observations may be advantageously premised on some of the
distinguishing characters of the Camelidæ. It is well known that the
Camels and Llamas deviate in their dentition, viz., in the presence of
two incisors in the upper jaw, from the true Ruminants; and we cannot
avoid perceiving that in this particular the direction in which they
deviate tends towards the conterminous Ungulate Order, in which incisor
teeth are rarely absent in the upper jaw. They also further deviate from
the Ruminants and approach the Pachyderms in the absence of cotyledons
in the uterus and fetal membranes; having, instead thereof, a diffused
vascular villosity of the chorion, as in the sow and mare.
But besides these characters, by which, in receding from one type of
hoofed mammalia, the Camelidæ claim affinity with another, there are
many parts of their organization peculiar to themselves; of some of
these peculiarities, the relation to the circumstances under which the
animal exists, can be satisfactorily traced; in others, the connection
of the structure with the exigencies of the species, is by no means
obvious, and in this predicament stands the osteological peculiarity,
which is immediately connected with our present subject—a peculiarity in
which the Camelidæ differ not only from the other Ruminants, but from
all other existing Mammalia, and which consists in the absence of
perforations for the vertebral arteries in the transverse processes of
the cervical vertebræ, the atlas excepted.
I may observe that what is described as a perforation of a single
transverse process in a cervical vertebra is essentially a space
intervening between two transverse processes, a rudimental rib, and the
body of the vertebra. In the cold-blooded Saurians,—in which the
confluence of the separate elements of a vertebra takes place tardily
and imperfectly, if at all,—the nature of the so called perforation of
the transverse process is very clearly manifested, as in the cervical
vertebræ of the Crocodile, in which the interspace of the inferior and
superior transverse processes is closed externally by a separate short
moveable cervical rib. In the Ornithorhynchus paradoxus the vertebra
dentata also preserves throughout life this condition of its lateral
appendages: in other Mammalia it is only in the fœtal state that the two
transverse processes are manifested on each side with their extremities
united by a distinct cartilage, which afterwards becomes ossified and
anchylosed to them.
In the Hippopotamus the inferior transverse process sends downwards a
broad flat plate extended nearly in the axis of the neck, but so
obliquely, that the posterior margins of these processes, in one
vertebra, overlap the anterior ones of the succeeding vertebra below,
like the cervical ribs in the Crocodile; the same structure obtains in
many other mammalia, especially in the Marsupials. In the Giraffe, the
inferior transverse processes are represented by relatively smaller
compressed laminæ, projecting obliquely downwards and outwards from the
anterior and inferior extremity of the body of the vertebra. The
superior transverse processes in this animal are very slightly developed
in any of the cervical vertebræ, and the perforation for the vertebral
artery is above and generally in front of the rudiment of this process,
being continued as it were through the side of the substance of the body
of the vertebræ.
In the long cervical vertebræ of the Camel and Llama, the upper and
lower transverse processes are not developed in the same perpendicular
plane on the sides of the vertebræ, but at some distance from each
other; the lower transverse processes (_a_, fig. 1, Pl. VI.; _a_, fig.
1, 3, 4, Pl. VII.) being given off from the lower part of the anterior
extremity of the body of the vertebra; the upper ones (_b_, fig. 1, Pl.
VI.; _a_, fig. 1, 3, 4, Pl. VII.) from the base of the superior arch
near the posterior part of the vertebra, or from the sides of the
posterior part of the body of the vertebræ. The extremities of these
transverse processes do not become united together, but they either pass
into each other at their base, or continue throughout life separated by
an oblique groove (as in fig. 1, Pl. VI.) This groove would not,
however, afford sufficient defence for the important arteries supplying
those parts of the brain which are most essential to life; and,
accordingly the vertebral arteries here deviate from their usual course,
in order that adequate protection may be afforded to them in their
course along the neck. From the sixth to the second cervical vertebræ
inclusive in the _Aucheniæ_, and from the fifth to the second inclusive
in the _Cameli_,[17] the vertebral arteries enter the vertebral canal
itself, along with the spinal chord, at the posterior aperture in each
vertebra, run forwards on the outside of the dura mater of the chord
between it and the vertebral arch, and when they have thus traversed
about two-thirds of the spinal canal, they perforate respectively the
superior vertebral laminæ, and emerge directly beneath the anterior
oblique or articulating processes, whence they are continued along with
the spinal chord into the vertebral canal of the succeeding vertebra,
and perforate the sides of the anterior part of the superior arch in
like manner; and so on through all the cervical vertebræ until they
reach the atlas, in which their disposition, and consequently the
structure of the arterial canals, resemble those in other Ruminants.
The two cervical vertebræ of the Macrauchenia present precisely the
structure and disposition of the bony canals for the vertebral arteries
which are peculiarly characteristic of the Camelidæ among existing
Mammalia. In Plate VI. fig. 2, the groove and orifices of the canal for
the vertebral artery are shown in a section exposing the spinal canal:
in Plate VII. figures 1 and 3 exhibit the orifices at the commencement
of the arterial canals, as seen in a posterior view of the vertebræ; in
figs. 2 and 4, the terminations of the same canals are shown, in the
anterior view of the same vertebræ; the smaller figures (3 and 4) are
taken from the fourth cervical vertebra of a Llama. The vertebræ of the
Macrauchenia also closely resemble the middle cervical vertebræ of the
Vicugna and Llama in their elongated form; approaching the Auchenial
division of the Camelidæ, and deviating from the true Camels in the
relations of the length of the body of the vertebra to its breadth and
depth, and in the much smaller size of the inferior processes. Excepting
the Giraffe, there is no existing mammal which possesses cervical
vertebræ so long as the Macrauchenia; but the cervical vertebræ of the
Giraffe, differ in the situation of the perforations for the vertebral
arteries, and in the form of the terminal articular surfaces, as will be
presently noticed.
Both of the cervical vertebræ of the _Macrauchenia_ here described, are
of the same size, each measures six inches and a half in extreme length,
two inches, ten lines in breadth, and two inches, four lines in depth.
In the Giraffe and the Camelidæ, the spinous processes are thin laminæ
of considerable extent in the axis of the vertebra, but rising to a very
short distance above the level of the vertebral arch: the spinous
processes have the same form in the corresponding vertebræ of the
Macrauchenia, but present a still greater longitudinal extent; they
commence at the interspace of the anterior oblique processes, and extend
to opposite the base of the posterior oblique processes; the upper
margin describing a gentle curve, as shown in fig. 1, Pl. VI. The
transverse processes also present the form of slightly produced, but
longitudinally extended, laminæ: their disposition is essentially the
same as in the Camelidæ, but more nearly corresponds with the
modifications presented by the Aucheniæ. The inferior transverse
processes,—those which are alone developed in fish, but which are not
present in any other vertebræ save the cervical, in mammalia,—these
processes in the Macrauchenia are continued from the sides of the under
surface of the anterior part of the body of the vertebra; their
extremities being broken off, it cannot be determined how far they
extended from the body of the vertebræ, but they gradually subside as
they pass backwards: the superior transverse processes are continued
outwards from the sides of the posterior part of the body of the
vertebra, and gradually subside as they advance forwards along
three-fourths of the body of the vertebra: they are not continued into
the anterior and inferior transverse processes, as in the Vicugna, but
are separated therefrom by a narrow and shallow groove. The articular,
or oblique processes, closely resemble those of the Auchenia in form,
and in the direction of the articular surfaces; those of the anterior
processes looking inwards and a little upwards; those of the posterior,
outwards and a little downwards.
In the Macrauchenia a small longitudinal process (_c_, fig. 2, Pl. VII.)
is given off immediately below the base of the anterior oblique process;
this structure is not observable in any of the cervical vertebræ of the
Giraffe or Camelidæ.
In the form of the articulating surfaces of the bodies of the vertebræ
the Macrauchenia deviates from the Giraffe and Camel, but resembles the
Aucheniæ. In the Giraffe and Camel the anterior articulating surface is
convex and almost hemispheric, the posterior surface is proportionally
concave, so that the cervical vertebræ are articulated by ball and
socket joints; yet not, as in most Reptiles, with intervening synovial
cavities, but by the concentric ligamentous intervertebral substance
characteristic of the Mammiferous class. In the Llama and Vicugna, the
degree of convexity and concavity in the articular surface of the bodies
of the cervical vertebræ is much less than in the Camels; and in
consequence they carry their necks more stiffly and more in a straight
line. In Macrauchenia the anterior articulating surface (fig. 2, Pl.
VII.) presents a still slighter convexity than in the Llama (fig. 4, Pl.
VII.), and the posterior surface (fig. 1, Pl. VII.) presents a
correspondingly shallower concavity. The form of the extremities of the
body of the vertebræ, especially of the posterior, is sub-hexagonal, the
breadth being to the depth as eight to five. The sides and under part of
the vertebræ are slightly concave; on the inferior surface there are two
ridges, continued forwards from the posterior margin of the vertebra,
each situated about an inch distant from the middle line; they converge
as they pass forwards, and are gradually lost in the level of the
vertebra; their greatest elevation does not exceed half an inch. In the
Aucheniæ there is a longitudinal protuberance in the mesial line,
instead of the two ridges. The two long cervical vertebræ of the
Macrauchenia are also characterized by the maintenance of an almost
uniform diameter of the body, both in its vertical and transverse
extent; the cervical vertebræ of the Vicugna come nearest to them in
this respect; those of the Camel deviate further in the large excavation
at the under part of the body.
The long vertebral or spinal canal offers a slight enlargement at the
two extremities; this structure which is generally in the ratio of the
extent of motion of the vertebræ on each other is more marked in the
Camel, where the form and mode of articulation of the bodies of the
vertebræ are designed to admit of a free and extensive inflection of the
cervical vertebræ; and the result of this structure is very obvious in
the sigmoid flexure of the neck in the living animal. In the Aucheniæ,
on the contrary, the neck is carried less gracefully erect and in an
almost straight line, and the form of the vertebræ and the nature of
their joints correspond, as we have seen, to this condition. From the
length of the bodies of the cervical vertebræ of the Macrauchenia, and
the almost flattened form of their anterior and posterior articular
surfaces, I infer that the long neck in this singular quadruped must
have been carried in the same stiff and upright position as in the
Vicugna and Guanaco.
The following individual differences are observable in the two cervical
vertebræ of the Macrauchenia;—in the posterior one the superior arch is
wider and with thicker parietes, the body is more concave below, and the
inferior transverse processes have a more lengthened origin.
Not a fragment of dorsal vertebræ, ribs or sternum, is included in the
collection of the bones of the Macrauchenia; but fortunately seven
lumbar vertebræ, forming a consecutive series of the same individual as
that to which the cervical vertebræ belonged, were obtained, all more or
less fractured, but all sufficiently perfect to demonstrate their true
nature. These vertebræ, although not possessing such distinctive
characters as the cervical, contribute by no means an unimportant
element towards the illustration of the osteology of the Macrauchenia,
and support the view which I have taken of its affinities; for,
although, as will be seen from the structure of its extremities, this
animal must be referred to the Order Pachydermata, yet no existing
species of that order has more than six lumbar vertebræ; whilst among
the Ruminants it is only in the Camel, Dromedary, Llama and Vicugna,
that the lumbar vertebræ reach the number seven,—the same number which
characterizes the extinct annectant species in question. The dimensions
of the vertebræ in the Macrauchenia present the same relations to the
two cervical vertebræ above described, which the lumbar vertebræ of the
Vicugna bear to the third, fourth, or fifth of its cervical vertebræ.
But here we begin to discover modifications of form, in which the
Macrauchenia deviates from the Camelidæ, and approaches the Pachyderms,
as the Horse and Hippopotamus; and these indications become stronger as
the vertebræ approach the sacrum.
In the Camel, as well as in the Horse and Hippopotamus, the bodies of
the lumbar vertebræ diminish in vertical extent, or become flatter, as
they approach the sacrum; but this character is more strongly marked in
the Macrauchenia than in either of the above species. But in the
Camelidæ the transverse processes of the lumbar vertebræ, are elongated,
flattened, and narrow, resembling ribs, except that they are nearly
straight; and this is more particularly the case with the transverse
processes of the last lumbar vertebræ, which are the narrowest of all in
proportion to their length, and stand freely out without touching the
sacrum. The transverse processes of the lumbar vertebræ of the Giraffe
resemble those of the Camel, but are relatively smaller and shorter. In
the Hippopotamus the transverse processes of the lumbar vertebræ are
much broader in proportion to their length than in any of the Ruminants,
and they increase in breadth to the last lumbar vertebra, which presents
in addition, the following characters; each transverse process sends off
from its posterior margin a thickened and transversely elongated
protuberance, which supports a flattened articular surface adapted to a
corresponding surface on the anterior part of the transverse process of
the first sacral vertebra: it likewise presents on its anterior edge a
flattened and rough surface, which is closely attached by ligamentous
substance to the opposite part of the transverse process of the
penultimate lumbar vertebra. A similar structure exists in the last two
lumbar vertebræ of the Rhinoceros, Tapir, and Horse. In the latter
animal, anchylosis of these articulating surfaces of the lumbar and
sacral vertebræ generally takes place with age, and, judging from the
character of the same surfaces in the Hippopotamus, the motion of its
lumbar vertebræ upon the sacrum may in like manner become ultimately
arrested.
Now in the Macrauchenia, as in the Pachyderms above cited, the
transverse processes of the last lumbar vertebræ are of considerable
thickness and extent, and are joined by enarthrosis to the transverse
processes of the sacrum; but the bony structure of these joints would
indicate that they were not subject to be obliterated by anchylosis. The
articular surfaces which project from the posterior part of the
transverse processes of the last lumbar vertebræ present a regular and
smooth concavity, adapted to a corresponding convexity in the transverse
processes of the first sacral vertebra. These articulating surfaces have
evidently been covered with smooth cartilage; they present a pretty
regular transverse ellipsoid form. A view of the three joints by which,
independently of the two oblique processes, the last lumbar vertebra of
the Macrauchenia was articulated with the sacrum, is given in Plate
VIII. fig. 1. The transverse processes of the posterior lumbar vertebra,
besides their agreement with those of the Horse and Hippopotamus in the
structure just described, also correspond with them in general form, and
deviate remarkably from those of the _Camelidæ_ in their great breadth.
It will be seen that the articulations on the body and transverse
processes of the last lumbar vertebra of the Macrauchenia differ from
the corresponding articular surfaces of the Horse, inasmuch as the
middle surface is convex, while the two lateral ones are concave, and
these are moreover relatively larger than either in the Horse or
Hippopotamus: by this structure the trunk was more firmly locked to that
segment of the vertebral column, which receives and transmits to the
rest of the body the motive impetus derived from the hinder extremities,
which are in all quadrupeds the chief powers in progression; while at
the same time the shock must have been diminished by the great extent of
interposed elastic cartilages; and a certain yielding or sliding motion
would be allowed between the lumbar vertebræ and sacrum.
The anterior oblique processes of the lumbar vertebræ of the
Macrauchenia (fig. 4, Pl. VIII.) have concave articular facets turned
towards, and nearly continued into, each other at their lower
extremities; so as to form together a deep semilunar notch, into which
the corresponding convex articular surfaces of the posterior oblique
processes of the adjoining vertebra (fig. 3, Pl. VIII.) are firmly
locked. In the close approximation of the two anterior concave articular
facets, which are separated from each other only by a vertical ridge,
and a rough surface of about three or four lines in breadth, the lumbar
vertebræ of the Macrauchene resemble those of the Horse, and differ from
those of the Camel tribe and Ruminants generally, in which those
surfaces are wider apart. In the hook-like form, however, of these
articular processes the lumbar vertebræ of the Macrauchene differ from
those of the Horse; and resemble those of many Ruminant species, and of
the Anoplothere;[18] but the degree of concavity of the articulating
surface is not so great in the Macrauchene. It would be interesting to
determine the relations which the lumbar vertebræ of the Macrauchene
bear to those of the Palæothere; but the indication which Cuvier gives
of the single lumbar vertebra, of which he had cognizance in the latter
genus[19] is too slight to enable me to enter upon the comparison.
The whole length of the lumbar region in the Macrauchene is twenty
inches. When the bodies of these vertebræ are naturally adapted
together, they form a slight curve, indicating that the loins of the
Macrauchene were arched, or bent downwards towards the sacrum. That the
lumbar vertebræ were rigidly connected together, or but slightly
flexible, is evident from the flatness of the articular surfaces of the
vertebral body, and by the circumstance of ossification having extended
along the anterior vertebral ligaments, and produced an anchylosis
between the fourth and fifth lumbar vertebræ; (fig. 2, _c_, Pl. VIII.)
This kind of ossification is frequent in aged horses, and I have seen an
example of a similar anchylosis of the lumbar vertebræ, by abnormal
deposition of bone in their anterior ligaments, in the skeleton of a
Hippopotamus preserved in the Senkenbergian Museum, at Frankfort.
In preparing the preceding account of the cervical and lumbar regions of
the vertebral column of the Macrauchene, I have felt frequently a strong
desire to enter into a comparison between them and the corresponding
vertebræ of the extinct Pachyderms of the Paris Basin. Some of these, as
the _Anoplotherium gracile_, in the length and slenderness of the
cervical vertebræ, resemble both _Auchenia_ and _Macrauchenia_; others,
as the _Palæotherium minus_, and probably the rest of the genus,
resemble the _Camelidæ_ and _Macrauchenia_ in having seven lumbar
vertebræ. Cuvier points out the resemblance which the atlas of the
Anoplothere bears to that of the Camel, and especially of the Llama;[20]
but he expressly notices the existence of the canals for the vertebral
artery in the fifth or sixth cervical vertebra of the _Anoplotherium
commune_.[21] Do the cervical vertebræ—say from the third to the sixth
inclusive—of the _Palæotherium_ present an imperforate condition of
their transverse processes, or exterior part of their sides? Cuvier, who
seems not to have been aware of this peculiarity in the _Camelidæ_,
merely notices the absence of these arterial foramina in the last
cervical vertebra of the _Palæotherium minus_,[22] which, unfortunately
for the comparison I am desirous of establishing, is that which most
commonly presents this imperforate condition in the Mammalia generally.
As, however, the cervical vertebræ of the Palæothere had the anterior
articular surface of the body convex, and the transverse processes
produced into descending laminæ, it is most probable that they
corresponded with the cervical vertebræ of the typical Pachyderms in the
condition of their arterial foramina.
The sacrum and ossa innominata in the present specimen of _Macrauchenia_
are very imperfect; but sufficient is preserved to show that the sacrum
was anchylosed to the ilia: the lower boundary of this anchylosis is
marked below by an external ridge, and by vascular canals and grooves in
the substance of the bone, as in the Hippopotamus. The body of the
sacrum is lost, but the smooth articular convexities upon the transverse
processes adapted to the articular depressions of the last lumbar
vertebra are fortunately preserved.