ON THE VERTEBRATE SKELETON. 329 



In the bird the expansion of the element 62 proceeds to a further extent, 

 and besides the proximal piece of the pleurapophysis of its own segment 62, 

 is brought into connection with the homologous stunted or proximal ends 

 of pleurapophyses of several contiguous segments, in the manner indicated 

 by the dotted line in fig. 28. Now, if the ilium, so expanded, were inter- 

 preted 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 heemapophyses 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 is divided, and the lower portion expanded to form the so-called 

 ' ilium ' (62). The haemapophysis (64) coalesces with that of the succeeding 

 vertebra (63), and with its own pleurapophysis (62). 



The second sacral vertebra has its haemapophysis (m, called e ischium ') 

 ossified, 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 haemapophysis (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 vertebra?, 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 

 other and form the anthropotomical bone called ' sacrum.' 



The first coccygeal vertebra in man consists of a centrum and of stunted 

 neurapophyses* 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 

 by exogenous tubercles of bone in the second coccygeal vertebra ; and the 

 third and fourth vertebrae are reduced to the centrums only. 



The cartilaginous deposits in the primitive blastema of this extremity of 

 the trunk indicate a greater number of caudal vertebrae, and the rudimental 

 tail 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 f. 



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, /. c. p. 142. f lb. p. 141. 



1846. z 



