ON THE VERTEBRATE SKELETON. 269 
» The hemal spine is much less constant as to its existence, and is subject 
to a much greater range of variety, when present, than is its vertical homo- 
type above, which completes the neural arch. Long, slender, and ‘ spinous’ 
in the tail, the hemal spine is reduced to a short and thick bone, often 
flattened, in the thorax of mammals, a series of thirteen such modified spines 
forming the so-called ‘sternum’ in the two-toed sloth: the thoracic hemal 
spines are few in number, and are expanded and perforated in the whales: 
the horizontal extension of this vertebral element is sometimes accompanied 
by a median division, or in other words, it is ossified from two lateral centres ; 
this is seen in the development of parts of the human sternum: the same vege- 
tative character is constant in the broader thoracic hemal spines of birds ; 
though, sometimes, as e. g. in the struthionide, ossification extends from the 
same lateral centre lengthwise, ¢. e. forwards and backwards, calcifying the 
connate cartilaginous homologues of halves of four or five hzemal spines, 
before these finally coalesce with their fellows at the median line. In some 
other birds, however, there are two or more lateral centres, and usually, 
also, a median one, from which the ossification of the keel extends down- 
wards, prior to its confluence with the rest of the ‘sternum. In the thorax 
of chelonians four hemal spines are established, each by two lateral centres 
of ossification, forming four pairs of sternal bones with a ninth ‘ entosternal’ 
piece between the first and second pairs. ~The ‘ plastron’ is the result of 
this extreme development of the hemal spines :—the modified moieties of 
which, remaining permanently distinct and united by suture, have received 
from Geoffroy St. Hilaire* the convenient special names of ‘ episternals,’ 
‘hyosternals,’ ‘ hyposternals’ and ‘xiphisternals,’ respectively, as they suc- 
ceed each other from before backwards. 
The diverging appendages are, as might be expected, of all the elements 
of the vertebral segment, the least constant in regard to their existence, and 
the subjects of the greatest amount and variety of modification. Simple 
slender spines or styles in fishes (fig. 17, aa), simple plates retaining long 
their cartilaginous condition in crocodiles, short flat slightly curved pieces in 
birds (fig. 15, a a), in some of the lowest species of which, e. g. Aptenodytes, 
they become expanded, like their homologues in the crocodile ; such, with 
one exception, is the range of the variety of form to which these parts are 
subject in the segments of the trunk. But that exception is a remarkable 
one: even under its normal ichthyic condition, as a simple style or filament, 
the diverging appendage of the insulated heemapophysial portion of the pelvie 
arch in the protopterust and lepidosiren{ is composed of many cartilaginous 
segments, and projects freely from the surface, carrying with it a smooth 
covering of integument. In other fishes similar filaments or jointed rays are 
progressively added to the sustaining arch, which cause a progressive expan- 
sion of the common investing fold of skin, forming the organ called the 
‘ventral fin,’ which is accordingly described by the ichthyologist as having 
two rays (Blennius), three rays (Zoarces), up to more than twenty rays, (as 
Acipenser in the sturgeons). 
When we quit the piscine class we find the diverging appendage of the pel- 
. * Du Sternum considerée dans les Oiseaux et dans les Poissons. Anatomie Philoso- 
phique, p. 69. pl. 2, fig. 21. Here Geoffroy contends that the parts of the hyoid arch (39, 
40 and 43) are the homologues of the modified hzmal spines which he calls episternals, hyo- 
sternals and hyposternals in the plastron of the turtle: but these names may well be retained, 
that of ‘ hyosternal’ being used in an arbitrary sense, without reference to the hypothesis 
which first suggested it. 
_.¥ Linn. Trans. vol. xviii. pl. 23, fig. 4, z. Lectures on Vertebrata, p. 79, figs. 27, 66. 
~ = Bischoff, op. git. pl. 2, fig. 5, c. 
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