205
CHAPTER SEVEN
Skeletal System
7.5
SKELETAL ORGANIZATION
Number of Bones
The number of bones in a human skeleton is often reported to
be 206
(table 7.3)
, but the number varies from person to per-
son. People may lack certain bones or have extra ones. For
example, the fl at bones of the skull usually grow together and
tightly join along irregular lines called
sutures.
Occasionally,
extra bones called sutural bones (wormian bones) develop in
Stimulus
Blood calcium
level increases.
too high
too low
Normal blood
calcium level
Control center
Thyroid gland
releases calcitonin.
Control center
Parathyroid glands
release parathyroid
hormone.
Receptors
Cells in the thyroid
gland sense the
increase in blood
calcium.
Receptors
Cells in the parathyroid
gland sense the
decrease in blood
calcium.
Effectors
Osteoblasts deposit
calcium in bones.
Effectors
Osteoclasts break
down bone to release
calcium.
Stimulus
Blood calcium
level decreases.
Response
Blood calcium
level is returned
to normal.
Response
Blood calcium
level is returned
toward normal.
FIGURE 7.13
Hormonal regulation of bone calcium resorption and
deposition.
TABLE
7.3
|
Bones of the Adult Skeleton
1. Axial Skeleton
a. Skull
22 bones
8 cranial bones
frontal 1
parietal 2
occipital 1
temporal 2
sphenoid 1
ethmoid 1
14 facial bones
maxilla 2
palatine 2
zygomatic 2
lacrimal 2
nasal 2
vomer 1
inferior nasal concha 2
mandible 1
b. Middle ear bones
6 bones
malleus 2
incus 2
stapes 2
c. Hyoid
1 bone
hyoid bone 1
d. Vertebral column
26 bones
cervical vertebra 7
thoracic vertebra 12
lumbar vertebra 5
sacrum 1
coccyx 1
e. Thoracic cage
25 bones
rib 24
sternum 1
2. Appendicular Skeleton
a. Pectoral girdle
4 bones
scapula 2
clavicle 2
b. Upper limbs
60 bones
humerus 2
radius 2
ulna 2
carpal 16
metacarpal 10
phalanx 28
c. Pelvic girdle
2 bones
hip bone 2
d. Lower limbs
60 bones
femur 2
tibia 2
F
bula 2
patella 2
tarsal 14
metatarsal 10
phalanx 28
Total
206 bones
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