ON THE VERTEBRATE SKELETON. 275 
those riext in advance, we have the-circle, or the base bone (1) and arch 
(2,3, 4), represented in figure 1, and we also bring away, articulated therewith, 
an inferior or inverted arch with its appendages, represented in profile outline 
in fig. 5, 50-57: the arrow indicating the course of convergence, and its head 
the point of union, of the two flanks or crura, forming the closing point or 
crown of such inverted arch. 
We have thus removed a segment of the skull, and with as little or even 
less violence or disturbance to the other bones, than must have been used in 
detaching a similar segment from the thorax or pelvis of a land-animal. If 
we compare this cranial segment with the typical vertebra fig. 14, we recog- 
nise in the single median bone (1, fig. |) the centrum, by its relative position 
and its articular surface for the atlas, which retains, moreover, the concave 
form characteristic of the vertebre in the piscine class: in the pair of bones 
(2, 2), which articulate with the upper surface of the centrum, protect the 
sides of the epencephalon, and are perforated by the ‘ nervi vagi,’ we have the 
neurapophyses: in the single symmetrical bone (3) which completes the 
arch, and terminates in a crest for the attachment of the uppermost or dorsal 
portions of the vertebral muscles continued from the trunk, we have the newral 
spine: and in the pair of bones.(4, 4), wedged between this spine and the 
neurapophyses, which give attachment to the inferior arch of the segment 
(fig. 5, H i), and terminate in a free crest or spine for the attachment of the 
upper and lateral portions of the vertebral muscles, we have the parapo- 
physes ; for whose elevated position we have been prepared by their gradual 
ascent in the anterior vertebrz of the trunk. The rest of this natural segment 
has undergone the same kind of modification as the thoracic vertebre present 
in higher animals (fig. 15), and which consists in the great expansion of the 
hzmal arch, the removal of the hemapophyses (fig. 5, 52) from the centrum 
(ib. 1), and the interposition of elongated and deflected plewrapophyses (50, 51): 
finally, the great inverted arch, so formed, enconipasses, supports.and protects 
the heart, or centre of the hemal axis. The elements of this arch are open 
to two interpretations according to the type of figure 15: either 50 may be 
pl, 51, h and s2 hs; or 50 and 51 may be a divided (teleologically compound) 
pleurapophysis, and 52 an unusually developed heemapophysis : and this latter 
conclusion is more agreeable with the character of the vertebral segments of 
the trunk in fishes, in which the hemal spines are absent, the hemapophyses, 
when ossified, long and sometimes joined together at their lower ends, as é. g: 
in the first trunk- vertebra of Argyreiosus vomer, and the pleurapophyses some- 
times, as e. g. in the sturgeon, composed of two or more pieces, set end to 
end. The condition of the pleurapophysis of the pelvic arch in the meno- 
pome (fig. 28, 62, pl), which sustains a radiated appendage (ib. A) of the 
_ chemal arch of the occipital vertebra, indicates the true character of the 
_ pleurapophysis: and the modifications of this arch in the higher classes will 
be found to establish the accuracy of the general homvlogy of the bone 52; 
with the hemapophysial element, since the lower extremities of 52 are actu- 
ally drawn apart and articulated to a hzemal spine, which completes the arch 
_ below in reptiles and birds (fig. 22, Hs). boit | 
Even should there be error in assuming the subdivision of the pleurapo- 
physes and the absence of the hemal spine, in the particular determination of 
the constituent elements of the arch in question, yet the alternative is still 
within the recognised limits of the vertebral modifications of the trunk; and 
the want of unquestionable proof of the precise elements forms no valid ob- 
jection to its general homology as a hemal vertebral arch, expanded and modi- 
fied after one or other of the types of those which, in the thorax of the air- 
breathing vertebrates, encompass and protect the more backwardly placed 
