ON THE VERTEBRATE SKELETON, 329 
ie bird the expansion of the element 62 proceeds to a further extent, 
1 besides the proximal piece of the pleurapophysis of its own segment o2, 
is rought. into connection with the homologous stunted or proximal ends 
pl pleurapophyses of several contiguous segments, in the manner indicated 
the dotted line in fig. 28. Now, if the ilium, so expanded, were inter- 
tg as the coalesced complementary portions of all the short pleurapo- 
physes with which it articulates, its condition would be very similar to that 
which Oken has attributed to the scapula. But its ossification radiates, as 
in the simple rib-like ilium of the menopome, from a common centre: there 
are no corresponding multiplications of haemapophyses below; these are 
restricted in the pelvis of all animals to the number which they present in 
the crocodile. And since the scapula has been proved to be, under its most 
expanded form, the homologue of a single pleurapophysis, so also I am dis- 
posed to regard its homotype, the ilium, as maintaining under every variety 
of form and proportion, the same fundamental singleness of character, as it 
presents on its first appearance in the perennibranchiate batrachian. 
The first sacral vertebra, then, in man is complete; but its pleurapo- 
physis i is. divided, and the lower portion expanded to form the so-called 
‘ilium’ (62). The heemapophysis (6a) coalesces with that of the succeeding 
yertebra (63), and with its own pleurapophysis (62). 
_ The second sacral vertebra has its hemapophysis (63, called ‘ ischium ’) 
gssified, but separated from its proper pleurapophysis by the expanded (iliac) 
portion of that of the preceding vertebra, with which it coalesces, as well as 
with the preceding hemapophysis (pubis). The short and thick pleurapo- 
physes of the third sacral vertebra also articulate in the adult with the ex- 
panded distal portions of those of the first sacral vertebra: but these (iliac 
bones) are restricted in infancy and early childhood to their connections 
with the first and second sacral vertebre, which connections are permanent 
in most reptiles. 
The fourth sacral vertebra consists of centrum, neurapophyses, and rudi- 
mental pleurapophyses; the fifth sacral vertebra of centrum and rudimental 
neurapophyses, which rarely meet above the neural canal. 
_ In each sacral vertebra the elements of the neural arch and rudimental 
ribs first coalesce together; and afterwards the vertebrae unite with each 
ether and form the anthropotomical bone called ‘ sacrum.’ 
The first coceygeal vertebra in man consists of a centrum and of stunted 
eR wide apart above, but developing zygapophyses, which join 
those of the last sacral vertebra, and diapophyses which extend outwards 
further than those of the same vertebra. The neurapophyses are represented 
exogenous tubercles of bone in the second coccygeal vertebra ; and the 
ird and fourth vertebre are reduced to the centrums only. 
ae The cartilaginous deposits in the primitive blastema of this extremity of 
the. trunk indicate a greater number of caudal vertebre, and the rudimental 
tail i is proportionally longer in the embryo than in the adult. It is shortened, 
however, by absorption prior to the commencement of ossification, and but 
four segments are indicated by depositions of the earthy salts in the situa- 
tions proper to the above-specified elements of a typical vertebra: these 
finally coalesce into a single bone “ of a crooked pyramidal figure,” which 
got its name of ‘os coccygis’ from its supposed resemblance to a cuckoo’s 
beak +. 
The early recognition of these and other specialities arising out of the va- 
rious adaptive modifications of the typical segments of the human skeleton 
found its expression, necessarily, in special terms, the convenience of which 
* “ Shoulders of the os coccygis.”-—Monro, J. c. p. 142. t i. p. 141. 
1846. Zz 
