Biomedical Engineering Reference
In-Depth Information
connects the hyoid bone to the mandible superiorly and the
larynx, trachea, sternum, clavicle, and ribs inferiorly. All
play an active role in movements of the hyoid bone,
particularly during swallowing.
As in humans, the thyroid gland consists of an isthmus
spanning the midline of the neck, expanding laterally and
superiorly into left and right lobes which embrace the
trachea and lateral esophagus inferiorly, and the laryngeal
cartilages and inferior constrictor muscles of the pharynx
superiorly ( Figure 4.8B ). In rhesus macaques, the isthmus
straddles the midline at the third tracheal ring, while the
lobes approach the hyoid bone cranially ( Miller and
Leonard, 1933 ). The gland is tightly bound to these respi-
ratory and digestive viscera by the pretracheal fascia of the
neck.
package, the pre-vertebral muscles are a deep group that
attach to the bodies, pedicles, and transverse processes of
the cervical vertebrae and the basioccipital of the skull.
Functioning as a unit they flex the neck and bend the head
forward.
The most dorsal (posterior) nuchal muscles of the neck
include superficial muscles, many of which during
embryonic development migrated secondarily into the
region. as well as deep muscles which are intrinsic to the
region. Superficially, the m. trapezius has a broad attach-
ment on the occipital bone and in many species of higher
primates this attachment is raised into a distinctive nuchal
crest. The size of the nuchal crest ( Figure 4.2 ) varies greatly
among species and between sexes ( Ashton and Zuckerman,
1956 ). The extent of its development is correlated with the
postural need to counteract the weight of the anterior part of
the skull and face in balancing the head on the neck. The
relatively short neck, broad chest, and concurrent dorsal
placement of the scapula seen in most primates results in
the superficial neck muscles having a slightly different
orientation than that commonly seen in nonprimate quad-
rupeds. The deeper, intrinsic muscles of the dorsal (poste-
rior) neck are continuous with, and a part of, the
longitudinal musculature which extends from the occipital
of the skull to the distal tip of the tail and are discussed in
detail in the section “Musculature” below.
Deeper Musculature
The deeper musculature of the head and neck region
( Figure 4.8B ) is divided into groups according to location
and function. The first group comprises the muscles of
mastication. The four pairs of muscles in this group are
mm. temporalis, mm. masseter, mm. medial pterygoid, and
mm. lateral pterygoid. Use of the first three of these pairs in
a single coordinated action closes the jaws. The last pair,
mm. lateral pterygoid, assist neck muscles in opening the
jaw. Various combinations of these muscles also result in
protraction, retraction, and side-to-side movement of the
mandible. These muscles are found in relatively the same
positions as in humans. However, they are consistently
developed to a far greater extent, often producing dramatic
cranial cresting, due to the preservation of a crucial DNA
segment coding for the powerful myosin MYH16 in
nonhuman primates ( Stedman et al., 2004 ).
The ventral (anterior) and lateral neck musculature in
higher primates is subdivided according to its position
relative to the visceral package (see also the section “Neck
viscera and thyroid and parathyroid gland” above).
(Contents of the visceral package are discussed in detail
elsewhere: pharynx with the oral cavity (see the section
“Oral cavity” above); trachea and larynx with the respira-
tory system (see the section “Respiratory system” below)
and pharynx and esophagus with the gastrointestinal
system (see the section “Gastrointestinal system and
spleen” below).).
Superficial and lateral to the visceral package in the
neck is what in humans is called the m. sternocleidomas-
toideus and deeper are the mm. scalenii. As a group these
muscles act either to move the head on the neck, flex the
neck, or elevate the rib cage, depending on which end of the
muscles is fixed. When used ipsilaterally they can laterally
bend the neck. By contracting muscles with similar fiber
directions, various combinations of muscles can also rotate
the neck and head. Dorsal (posterior) to the visceral
Skeleton
The skull of higher primates differs from that of other
animals in several features ( Figures 4.1, 4.2 ). First, the eyes
are completely surrounded by bony orbits. Lateral to each
eye, a complete postorbital bar is connected to the brain-
case by a bony postorbital septum which completely
divides the orbital cavity from the temporal region. This
change in the orbit results in the dissociation of the rostral
part of the zygomatic arch from the inferior aspect of the
orbit and makes the infratemporal fossa a discrete area. The
cranial vault is expanded to allow for the enlarged brain
found in higher primates. The external surface of the vault
may be marked superiorly by a sagittal crest delineating the
borders of well-developed mm. temporalis or posteriorly by
a nuchal crest marking the abutment of the attachments of
the mm. trapezius and posterior temporalis. These two
crests may appear separately or coalesce and may differ
greatly in size among closely related species and between
sexes. The degree of development of the sagittal crest is
thought to be related to the forces generated by the m.
temporalis, whereas the development of the nuchal crest
reflects the need to counter-balance a heavier face and
anterior skull on the neck ( Ashton and Zuckerman, 1956 ).
The overall size and shape of the primate skull differs
among species and between sexes. The skulls of Old World
( Figures 4.1, 4.2 ) and New World ( Figures 4.5, 4.9 )
Search WWH ::




Custom Search