Vertebral Column

 

 

 

VERTEBRAL COLUMN

(Girish Chandra)

Vertebral column is part of the axial skeleton that supports the body weight and balances the body on its axis. Primitive chordates had a single rod-like notochord as axial skeleton but in later groups a vertebral column consisting of a series of vertebrae replaced it, increasing flexibility and bending of body possible.

TYPICAL VERTEBRA

            A typical vertebra has an axial cylindrical piece called centrum to which different processes are attached.  Centrum provides strength to vertebra and connects one vertebra with another. There are many types of centra in vertebrates.

Amphicoelous vertebrae are found in fishes apoda and some urodela. These have concavities on anterior as well as posterior side of the centrum.

Procoelous vertebrae are concave on the anterior side and convex on the posterior side of the centrum. Ex. Anura and reptiles.

Opisthocoelous vertebrae possess concavity on the posterior side while convexity on the anterior side, as found in salamanders, parrots and ungulates.

Amphiplatyan vertebrae are flat on both sides as in mammals.

Heterocoelous vertebrae are characteristic of birds neck. The centrum is saddle-shaped to provide high flexibility to the neck movement.

Biconvex vertebra is the 9th vertebra of frog, whose centrum has convexity on both anterior as well as posterior side.

Platycoelous vertebrae are flat on the anterior side and concave on the posterior side and are found in some mammals.

Coeloplatyan vertebrae are also found in some mammals. They are concave on the anterior side and flat on the posterior side.

           Neural arch and neural spine are attached on the dorsal side of the centrum to protect the spinal cord, while haemal spine and haemal arch are attached on the ventral side to protect the blood vessels going to tail.  There are two transverse processes attached on the lateral side of the centrum, which are enlarged in frog for protection of the back. But in other vertebrates it may have one or two attachment places for ribs, which are called diapophysis and parapophysis.

 CYCLOSTOMES

            Notochord persists and the cartilaginous vertebral column encloses it from the ventral and lateral sides while on the dorsal side there is a connective tissue covering.

 ELASMOBRANCHS

Vertebral column is cartilaginous and continuously wrapped around the notochord. There are only two types of vertebrae, namely trunk and caudal, the latter possess haemal arch and haemal spine. Vertebrae are diplospondylous type, having anterior hypocentrum and posterior pleurocentrum.

 TELEOSTS

            Vertebrae are bony and identifiable into trunk and caudal vertebrae. Caudal vertebrae possess long neural and haemal spines. Centra are amphicoelous and notochord is extremely constricted in the middle of the centrum. Ribs are single headed attached to the trunk vertebrae.

 AMPHIBIA

            In apoda centra are amphicoelous and notochord persists. Number of vertebrae can be as many as 250 and caudal region is short. Sacral region is absent.

            In urodela also vertebrae are amphicoelous except in salamanders in which they are opisthocoelous. Number can be as high as one hundred and haemal arch present in tail.

            In anura vertebrae are procoelous and caudal region is absent. In frog first vertebra is atlas or cervical which is ring shaped to support the skull. Second to 7th vertebrae are procoelous with elongated transverse processes. Eighth vertebra is amphicoelous and the 9th vertebra is biconvex and is attached to urostyle on the posterior side. Its transverse processes are long and broad to brace against the ilium bone of pelvic girdle. Vertebrae in frog are stereospondylous with hypocentrum supporting the neural arch.

 REPTILES

            Vertebral column is differentiated into atlas, axis, cervical, thoracic, lumbar, sacral and caudal regions. Atlas and axis make the flexible skull joint that allows the skull to move up-down and sideways. Thoracic vertebrae carry single-headed ribs and caudal vertebrae have chevron bone on the ventral side. Sacral vertebrae are always two and fused together to provide strong articulating joint with pelvic girdle.

In chelonia, 10 thoracic vertebrae including their ribs are fused with the carapace.

In lizards there are about 22 dorsal vertebrae and all of them carry ribs. Even cervical ribs are present. Sphenodon has amphicoelous vertebrae.

In snakes, there can be up to 435 vertebrae, cervical and dorsal vertebrae all carry ribs. There are no sacral  vertebrae. All vertebrae carry a peg-like process called zygosphene on the anterior side, which fits into a cavity called zygantrum on the posterior side of the anterior vertebra, making the attachment between vertebrae very strong.

In crocodiles thoracic and lumbar vertebrae are differentiated and ribs are double-headed with an uncinate process. Tail carries chevron bone.

 AVES

            Most of the vertebrae, except in the neck and tail region are fused together and are gasterocentrous. Cervical vertebrae are heterocoelous to provide flexibility to the neck. Thoracic vertebrae are fused and carry double-headed ribs with uncinate process. Foramen transversalia is characteristic of cervical vertebrae and also there are reduced cervical ribs. Synsacrum is made by the fusion of thoracic, lumbar, sacral and few caudal vertebrae. Free caudal vertebrae are amphicoelous and the terminal caudals are fused together to form pygostyle for giving support to rectrices.

 MAMMALS

            Mammals have 7 cervical vertebrae, except 6 in manatee, 8 in ant bear and 9 in sloth. Dorsal vertebrae are differentiated into thoracic and lumbar which vary in number. Sacral vertebrae are 2-5 and are fused to form a synsacrum. Sacrum is absent in whales and dolphins. Chevron bone in caudal vertebrae is present in monotremes, marsupials, sirenians, cetaceans, some carnivores and rodents. Ribs are double-headed with uncinate process. Lumbar vertebrae in mammals possess no ribs and have a pair of additional long processes called metapophysis for the attachment of muscles.