58 



SPECIAL ANATOMY OF THE SKELETON 



versalis colli, Trachelomastoid, Complexus, Biventer cervicis, Semispinalis clorsi and colli, 

 Multifidus spinae, Rotatores spinae, Interspinales, Supraspinales, Intertransversales, Levatores 

 costarum. 



The Sacral and Coccygeal Vertebrae. 



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

 separate pieces, which are united in the adult so as to form two bones, five enter- 

 ing into the formation of the sacrum, four into that of the coccyx. Occasionally, 

 the coccyx consists of five bones. 1 



Sacrum (os sacrum). The sacrum is a large, triangular bone (Fig. 23), 

 situated at 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 in- 

 nominate bones; its upper part or base articulating with the last lumbar vertebra, 



FIG. 23. Sacrum, anterior surface. 



its apex with the coccyx. It is composed of five segments of bone. The sacrum 

 is curved upon itself, and placed very obliquely, its upper extremity projecting 

 forward, and forming, with the last lumbar vertebra, a very prominent angle, 

 called the promontory (promontorium), or sacrovertebral angle; while its central 

 part is directed backward, so as to give increased capacity to the pelvic cavity. 

 It presents for examination an anterior and posterior surface, two lateral surfaces, 

 a base, an apex, and a central canal. 



Surfaces. Anterior or Pelvic Surface (fades pelvina). The anterior surface 

 is concave from above downward, and slightly so from side to side. In the 



1 Sir George Humphry describes this its the usual composition of the coccyx. On the Skeleton, p. 450. 



