188 ANIMAL ACTIVITIES. 
( 
\ 
A muscle shortens because the cells of protoplasm 
tility. The cells 
(~) all contract in one 
SZ SZ the whole muscle 
Fic. 144.—Growth of Frog’s Egg. shorter and thicker. 
are under the control of the nerves. 
A number of muscle-cells make one of the fibrille. 
fibre. The fibres are wrapped in bundles, and these 
bundles are covered by a thin layer of tough connective 
and fibres unite to form tendons by which the muscles 
are attached to the bones (Figs. 149 and 150). 
the leg of a frog or of a man, show peculiar cross-mark- 
ings. Muscles not under the control of the will, like 
are commonly unstriped. 
The bony skeleton of the frog, 
the movements of the body, has 
an axis of nine movable bones, 
long bone, the urostyle, reaches 
to the hips, and in front is at- 
skull and large jaws. On the 
base of the skull are two rounded 
which fit into corresponding de- Tadpoles. 
pressions on the atlas-bone, the 
The fore limbs are attached to the axial skeleton by 
muscles and ligaments. A shoulder, or pectoral girdle, 
of which it is composed all have the power of contrac- 
direction, making 
These contractions 
Several of these fibrilla wrapped in a sheath form a 
tissue. The sheaths or coverings of muscles, bundles, 
Muscles under the control of the will, like those in 
those of the intestine or stomach, 
on which the muscles act to cause 
the vertebra. Behind these a 
tached the head with its broad 
prominences called condyles, Fre, 145.—Very Young 
first bone of the vertebral axis (Fig. 151). 
consisting of several bones is present. The hinder 
