i88 
ANIMAL ACTIVITIES. 
A muscle shortens because the cells of protoplasm 
of which it is composed all have the power of contrac¬ 
tility. The cells 
all contract in one 
D | V*# V*.y direction, making 
^ the whole muscle 
Fig. 144.— Growth of Frog’s Egg. shorter and thicker. 
These contractions 
are under the control of the nerves. 
A number of muscle-cells make one of the fibrillae. 
Several of these fibrillar wrapped in a sheath form a 
fibre. The fibres are wrapped in bundles, and these 
bundles are covered by a thin layer of tough connective 
tissue. The sheaths or coverings of muscles, bundles, 
and fibres unite to form tendons by which the muscles 
are attached to the bones (Figs. 149 and 150). 
Muscles under the control of the will, like those in 
the leg of a frog or of a man, show peculiar cross-mark¬ 
ings. Muscles not under the control of the will, like 
those of the intestine or stomach, 
are commonly unstriped. 
The bony skeleton of the frog, 
on which the muscles act to cause 
the movements of the body, has 
an axis of nine movable bones, 
the vertebrae. Behind these a 
long bone, the urostyle, reaches 
to the hips, and in front is at¬ 
tached the head with its broad 
skull and large jaws. On the 
base of the skull are two rounded 
prominences called condyles, fig. 145.—Very Young 
which fit into corresponding de- Tadpoles, 
pressions on the atlas-bone, the 
first bone of the vertebral axis (Fig. 15 1). 
The fore limbs are attached to the axial skeleton by 
muscles and ligaments. A shoulder, or pectoral girdle, 
consisting of several bones is present. The hinder 
