50 VERTEBRAL COLUMN. 



The hyoid arch. 4. The thoracic arch and upper extremities. 5. The 

 pelvic arch and lower extremities. 



VERTEBRAL COLUMN. 



The vertebral column is the first and only rudiment of Internal skeleton 

 in the lower Vertebrata, and constitutes the type of that great division of 

 the animal kingdom. It is also the first developed portion of the skeleton 

 in man, and the centre around which all the other parts are produced. 

 In its earliest formation it is a simple cartilaginous cylinder, surrounding 

 and protecting the primitive trace of the nervous system ; but, as it advances 

 in growth and organisation, it becomes divided into distinct pieces, which 

 constitute vertebra. 



The vertebrae are divided into true and false. The true vertebrae are 

 twenty-four in number, and are classified, according to the three regions 

 of the trunk which they occupy, into cervical, dorsal, and lumbar. The 

 false vertebra consist of nine pieces united into tw T o bones, the sacrum and 

 coccyx The arrangement of the vertebrae may be better comprehended 

 by means of the accompanying table : 



C 7 Cervical, 



True vertebrae 24 3 12 Dorsal, 

 ( 5 Lumbar. 



False vertebra, 9 j J 



Characters of a Vertebra. A vertebra consists of a body, two laminee, 

 a spinous process, two transverse processes, and four articular processes. 

 The body is the solid part of the vertebra ; and, by its articulation with 

 adjoining vertebrae, gives strength and support to the trunk. It is /flattened 

 above and below, convex in front, and slightly concave behind. Its an- 

 terior surface is constricted around the middle, and pierced by a number 

 of small openings which give passage to nutritious vessels. Upon its pos- 

 terior surface is a single irregular opening, or several, for the exit of the 

 venae basis vertebrae. 



The lamina commence upon the sides of the posterior part of the body 

 of the vertebra by two pedicles; they then expand; and, arching back- 

 wards, enclose a foramen which serves for the protection of the spinal 

 cord. The upper and lower borders of the laminae are rough for the at- 

 tachment of the ligamenta subflava. The concavities above and below the 

 pedicles- are the iniervertebral notches. The spinous process stands back- 

 wards from the angle of union of the laminae of the vertebra. It is the 

 succession of these projecting processes along the middle line of the back, 

 that has giv^i rise to the common designation of the vertebral column, the 

 spine. The use of the spinous process is for the attachment of muscles. 

 The transverse processes project one at each side from the laminae of the 

 vertebra ; they are intended for the attachment of the muscles. The arti- 

 cular processes , four in number, stand upwards and downwards from the 

 laminae of the vertebra, to articulate with the vertebra above and belo\v. 



Cervical Vertebrae. In a cervical vertebra the body is smaller than 

 in the other regions ; it is thicker before than behind, broad from side 

 lo side, concave on the upper surface, and convex below; so that, when 

 articulated, the vertebrae lock the one into the other. The lamina are 



