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piece, (neural spine, or neuracantha), which may be subdivided in 
the median plane. 
The inferior or heemal arch is also constituted when most developed 
(tail of the lepidosiren) by three elements ; the two lateral (heemal 
laminze or angiopomata) and the azygos inferior one (angiacantha 
or heemal spine), which is never subdivided. This arch is most 
generally present in the caudal region, disappears in the trunk, and 
reappears in the cervix. In man it only exists at the junction of the 
occipital and atlantal vertebrae, forming the so-called ‘body of the 
atlas,’ which is regarded by me as the hemal arch of the third 
cranial vertebra displaced backwards to the intervertebral interspace, 
as in the caudal region. 
The visceral arch, which is also inferior but external to the last, 
may be regarded as composed of an azygos inferior and two lateral 
elements. The former is the sternal segment and may be subdivided 
mesially. Each lateral piece is also resolvable generally into an upper 
segment (vertebral rib or pleura); and a lower one usually cartila- 
ginous (sternal rib or hypopleura), which may be subdivided into two 
or three pieces (three in Plesiosaurus). 
The segmentation of the vertebree is partly due to the laws which 
preside over their genesis, and partly determined by teleological causes. 
Several of the elements unite to form the vertebra of the anthro- 
potomist ; thus the constituents of the neural arch coalesce with the 
centrum in the dorsal vertebrze ; while in those of the cervical, lum- 
bar and sacral regions, the abortive pleural complements also are an- 
chylosed to the elements just mentioned. 
In fishes, the lower part of the vertebral body is formed by the ex- 
panded bases of the angiopomata, which meet those of the neuropo- - 
mata and enclose the proper centrum; but in the higher vertebrata 
the greater development of the centrum excludes the angiopomata 
from any share in the body, and displaces them backwards to the in- 
tervertebral interspace next in succession. 
The coexistence of the visceral and hzemal arches is seen in fishes, 
in the cervical region of many lacertz, and in the tails of the lizards 
and crocodiles, &c. 
Therefore the one is not convertible into the other, as has been 
supposed by Professor Owen, who regards the sternum and sternal 
ribs in the thorax as the equivalents of the angiacantha and angiopo- 
mata, the latter being dislocated from their normal attachment to the 
centrum and suspended to the extremities of the corresponding pleural 
elements constituting the sternal ribs, while the former is expanded 
and sometimes divided mesially to form the sternum. 
I am therefore compelled to suggest a new nomenclature of the 
elements of a typical vertebra more conformable to nature than 
that employed by Professor Owen, who has used the same term for 
several distinct objects, and given two different appellations to the one 
and the same element. 
My view of the typical vertebra is that which has been adopted 
by the distinguished German anatomists Miiller, Rathke, &c. 
