152 THE SKELETON. 



presenting prominent margins, which afford a broad basis for the support of the 

 superincumbent weight. The pedicles are very strong, directed backward from 

 the upper part of the bodies ; consequently, the inferior intervertebral notches are 

 of considerable depth. The lamince are broad, short, and strong, and the spinal 

 foramen triangular, larger than in the dorsal, smaller than in the cervical, region. 

 The spinous processes are thick arid broad, somewhat quadrilateral, horizontal in 

 direction, thicker below than above, and terminating by a rough,- uneven border. 

 The superior articular processes are concave, and look backward and inward ; 

 the inferior, convex, look forward and outward ; the former are separated by a 

 much wider interval than the latter, embracing the lower articulating processes 

 of the vertebra above. The transverse processes are long, slender, directed trans- 

 versely outward in the upper three lumbar vertebrae, slanting a little upward in 

 the lower two. They are situated in front of the articular processes, instead of 

 behind them as in the dorsal vertebrae, and are homologous with the ribs. Of the 

 three tubercles noticed in connection with the transverse processes of the twelfth 

 dorsal vertebra, the superior ones become connected in this region with the back 

 part of the superior articular processes, and have received the name of mammttlary 

 processes ; the inferior are represented by a small process pointing downward, 

 situated at the back part of the base of the transverse process, and called the 

 accessory processes : these are the true transverse processes, which are rudimental 

 in this region of the spine ; the external ones are the so-called transverse processes, 

 the homologue of the rib, and hence sometimes called costal processes. Although 

 in man these are comparatively small, in some animals they attain considerable 

 size, and serve to lock the vertebrae more closely together. 



The Fifth Lumbar vertebra is characterized by having the body much thicker 

 in front than behind, which accords with the prominence of the sacro-vertebral 

 articulation; by the smaller size of its spinous process; by the wide interval 

 between the inferior articulating processes ; and by the greater size and thickness 

 of its transverse processes. 



Structure of the Vertebrae. The structure of a vertebra differs in different 

 parts. The body is composed of light, spongy, cancellous tissue, having a thin 

 coating of compact tissue on its external surface perforated by numerous orifices, 

 some of large size, for the passage of vessels ; its interior is traversed by one or 

 two large canals, for the reception of veins, which converge toward a single large, 

 irregular aperture or several small apertures at the posterior part of the body of 

 each bone. The arch and processes projecting from it have, on the contrary, an 

 exceedingly thick covering of compact tissue. 



Development. Each vertebra is formed of four primary cartilaginous portions 

 (Fig. 116), one for each lamina and its processes, and two for the body. Ossifica- 

 tion commences in the laminae about the sixth week of foetal life, in the situation 

 where the transverse processes afterward project, the ossific granules shooting 

 backward to the spine, forward into the pedicles, and outward into the transverse 

 and articular processes. Ossification in the body commences in the middle of the 

 cartilage about the eighth week by two closely approximated centres, which speedily 

 coalesce to form one central ossific point. According to some authors, ossifica- 

 tion commences in the laminae only in the upper vertebrae i. e. in the cervical and 

 upper dorsal. The first ossific points in the lower vertebrae are those which are to 

 form the body, the osseous centres for the laminae appearing at a subsequent period. 

 At birth these three pieces are perfectly separate. During the first year the laminae 

 become united behind by a portion of cartilage in which the spinous process is 

 ultimately formed, and thus the arch is completed. About the third year the 

 body is joined to the arch on each side, in such a manner that the body is formed 

 from the three original centres of ossification, the amount contributed by the ped- 

 icles increasing in extent from below upward. Thus the bodies of the sacral 

 vertebrae are formed almost entirely from the central nuclei ; the bodies of the 

 lumbar are formed laterally and behind by the pedicles ; in the dorsal region the 

 pedicles advance as far forward as the articular depressions for the head of the 



