Biomedical Engineering Reference
In-Depth Information
a fifth extremity. Other families also have some functional
prehensibility in their tails but lack the pressure pads with
accompanying sensory receptors necessary for true
prehension. At one time, prehensile tails were thought to be
used primarily as a safety rope ( Jungers and Stern, 1981 ),
but recent studies have found they can be active participants
in the overall locomotor pattern especially during loco-
motion with the forelimbs in tension ( Schmitt et al., 2005 ).
Among Old World monkeys there is tremendous vari-
ability in tail length even among closely related species. No
Old World monkey has a prehensile tail even though in
some species young animals may occasionally wrap their
tails around supports and use their tails for balancing. In
general, tails of Old World monkeys appear to be used
primarily for balance ( Larson and Stern, 2006 ).
the upper thoracic vertebrae (generally T2 through T9) also
have a caudal (inferior) articulation on the body for contact
with the rib of the adjacent vertebrae. Spinous processes of
thoracic vertebrae are usually long and narrow and overlap
the spine of the adjacent more caudal (inferior) vertebra
( Figure 4.8D ). Most rotation of the vertebral column occurs
in the thoracic region, but flexion is negligible due to the
configuration of the articulations between the vertebral
arches, the presence of the ribs, and the thin intervertebral
discs.
The lumbar vertebrae (L1 to L7) constitute the region of
the vertebral column in higher primates with the most
variable number of segments. Some species usually have as
few as four lumbar vertebrae while others usually have
seven ( Figure 4.10F,G ). All lumbar vertebrae have large
bodies and large broad spinous processes. Their transverse
processes become progressively longer and more massive
from cranial to caudal. An exception is the last lumbar
vertebra, which may be smaller and in close proximity to
the adjacent borders of the ilia.
The number of lumbar vertebrae not only varies greatly
among species but there is also considerablevariabilitywithin
a single species. The most common number for Old World
monkeys is seven, for lesser apes five, and for great apes four,
whereas New World monkeys range from four in Ateles and
Lagothrix to six or seven in Cebus. The number of actual
vertebrae in this region can be roughly, but not precisely,
correlated with the locomotor behavior or functional role of
the region in a particular species. Erikson (1963) demon-
strated this correlation between the use of the region in
common patterns of locomotion and its functional length. The
functional (as opposed to morphologic) length of the lumbar
region is evaluated not by the presence or absence of ribs but
rather by the position of the articular facets, the length of the
spinous processes, and the location of the anticlinal vertebra.
Thus, in some species the functional length of the lumbar
region also includes a number of the lower thoracic vertebrae.
Although not a perfect fit, Erikson's analysis does provide
evidence of functional differences associated with morpho-
logical variability in the region. For example, among leapers
(e.g. Aotus) the functional lumbar region may exceed the
thoracic length while in brachiators (e.g. Ateles) the lumbar
region may be only slightly greater than half the thoracic
length ( Erikson, 1963 ). Most of the flexion and extension as
well as a large amount of the lateral bending of the vertebral
column occur in the lumbar region.
The sacral vertebrae (S1 to S5) of higher primates are
fused after infancy ( Figure 4.10H,I ). The sacrum of great
and lesser apes generally are the result of fusion of four or
five sacral vertebrae, whereas those of most Old World and
New World monkeys generally incorporate only three
sacral elements. The ala of the more cranial segments of the
sacrum are broad and offer an extensive articulation with
the ilia. The first sacral segment is always the largest with
Skeleton
The vertebral column is divided into five distinct anatom-
ical and functional regions. At birth the body, transverse
processes, spine, and lamina of the individual vertebrae are
generally separate, but soon the components of vertebral
arch ossify and fuse to form the vertebral canal. Failure to
do so will result in spina bifida (described in the section
“Prenatal development, congenital malformations, and
molecular basis of primate morphology” above). The
epiphyseal plates of the vertebral bodies face the interver-
tebral discs and are among the last to fuse in adulthood.
Most primates have seven cervical vertebrae (C1 to C7)
which are characterized by bodies with concave cranial
(superior) surfaces mirrored by convex caudal (inferior)
surfaces and slender, caudally (inferiorly) angled spines.
Details of clinically important modifications of C1 and C2
(Figures 10A
C, 4.11) are described in the section “Head
and neck morphology” (last paragraph in section “Skel-
eton”). The first six cervical vertebrae typically have
a foramen transversarium perforating each transverse
process and usually the vertebral artery enters the resultant
canal at the level of C6. The transverse processes and spine
of C7 are generally long and slender. The articulations
between the skull and C1 allow for nodding movements of
the head as if indicating “yes.” The articulations between
C1 and C2 allow for rotation or movement of the head as if
indicating “no.” Movements between the remaining
cervical vertebrae are primarily flexion and extension,
although lateral bending is also possible due to the thick-
ness of the intervertebral discs in this region.
Most primates have 12 thoracic vertebrae (T1 through
T12), although some individuals may have as many as 13 or
as few as 10. All thoracic vertebrae provide articulations for
a pair of ribs ( Figure 4.10D,E ). The rib of the same number
generally articulates with both the cranial (superior) part of
the body and the transverse process of the thoracic verte-
brae of the same number. In addition to these articulations,
e
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