ON THE VERTEBRATE SKELETON. 265 
articular surface, and a thicker intermediate piece, as in all foetal mammals, 
and throughout life in some cetaceans. 
With respect to function, the centrum forms the axis of the vertebral 
column, and commonly the central bond of union of the peripheral elements 
of the vertebra: as a general rule it supports, either immediately or through 
the medium of the approximated or conjoined bases of the neurapophyses, 
the neural axis (in the trunk called myelon, or spinal marrow, and its mem- 
branes); the terminal centrums being usually deprived of this function by 
the withdrawal of that axis from them in the course of its centripetal or con- 
centrative movement. 
The newrapophyses are more constant as osseous or cartilaginous elements 
of the vertebrz than the centrums; but they are absent, under both histolo- 
gical conditions, at the end of the tail in most air-breathing vertebrates, where 
the segments are reduced to their central elements. The neurapophyses lose 
their primitive individuality by various kinds and degrees of confluence ; as 
e. g. first, of the bases of each pair with their supporting centrum ; secondly, 
of the apices of each pair with one another and with the neural spine,—the 
lepidosiren affording a rare exception of the persistent individuality of this 
element and of each neurapophysis throughout the trunk; thirdly, of two 
or more neural arches with one another, as in the neck of some fishes, cetacea, 
and armadillos, and in the sacrum of birds and mammals; where they also 
often coalesce with the pleurapophyses, as they do in the neck of most mam- 
mals and birds. The neurapophyses rarely depart from the form of plates, 
either broad or high, or both ; sometimes they are straight, sometimes arched, 
sometimes bent ; sometimes by the inward extension of their bases, they form 
together a bony ring above the centrum, excluding both that and the spine 
from the neural canal. The neurapophyses may develope, as exogenous pro- 
cesses, either diapophyses or zygapophyses, and the latter are sometimes 
double from both the anterior and posterior borders of the plates ; as e. g. in 
the vertebrz of Mugil, in some serpents, and in the lumbar vertebrz of some 
mammals. The observed extent of variation of position of the neurapophyses 
_ is from the upper surface of their own centrum to above the next intervertebral 
space, so as to rest equally on two centrums; or they may be uplifted bodily 
from their centrum, and wedged or suspended between the two contiguous 
neural arches, as e. g. in the atlas of ephippus and other deep-bodied fishes. 
Except in the cartilaginous neurapophyses of the sturgeon, I am not aware 
of any instance of the subdivision of this element into two pieces, placed 
vertically upon each other. Some plagiostomes show the principle of vegetative 
repetition in two or three star-like centres of ossification, side by side, in the 
primitive basis of the neurapophysis, but the second of the two cartilaginous 
_ plates on each side of the neural canal, coextensive with the single centrum, 
in most sharks, which second piece has the form of a wedge with the small 
end directed down over the intervertebral space, seems to answer, as Prof. 
Miller has suggested, to the intercalary or interneural piece in bony fishes. 
The most constant functional relation of the neurapophysis is to protect 
the spinal nerve in its exit from the spinal canal, either by a direct perfora- 
tion of the neurapophysis (many fishes, and some mammals), by a notch in 
the margin, or by the interspace between two neurapophyses. This function 
alone is performed, in reference to the nervous system, at the posterior part 
of the vertebral column in many animals, where the place of the shortened 
_ myelon is occupied by the lengthened roots of the nerves: in the rest of the 
trunk the neurapophyses protect also the neural axis. The original relation 
_ of each neurapophysis to the segments of that axis is determined by the place 
_ of connection of the perforating nerve with the shortened myelon. 
1846. T 
