90 OSTEOLOGY. 
fusion of the odontoid with the body is defined by a small dise of cartilage which persists 
within the substance of the bone till an advanced period of life. 
A pair of epiphyses placed over the tubercles of the spine, if not always present, are 
at least frequent. 
Sacrum.—Each of the sacral segments is ossified from three centres: one for the 
body, and two for the neural arch—that for the body, which makes its appearance in the 
first three sacral vertebrie 
about the end of the third 
month, about the fifth to the 
eighth month for the last 
two segments. From the two 
centres for the neural arches, 
which make their appearance 
Fic. 69,—OssIFICATION OF SACRUM. —_ aout the fifth or sixth month 
a.d. Centres for bodies ; bb. Epiphysial in the higher segments, the 
yates on odie ca, Cees fr cet Tamim, articular processes 
e.e. Lateral epiphyses. and the posterior half of 
the ale on either side are 
developed. The spinal canal is not enclosed till the seventh to the tenth year, the 
laminze usually failing to meet in the lowest segment, and occasionally, to a greater or 
less extent, in some of the higher segments. The anterior part of the lateral masses is 
developed from separate centres which represent the costal elements (Gegenbauer). These 
appear about the sixth to the eighth month, and may develop in relation to the upper 
four sacral segments ; more usually they are met with in connexion with the first three, 
and exceptionally they may be found only in the upper two. It is by fusion of these 
with the posterior arches that the lateral masses which support the innominate bones 
are formed. The costal elements fuse about the second to the fifth year with the neural 
arches, prior to their union with the centra ; and the segments of the lateral masses unite 
with each other sooner than the union of the bodies is effected. The latter only takes 
place after puberty by the fusion of the epiphysial plates, a pair of which make their 
appearance between the centra of each segment. The lower segments begin to unite 
together about the eighteenth year, but fusion between the first and second sacral verte- 
bre is not completed ‘till the tw enty-fifth year or after. In addition to the foregoing, two 
thin osseous laminze are developed in the cartilage covering the outer surface of the alar 
mass. The upper of these overspreads the auricular surface, whilst the lower forms the 
sharp edge below. The extremities of the upper spinous processes are occasionally 
developed from independent epiphyses. On making a mesial section of an adult bone 
the persistence of the intervertebral discs between the centra is indicated by a series of 
oval cavities. 
Coccygeal Vertebrz.—These are cartilaginous at birth. Each has a separate 
centre ; the first appears from the first to the fourth year, the second from the sixth to 
the tenth year, the third and fourth segments at or about puberty. Secondary centres, 
for the coccygeal cornua and epiphysial plates for the bodies are also described. Fusion 
of the various segments begins below and proceeds upwards, but is liable to great indi- 
vidual variation. In advanced life the coccyx is often ossified to the sacrum. 
SERIAL HOMOLOGIES OF THE VERTEBR. 
It is a self-evident fact that the vertebral column consists of a number of segments or verte- 
brie all possessing some characters in common. These vertebre or segments undergo modifications 
according to the region they occupy and the functions they are called upon to serve, so that 
their correspondence and identity is thereby obscured. There is no difficulty in recognising the 
correspondence of the bodies and neural arches throughout the column. “According to some 
anatomists the neural arch is the more primitive element i in the formation of a ver tebra, whilst 
others hold that the centra are the foundation of the column. Be that as it may, we find that in 
the higher vertebrates, at least, the bodies are the parts which most persist. They are, however, 
subject to modifications dependent on their fusion with one another. This occurs in the cervical 
part of the column where the centrum of the first cervical or atlas vertebra has for functional 
reasons become fused with the body of the second or axis vertebra to form the odontoid process 
of that segment. For similar reasons, and in association with the union of the girdle of the 
hind-limb with the column, the bodies of the vertebree which correspond to the sacral segment 
become fused together to form a solid mass. In the terminal portion of the caudal region 
the centra alone represent the vertebral segments. ; 
As regards the neural arch, this in man “pecomes deficient in the lower sacral region, and absent 
altogether i in the lower coceygeal segments. The spinous processes are absent in “the case of the 
