Additional vertebrae may be present. The sacrum may show six segmernts.

1. Additional vertebrae may be present. • The sacrum may show six segmernts. • Part of a vertebra may be missing. • (a) The two halves of the neural arch may fail to fuse in the midline, condition is called spina bifida.

2. The gap between the neural arch may not be obvious (spina bifida occulta), or may be large enough for meninges and neural elements to bulge out of it. • Spina bifida in a fetus can be recognized by ultrasound examination( Examination of amniotic fluid shows increased levels of alpha feto proteins (AFP) in a case with spina bifida.

3. The vertebral body may ossify from two primary centres which so fuse. One of these parts may fail to develop, resulting in only half I the body being present. This is called hemivertebra. It is usually associated with absence of the corresponding rib. • The two halves of the vertebral body may be formed normally but may fail to fuse. The vertebral body then consists of two hemivertebrae. • Sometimes the gap between the two halves is large enough for meninges and nerves to bulge forward between them (anterior spina bifida).

4. Two or more vertebrae that are normally separate may be fused to each other. • Such fusion may occur in the cervical region (Klippel-Feil syndrome. • The atlas vertebra may be fused to the occipital bone (occipitilization atlas).

5. The fifth lumbar vertebra may be partially or completely fused the sacrum (sacralization of fifth lumbar vertebra). • Parts of the vertebral column that are normally fused to each other may separate. • The first sacral vertebra may be separate from the rest of the sacrum (lumbarization of the first sacral vertebra).

6. The odontoid process may be separate from the rest of the axis vertebra. • The articular facets may be abnormal in orientation, or may be deficient. When this happens in the lower lumbar region, the body of the fifth lumbar vertebra may slip forwards over the sacrum. This is called spondylolisthesis.

7. Diastematomylia. • The vertebral canal may be divided into two lateral halves by a projecting shelf of bone, which splits the spinal cord longitudinally into two halves

8. chondro-osteo-dystrophy • Ossification of the vertebral bodies may be defective thus reducing the to­tal length of the spine. • This can lead to the formation of dwarfs who have a short trunk but have limbs of normal length.

9. sacrococcygeal teratoma. • A peculiar tumour arising from cells of the primitive knot may be seen attached to the lower end of the spine. Various tissues may be seen in it.

10. (a) They may cause deformities of the spine. The spine may be bent on itself (congenital scoliosis). • Deformities of cervical vertebrae may lead to tilting of the head to one side and its rotation to the opposite side (congenital torticolis). • This deformity may be secondary to a contracture of the sternocleidomastoid muscle.

11. (b) The spinal'nerves, or even the spinal cord, may be implicated. They may be subjected to abnormal pressure leading to paralysis. • (c) They are frequently the cause of backache.

12. Anomalies of the Sternum and Ribs • Some ribs that are normally present may be missing. Unilateral absence of a rib is often associated with hemivertebra. • Accessory ribs may be present. Such a rib may be attached to the seventh cervical vertebra (cervical rib), or to the first lumbar vertebra (lumbar rib).

13. When the fusion of the two sternal bars is faulty, the body of the sternum shows a partial or even a complete midline cleft. • Minor degrees of non­fusion may result in a bifid xiphoid process or in midline foramina. • Transverse clefts may also occur.

14. In the condition called funnel chest, the lower part of the sternum and the attached ribs are drawn inwards into the thorax. The primary defect is that the central tendon of the diaphragm is abnormally short. • The upper part of the sternum (and related costal cartilages) may project forwards (pigeon breast).

15. Bones that are Completely Formed in Membrane (a) The frontal and parietal bones are formed in relation to mesenchyme covering the developing brain. (b) The maxilla (excluding the premaxilla), zygomatic and palatine bones, and part of the temporal bones, are formed by intramembranous ossification of the mesenchyme of the maxillary process. (c) The nasal, lacrimal and vomer bones are ossified in the membrane covering the nasal capsule.

16. Completely Formed in Cartilage ethmoid bone and the inferior nasal concha are derived from the cartilage of nasal capsule. The septal and alar cartilages of the nose represent parts of the capsule that do not undergo ossification.

17. Bones that are Partly Formed in Cartilage and Partly in Membrane (a) Occipital: The interparietal part is formed in membrane; the rest of the bone is formed by endochondral ossification. (b) Sphenoid: The lateral part of the greater wing, and the pterygoid lami­nae, are formed in membrane; the rest is cartilage bone.

18. (c) Temporal: The squamous and tympanic parts are formed in membrane. • The petrous and mastoid parts are formed by ossification of the cartilage of the otic capsule. The styloid process is derived from the cartilage of the second branchial arch.

19. (d) Mandible: Most of the bone is formed in membrane in the mesenchyme of the mandibular process. • The ventral part of Meckel's cartilage gets embedded in the bone. The condylar and coronoid processes are ossified from secondary cartilages that appear in these situations.

20. The greater part of the vault of the skull is missing in cases of anencephaly. • In one syndrome, deformities of the skull are associated with absence of the clavicle (cleidocranial dysostosis ). • Premature union of the sagittal suture gives rise to a boat-shaped skull (scaphocephaly).

21. Early union of the coronal suture results in a pointed skull (acrocephaly). • Asymmetrical union of sutures re­sults in a twisted skull (plagiocephaly). • When the brain fails to grow the skull remains small (microcephaly).

22. The bones of the vault of the skull may be widely separated by expansion of the cranial cavity in congenital hydrocephalus. • In a rare congenital condition called Hand-Schulter-Christian disease, large defects are seen in the skull bones. • The occipital bone may be fused to the atlas vertebra.

23. Anomalies of Limbs • One or more limbs of the body may be partially, or completely, absent (phocomelia, amelia). These conditions may be produced by harmful drugs. • Part of a limb may be deformed. Deformities are most frequently seen in the region of the ankle and foot, and are of various types.

24. In the most common variety of deformity, the foot shows marked plantar flexion (equinus: like the horse), and inversion (varus). Hence this condition is called talipes equinovarus, or club foot. • Congenital strictures, congenital amputations or congenital contractures may be present.

25. There may be abnormal fusion (bony or fibrous) between different bones of the limb. • Adjoining digits may be fused (syndactyly). • The phalanges of a digit may be fused to one another (synphalangia). • A digit may be abnormally large (macrodactyly), or • abnormally short (brachydactyly). • In arachnodactyly, the fingers are long and thin (spider fingers).

26. Supernumerary digits may be present (polydactyly). A digit (most commonly the thumb) may have an extra phalanx. • The palm or sole may show a deep longitudinal cleft (lobster claw). • The limbs may remain short in achondroplasia .

27. Sometimes the bone ends forming a joint are imperfectly formed (con­genital dysplasia). This can lead to congenital dislocation. • The hip joint is most commonly affected.