Chapter 8
Chapter 8
Chapter 8
Axial Skeleton
Skull
Form in lateral, ventral body
Ribs
Sterna
walls
Vertebral column forms around, sometimes invade
notochord during ontogenesis
VERTEBRAL COLUMN
Keystone of vertebrate skeleton
Segmented, arched rod flanked by axial musculature (where
head is attached):
Hypocentrum (intercentrum) large, median, Ushaped anterior bone that cradled notochord
8 cervical vertebrae
Remainder column of birds and turtles (except caudal
vertebrae) rigid
Mammals always 7 cervical vertebrae
Tail regenerates
Fishes
-
Tetrapods
Early tetrapods ribs associated w/ vertebrae from atlas to
nearly base of tail; later became restricted in length and
number (short ones ankylosing with transverse processes)
Long ribs of anterior thoracic region sternum
Tetrapod rib occupies same location of ventral ribs of fishes
encircle coelomic cavities
Tetrapod rib and ventral fish rib not homologous (tetrapod rib
homologous to dorsal fish rib)
Most are bicipital
Birds
-
Mammals
Recognizable ribs in mammals confined to thorax
Number ranges from 9 pairs in some whales to 24 in some
sloths; 12 pairs are common
Larger then 10, rest are floating; costal cartilages fail to
reach the sternum
Humans and apes other than orangutans 12 pairs of thoracic
ribs; apes addl 2 pairs of lumbars, humans frequently have
an addl rib , cervical or lumbar, as an anomaly
Mammalian embryos 2 heads of a vestigial bicipital rib
develop in association w/ each cervical vertebra. One of the
heads attached to a transverse process, the other to a centrum
Reptiles