30 



A MANUAL OF ANATOMY 



on the upper surface. The union between the odontoid process and the 

 body is indicated by a small cartilaginous disc in the centre, which persists 

 until advanced life. The odontoid process is to be regarded as the original 

 body of the atlas for the following reasons: (i) in the embryo the notochord 

 passes through its cartilage; (2) the notochord presents a swelling between 



. Appears in the 4th 



Year, and joins in 



the 1 2th Ytar 



Appear in the 5th Month 



and coalesce in the 



6th Month 



Appears in the 4th 



Month of 

 intra-uterine life 



Body Odontoid Process (joins Body 

 about the 4th Year) 



Fig. 21. — Ossification of the Axis. 

 A, At the Fifth Month; B, at the Fourth Year. 



the cartilage of the odontoid process and that of the body of the axis, as it 

 does in the case of other vertebrae; (3) there is a cartilaginous disc concealed 

 within the odontoid process, which is persistent until advanced Ufe; (4) the 

 odontoid process has two primary centres of ossification ; (5) in chelonians 

 it forms a separate ossicle; (6) a permanently separate odontoid process 

 in man has been recorded. 



The False Vertobrse. 



The false vertebrae are usually nine in number, the upper five of 

 which form the sacrum, and the lower four the coccyx. 



The Sacrum. 



The sacrum lies below the fifth lumbar vertebra, and is wedged 

 in between the ossa innominata, where it forms the greater part 

 of the posterior wall of the pelvis, its direction being downwards 

 and backwards. The sacral vertebrae diminish in size from above 

 downwards, which renders the bone triangular, the base being 

 upwards. 



The ventral or pelvic surface, which is directed downwards and 

 forwards, is concave from above downwards, and from side to side. 

 It presents along the centre a solid mass, representing the ankylosed 

 bodies and ossified intervertebral discs, which is marked by four 

 transverse ridges situated at the places of junction. Superiorly it 

 presents a projecting lip, called the promontory. On either side there 

 is a row of anterior sacral foramina, four in number, which diminish 

 in size from above downwards, and are directed outwards and for- 

 wards from the intervertebral foramina, by means of which they 

 communicate with the sacral canal. They transmit the anterior 

 primary divisions of the first four sacral nerves. 



The lateral masses are situated external to the anterior sacral 



