30 ANATOMY OF VERTEBRATES. 



in the full-grown. The third sacral has no pleurapophysis : its 

 l)arapophysis is a stumpy process ; its diapophysis is longer and 

 abuts against the ilium. In the fourth sacral both par- and 

 di-apophyses abut against the ilium. 



The neural arch of the fifth sacral vertebra has advanced and 

 rests over the interspace between its own and the preceding 

 centrum : this interlockino- relation continues to the eleventh 

 vertebra, where the arch resumes its normal position and con- 

 nections. The pleurapophyses of the fifth to the eleventh sa- 

 cral vertebrre inclusive have undergone a corresponding change 

 of position, and are synchondrosed by an expanded head to a 

 rough flat surface formed by the base of the neurapophysis and 

 by a portion of their own and of the preceding centrum : their 

 distal extremities expand and coalesce, forming a broad abutment 

 applied to the iliac bones. The diapophyses are directed upward 

 and outward against the same part, and are of considerable 

 length, especially in the ninth to the fifteenth sacral vertebra. 

 The dilated part of the neural canal is formed by the increased 

 breadth and flatness of the centrums, and by the wide expanse of 

 the neural arches at the middle of the sacrum. In the seventh 

 to the ninth of these arches there is a wide aperture in each 

 between the diapophysis and the base of the spine. The outlets 

 for the nerves are single and at the interspace of the neural 

 arches, but those at the middle of the canal show two grooves for 

 the separate exit of the motor and sensory roots. 



The spines of all the vertebra? are lofty, and already confluent 

 with each other at the middle of the sacrum. They are com- 

 pressed from before backward, consist of little more than a lace- 

 work of osseous tissue, and diverge in curves from the neural 

 arches, through the interspace between the iliac bones, with both 

 of which their lateral margins are confluent, and which they 

 thus serve to bind firmly together. By the peculiar cellular 

 and pneumatic structure of the parts, not more osseous texture 

 is expended in performing the oflfice of tie-beams across the 

 elongated roof of the pelvis than is absolutely required. The 

 last seven vertebrae are seen between the narrow parts of the 

 ilia produced backward beyond the acetabula, until full-growth, 

 when ossification extends from the summits of the spines bridging- 

 over the interval, leaving only a linear fissure on each side, fig. 24. 

 In the Cassowary a few pairs of foramina similarly indicate the 

 last three or four sacral-vertebras. 



In the Apteryx the first four sacral vertebrae send outwards 

 parapophyses which abut against the ilia, and progressively 



