106 OSTEOLOGY 





a rough, uneven border, thickest below where it is occasionally notched. The 

 superior and inferior articular processes are well-defined, projecting respectively 

 upward and downward from the junctions of pedicles and laminae. The facets 

 on the superior processes are concave, and look backward and medialward; those 

 on the inferior are convex, and are directed forward and lateral ward. The former 

 are wider apart than the latter, since in the articulated column the inferior articular 

 processes are embraced by the superior processes of the subjacent vertebra. The 

 transverse processes are long, slender, and .horizontal in the upper three lumbar 

 vertebrae; they incline a little upward in the lower two. In the upper three verte- 

 brae they arise from the junctions of the pedicles and laminae, but in the lower 

 two they are set farther forw r ard and spring from the pedicles and posterior parts 

 of the bodies. They are situated in front of the articular processes instead of behind 

 them as in the thoracic vertebrae, and are homologous with the ribs. Of the three 

 tubercles noticed in connection with the transverse processes of the lower thoracic 

 vertebrae, the superior one is connected in the lumbar region with the back part 

 of the superior articular process, and is named the mammillary process; the inferior 

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

 accessory process (Fig. 93). 



The Fifth Lumbar Vertebra (Fig. 94) is characterized by its body being much 

 deeper 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 articular processes; and by the thickness of its transverse 

 processes, which spring from the body as well as from the pedicles. 



The Sacral and Coccygeal Vertebrae. 



The sacral and coccygeal vertebrae consist at an early period of life of nine 

 separate segments which are united in the adult, so as to form two bones, five 

 entering into the formation of the sacrum, four into that of the coccyx. Some- 

 times the coccyx consists of five bones; occasionally the number is reduced to 

 three. 



The Sacrum (os sacrum). The sacrum is a large, triangular bone, situated 

 in the lower part of the vertebral column and at the upper and back part of the 

 pelvic cavity, where it is inserted like a wedge between the two hip bones; its 

 upper part or base articulates with the last lumbar vertebra, its apex with the 

 coccyx. It is curved upon itself and placed very obliquely, its base projecting 

 forward and forming the prominent sacrovertebral angle when articulated with 

 the last lumbar vertebra; its central part is projected backward, so as to give 

 increased capacity to the pelvic cavity. 



Pelvic Surface (fades pelvina). The pelvic surface (Fig. 95) is concave from 

 above downward, and slightly so from side to side. Its middle part is crossed 

 by four transverse ridges, the positions of which correspond with the original 

 planes of separation between the five segments of the bone. The portions of bone 

 intervening between the ridges are the bodies of the sacral vertebrae. The body 

 of the first segment is of large size, and in form resembles that of a lumbar vertebra; 

 the succeeding ones diminish from above downward, are flattened from before 

 backward, and curved so as to accommodate themselves to the form of the sacrum, 

 being concave in front, convex behind. At the ends of the ridges are seen the 

 anterior sacral foramina, four in number on either side, somewhat rounded in form, 

 diminishing in size from above downward, and directed lateralward and forward; 

 they give exit to the anterior divisions of the sacral nerves and entrance to the 

 lateral sacral arteries. Lateral to these foramina are the lateral parts of the sacrum, 

 each consisting of five separate segments at an early period of life; in the adult, 

 these are blended with the bodies and with each other. Each lateral part is tra- 



