the special acromion of the scapula to the sternum. In many running (ungulates) and jumping (carnivorous) mammals, whose limb movements are limited to a single plane, the clavicles disappear. Thus the girdle is completely unattached to the axial skeleton, and sharp impacts are cushioned. In insectivores, rodents, chiropter-ans, and primates—whose limb movements are more complex—the clavicles are well developed.

The pelvic, or posterior, girdle serves for articulated attachment of the ventral fins or the posterior limbs (in humans, the legs). In fishes it usually consists of paired cartilaginous or bony plates. It is embedded in the musculature of the trunk but is free from the axial skeleton. In terrestrial vertebrates the dorsal section, or ilium, is well developed in each half of the pelvic girdle. It articulates initially with one sacral vertebra and subsequently with a large number of vertebrae. The ventral section of the girdle contains the pubis and the ischium. At the junction of the ilium, pubis, and ischium, the socket for the head of the femur is formed.

In extant amphibians the pubic part of the pelvis is cartilaginous, whereas in fossil stegocephalians the pubic bone was ossified. Both halves of the pelvis are fused medianly, initially forming a continuous plate. In reptiles a more or less pronounced opening forms between the pubic and ischial bones. In birds and some fossil Ornithischia the pubic bones are directed not forward but backward along the ischia. Neither of the bones is fused medianly, making possible the laying of large eggs with hard shells.

In mammals a special trochanter develops in the region of the articular socket. In adult animals all the bones of the pelvis fuse to form a single innominate bone. Although in most mammals the pubic and ischial bones of both sides are fused ventrally, they fuse only anteriorly in insectivores, predators, and primates. The pelvis of marsupials has a supplementary paired bone that supports the pouch. Because humans walk erect, their pelvis is somewhat modified and supports not only the limbs but also the internal organs.

REFERENCE

Shmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947.

B. V. SUKHANOV

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