} 



30 THE SKELETON 



subject the vertebrae are thirty-three in number. Of these, the upper twenty- 

 four remain separate throughout life, and are distinguished as movable or true 

 vertebrae. The succeeding five vertebrae become consolidated in the adult to form 

 one mass, called the sacrum, and at the terminal part of the column are four rudi- 

 mentary vertebrae, which also tend to become united as age advances, to form the 

 coccyx. The lower nine vertebrae thus lose their mobility as individual bones, 

 and are accordingly known as the fixed or false vertebrae. Of the true vertebrae, 

 the first seven are called cervical [cervicales], the succeeding twelve thoracic 

 [thoracales] or dorsal, and the remaining five lumbar [lumbales]. 



Although the vertebrae of the different regions of the column differ markedly 

 in many respects, each vertebra is constructed on a common plan, which is more 

 or less modified in different regions to meet special requirements. The essential 

 characters are well seen in the vertebrae near the middle of the thoracic region, and 

 it will be advantageous to commence the study of the vertebral structures with 

 one selected from this region. 



Description of a thoracic vertebra (figs. 33, 34). — The vertebra consists of two 

 essential parts — a body in front and an arch behind. 



Fig. 33. — A Thoracic Vertebra. (Side view.) 



Superior costal pit for head of rib 



Superior articular process 



Pedicle (root of arch) 



Costal pit for tubercle 

 of rib 



Transverse process 



Inferior costal pit for 

 head of rib 

 Inferior articular process 



■Spinous process 



The body [corpus vertebrae] or centrum is a solid disc of bone, somewhat 

 heart-shaped, deeper behind than in front, slightly concave on its superior and in- 

 ferior surfaces, and wider transversely than antero-posteriorly. The upper and 

 lower surfaces are rough for the intervertebral discs which are interposed between 

 the bodies of the vertebrae, and the margins are slightly lipped. The circum- 

 ference of the body is concave from above downward in front, convex fron side to 

 side, and perforated by numerous vascular foramina. Posteriorly it is concave 

 from side to side and presents one or two large foramina for the exit of veins from 

 the cancellous tissue. On each side of the body, at the place where it joins the 

 arch, are two costal pits (superior and inferior) [fovea costalis superior; inferior] 

 placed at the upper and lower borders, and when two vertebrae are superimposed, 

 the adjacent costal pits form a complete articular pit for the head of a rib. The 

 superior and inferior costal pits were formerly designated as " demi-f acets. " 



The arch [arcus vertebrae] is formed by two pedicles and two laminae, and 

 supports seven processes — one spinous, two transverse, and four articular. The 

 pedicles or roots of the vertebral arch [radices arcus vertebrae] are two short, con- 

 stricted columns of bone, projecting horizontally backward from the posterior sur- 

 face of the l)ody. The concavities on the upper and lower borders of each pedicle, 

 of which the lower is much the deeper, are named vertebral notches [incisurae], and 

 when t wo vertebrae are in position, the notches are converted into intervertebral 

 foramina for the transmission of the spinal nerves and blood-vessels. 



The laminae are two broad plates of bone which connect the spinous process 

 with the roots (pedicles) and complete the arch posteriorly. The superior border 

 and the lower part of the anterior surface of each lamina is rough for the insertion of 

 the ligamenta flava. The upper part of the anterior surface is smooth where it 

 forms the posterior boundary of the vertebral canal. When articulated, the 



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