28 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 13. — The sacrum seen from behind (dorsal surface) (J). 



Fig. 14. — The sacrum seen from in front (pelvic surface) (£). 



Fig. 15. — The sacrum seen from above (base) (£). 



Fig. 16.— Horizontal section of the sacrum at the level of the second sacral foramina (|). 



Fig. 17.— Median longitudinal section through a sacrum, showing the synchondroses between the indi- 

 vidual vertebras (f). 



Fig. 18. — The sacrum and coccyx seen from the side (£). 



Fig. 19. — The coccyx seen from in front ( T ). 



Fig. 20. — The coccyx seen from behind (y). 



The transverse processes of the lumbar vertebrae (Fig. 12) are long, directed almost at a 

 right angle from the spinal column, and distinctly compressed from before backward. They 

 represent rudimentary lumbar ribs and are therefore really the costal processes,* while a small 

 projection situated at the base of the "transverse process," the accessory process (Figs. 9 and 

 12), corresponds to the transverse process of the thoracic vertebras. This accessor}- process is 

 characteristic for the lumbar vertebrae, and although sometimes poorly marked, it usually ap- 

 pears even in the twelfth thoracic vertebra. The superior articular processes of the lumbar 

 vertebrae also exhibit another round roughened protuberance, the mammillary process (Figs. 

 9 and 12). 



The spinous processes (Figs. 9 and 12) are very strong and high and are distinctly com- 

 pressed from side to side. They extend directly backward and show a slight thickening at 

 their apices. 



The False Vertebrje. 



The false vertebra?, which are variable in number, form two bones, the sacrum and the 

 coccyx. 



THE SACRUM. 



The sacrum is that portion of the vertebral column which is connected with the pelvic 

 girdle and completes the latter posteriorly; it consequently forms a portion of the pelvis itself. 



It is a broad, curved, moderately flat, shovel-shaped bone (Figs. 13 and 14), which is broad 

 and thick above, and narrow and thin below (Fig. 18). In it there may be distinguished an 

 anterior relatively smooth surface, concave in both the sagittal and transverse directions, and 

 known as pelvic surface, because it looks toward the pelvic cavity, and a posterior extremely 

 rough dorsal surface. The broad upper surface of the sacrum is called the base and the lower 

 angle the apex. 



The pelvic surface presents a number of transverse ridges, usually four, which connect 

 four pairs of irregular rounded foramina known as the anterior sacral foramina. The ridges 

 correspond to the junctions of the five originally separate sacral vertebrae of which the bone 

 is composed, while the sacral foramina indicate the junctions of the bodies with the lateral pro- 

 cesses (costal and transverse processes). The anterior sacral foramina communicate pos- 



* These processes occasionally form so-called abdominal or lumbar ribs. 



