Biomedical Engineering Reference
In-Depth Information
the size decreasing in each successively more caudal
segment. The size of the last sacral segment and the height
of its neural arch are in part correlated with tail length and
the comparative thickness of the spinal segment nerves
innervating the tail (
Ankel-Simons, 2007
).
The caudal vertebrae vary tremendously in form and
number among the various species of primates and longer
length does not necessarily correlate with greater flexibility.
In all cases, however, caudal vertebrae become progres-
sively smaller in diameter and more modified in
morphology from the base of the tail to its tip (
German,
1982
). The most proximal caudal vertebrae closely
resemble other vertebrae and have relatively short bodies
allowing more mobility at
1933a
). The extrinsic muscles include hypaxial muscles of
the pectoral and pelvic girdles which have migrated to the
dorsal (posterior) midline and attached to the spines and
transverse processes of the vertebrae. In primates the
relatively short neck and dorsal (posterior) position of the
scapulae dictates the extent and positioning of these
pectoral girdle muscles (
Figure 4.8C
), particularly mm.
trapezius and latissimus dorsi, which are generally broader
in origin and shorter in length than in most other quadru-
peds. The innervation and blood supply to the extrinsic
muscles are established early in embryonic development
prior to their migration and thus their innervation is from
the ventral rami of spinal nerves. The only exception is m.
trapezius, which also receives innervation from the spinal
branch of cranial nerve XI (accessory nerve).
The intrinsic muscles of the back extend from the skull
to the tip of the tail (Figures
4
.
8
B
the base of
the tail. The
cranial
caudal dimension of the vertebral body initially
increases and becomes much greater than either the dorsal-
ventral or medial-lateral widths. Typical vertebral charac-
teristics are lost in an orderly fashion from proximal to
distal along the tail
e
F). In primates,
these muscles resemble typical mammalian epaxial
muscles with each muscle bundle extending for a limited
number of segments and overlapping with adjacent
bundles. Muscle fibers become progressively shorter in
more internally placed muscles. As in other mammals,
epaxial muscles are innervated in serial segments by dorsal
rami of spinal nerves.
The musculature of the tail is a direct continuation of the
intrinsic muscles attaching more cranially along the vertebral
column. In primates lacking external tails these muscles, like
the caudal vertebrae, are poorly developed. In other primates
these muscles are developed in direct relationship to the
length and dexterity of the tail. Among prehensile-tailed
New World monkeys these muscles are highly developed
(
Ankel-Simons, 2007; Lemelin, 1995; Organ, 2010
). As
already described, the epaxial musculature of the dorsal tail
is innervated by dorsal rami, which provide the nerve supply
to the more dorsally positioned lateral muscles of the tail as
well. The majority of the ventral and lateral musculature of
the tail is innervated by ventral rami of spinal nerves.
e
D,
12
A
e
spine, vertebral foramen, articular
processes, and transverse processes. The articulations
between adjacent vertebral bodies become rounded and
chevron bones are associated with the more proximal
elements. As morphology becomes more simplified the
cranial
e
caudal length of the vertebrae increases until the
longest vertebra and then decreases more distally.
The proximal and distal functional regions of the tail
correlate with morphological patterns on either side of the
longest vertebrae (Schmitt et al., 2005). Gradually the
entire size of the caudal segments becomes markedly
reduced. The length and amount of flexibility and mobility
in the caudal region vary considerably among species, but
all higher primates with external tails use them for balance
on arboreal substrates. The highly arboreal prehensile
tailed New World monkeys also use their tails as versatile
fifth appendages particularly during suspensory locomotion
or feeding, but overall among nonhuman primates tail
length and flexibility are not necessarily highly correlated.
The joints and ligaments of the vertebral column are
similar to those of humans with the exception that the
nuchal ligament is either absent or poorly developed in
nonhuman primates. Some ligaments such as the supra-
spinous and flavum ligaments contain a higher proportion
of elastic fibers than other ligaments. The back ligaments of
nonhuman primates are susceptible to the same patholo-
gies, including calcification, as humans. The positioning of
the articular facets in each region vary among species and
are correlated with functional regions, ranges of motion,
and locomotor and postural patterns.
e
Spinal Cord
The spinal cord is housed in the vertebral canal formed by
the arches of adjacent vertebrae (
Hines, 1933; Noback and
Moskowitz, 1963
). The meninges that surround the cord, as
well as the relative positions of the roots of spinal nerves to
their exits via intervertebral foramina, are similar to those
of other mammals including humans. Recent research on
spinal cord injury in nonhuman primates provides evidence
of marked similarities to humans (
Courtine et al., 2007
).
Clinically Significant Features of Back,
Spine and Tail Morphology
Clinically significant features of back and spine
morphology include the relative positions of spines of the
Musculature
Internally, the muscles of the back can be subdivided into
extrinsic and intrinsic groups with the latter group
including the muscles of the tail (
Howell and Straus,
