PELVIC VERTEBRAE. 453 



/ On the upper surface, in the centre, is an oval articula- 

 ting surface for uniting with the last lumbar vertebra. On 

 each side of this is seen a broad triangular expansion, upon 

 which rests the psoas magnus muscle, and lumbo-sacral 

 nerve. The anterior edge of this expansion is continuous 

 with the linea ileo-pectinea. Between this expansion and 

 the oblique process is a groove for the fifth lumbar nerve. 

 The lower extremity of the sacrum is truncated and pre- 

 sents a small oval surface for articulation with the coccyx. 



The structure of the sacrum is mostly spongy and cancel- 

 lated, thick, yet very light, and covered by a thin compact 

 layer. Its development occurs usually from twenty-one 

 points of ossification ; five for each of the three superior 

 divisions, one for the body, one for each lamina, and one 

 for each lateral portion. The two lower divisions have each 

 three points, one for the body and one for each lateral por- 

 tion. As many, however, as thirty-four and thirty-five 

 points of ossification have been observed. Ossification is 

 observed to commence in the bodies of the three upper sa- 

 cral vertebras, about the eighth or ninth week, a little later 

 than that of' the true vertebrae, in the two lower about the 

 fifth month, and in the lateral portions from the sixth to 

 the ninth month. The union of these latter with the body 

 is found to take place from below upward, and in the fol- 

 lowing order : the fifth piece about the second year, and the 

 first not before the fifth or sixth year. The epiphyses are 

 developed about the fifteenth or eighteenth year, and the 

 sacrum is completely fused into one piece by the thirtieth 

 year. 



It is articulated above with the last lumbar vertebrae, 

 below with the coccyx, and laterally with the two ossa 

 innominata. 



Coccyx (xoxxvf , cuckoo, from likeness to a cuckoo's beak,) 

 is situated at the lower extremity of the sacrum, and cor- 

 responds to the tail of the inferior animals, (Fig. 131.) It 

 consists of four caudal vertebrae, sometimes only three, 

 which in the old are often consolidated into two or a single 

 piece. Its shape is triangular, the base above, broad and 



