FALSE VERTEBRA. 151 



tlie muscles of the right side, stronger than those of the left, 

 draw the vertebra in that direction, as Ludwig first pointed out. — 



False VertebrcB. 



The lower pyramid or under part of the spine, consists of one 

 large triangular bone, called the os sacrum, and of some small 

 bones, denominated the os coccygis. 



These bones are called the false vertebrae, because the sacrum 

 in young subjects is composed of five distinct bones, each of 

 which has some resemblance to a vertebra ; but they are com- 

 pletely united in the adult, and form but one bone, which is 

 supposed to have been denominated sacrum, because it was 

 offered in sacrifice by the ancients. 



The OS sacrum is of a triangular form, with its base upwards. 

 It is concave anteriorly, and convex posteriorly. The middle 

 of the bone, when viewed anteriorly, appears to be composed of 

 the bodies of five vertebrae, united to each other, and their union 

 is marked by four transverse lines. At the two extremities of 

 each of these lines, are large round holes, which communicate 

 with the vertebral cavity of the bone. 



On the exterior sides of these holes the surface is free from 

 any marks of the original separation. 



The middle of the upper surface, or base of the bone, is 

 formed for articulating with the last lumbar vertebra, and has 

 two oblique processes, with a groove in each side, which forms 

 part of the foramen for transmitting the twenty-fourth pair of 

 nerves. 



The back part of the os sacrum is rough and convex ; in the 

 middle there are commonly three processes similar to the spinous 

 processes of the lumbar vertebrae, and a fourth, which is much 

 smaller. Below this, there is a deficiency of the bony spine, 

 and the vertebral cavity is consequently open behind, but the 

 sides of the canal continue lower down. 



On each side of the spinous processes are four smaller holes, 

 which are opposite to the larger holes on the anterior surface. 

 Between the spinous processes and the anterior part, which 



