Anatomy and Physiology - Introduction to Biomedical Engineering - page 118

coordinating skeletal muscle contractions and impulses for voluntary muscular movement
that originate in the cerebral cortex. The cerebellum is a processing center that is involved
with coordination of balance, body positions, and the precision and timing of movements.
The gray matter of the spinal cord is divided into the dorsal and ventral horns. In a
human, standing upright, the “dorsal” horn is posterior and the “ventral” horn is anterior.
Dorsal horn neurons receive and process sensory information from the skin, while ventral
horn neurons participate in the control of skeletal muscle contraction. The gray matter is
surrounded by columns (funiculi) of white matter containing ascending and descending
axons. The spinal cord communicates with the periphery via the dorsal and ventral root
fibers that exit between the bony vertebra. Dorsal root fibers bring information to the spinal
cord, and ventral root fibers carry information away from the spinal cord (Figure 3.30c).

3.4.4 The Skeletal System
The average adult skeleton contains 206 bones, but the actual number varies from
person to person and decreases with age as some bones become fused. Like the body, the
skeletal system is divided into two parts: the axial skeleton and the appendicular skeleton
(Figure 3.31). The axial skeleton contains 80 bones (skull, hyoid bone, vertebral column,
and thoracic cage), whereas the appendicular skeleton contains 126 (pectoral and pelvic
girdles and upper and lower extremities). The skeletal system protects and supports the

SKULL

CLAVICLE

MANDIBLE

SCAPULA

HUMERUS

STERNUM

RIB
VERTEBRAL
COLUMN

ULNA

PELVIS
SACRUM
COCCYX

RADIUS

CARPALS
METACARPALS
PHALANGES

FEMUR

PATELLA

TIBIA

FIBULA

METATARSALS

TARSALS
PHALANGES
FIGURE 3.31 The skull, hyoid bone (not shown), vertebral column, and thoracic cage (ribs, cartilage, and ster-
num) make up the axial skeleton, whereas the pectoral (scapula and clavicle) and pelvic girdles and upper and
lower extremities make up the appendicular skeleton.