THE SACRUM 



39 



than behind in consequence of being bevelled off to form with the sacrum the sacro- 

 vertebral angle. The transverse processes are short, thick, conical, and spring 

 from the body as well as from the roots of the arch. They are very strong for the 

 attachment of the ilio-lumbar ligaments. The spinous process is smaller than 

 that of any of the other lumbar vertebrae; the laminae project into the vertebral 

 foramen on each side; and the roots are stout and flattened from above down- 

 ward. The inferior articular processes are separated to such a degree as to be 

 wider apart than the superior, and they articulate with the first sacral vertebra. 



The roots of the arch in this vertebra are hable to a remarkable deviation from the condi- 

 tions found in other parts of the spine. The pecuUarity consists of a complete solution in the 

 continuity of the arch immediately behind the superior articular processes. In such speci- 

 mens the anterior part consists of the body carrying the roots, transverse and superior articu- 

 lar processes; whilst the posterior segment is composed of the lamime, spine, and inferior articu- 

 lar processes. The posterior segment of the ring of this vertebra may even consist of two pieces. 

 There is reason to beheve that this abnormality of the fifth lumbar vertebra occurs in five g^ 

 cent, of all subjects examined. Sir William Turner, in his report on the human skeletons m 

 the Challenger Reports, found seven examples among thirty skeletons examined. The skeletons 

 in which this occurred were: — a Malay, an Andamanese, a Chinese, two Bushmen, an Eskimo, 

 and a Negro. Turner has also seen it in the skeleton of a Sandwich Islander. A similar con- 

 dition is occasionally met with either unilaterally or bilaterally in the thoracic vertebrae. 



( 



THE SACRUM 



The five sacral vertebrae (figs. 43, 44) are united in the adult to form the os 

 sacrum, a large, curved, triangular bone, firmly wedged between the innominate 

 bones, and completing, together with, the coccyx, the posterior boundary of the 



The S.^crum and Cocc'i'x. (Anterior view.) 



Iliacus 



Inferior lateral 

 angle 



Piriformis 



Cocc>geus 



Coccygeus 



Levator ant 



minor (or small) pelvis. Of the five vertebrae which compose the sacrum the 

 uppermost is the largest, the succeeding ones become rapidly smaller, and the fifth 

 is quite rudimentary. In the erect posture the sacrum lies obliquely, being di- 

 rected from above downward and backward, and forms ^^^th the last lumbar 

 vertebra an anterior projection known as the promontory. The sacrum presents 

 for examination a pelvic and a dorsal surface, two lateral margins, a base, and 

 an apex. 



Surfaces. — The pelvic surface, directed do^\^lward and forward, is smooth, 

 concave from above downward and slightly from side to side. It is crossed m the 



