224 ANIMAL ACTIVITIES. 
and very many have two sets, the milk-teeth and the 
permanent teeth. The visceral cavity is divided by 
the diaphragm into two parts, the thorax or chest, and 
the abdomen. External ears are present in most cases. 
The heart has four chambers (Fig. 179). 
The nervous system differs from that of other verte- 
Fic. 183.—Fore Foot Fic. 184.—Fore Foot of Cat. 
of Mole. a, with claws extended; 
6, with claws drawn in. 
brates largely in the greater size of the cerebrum or 
forebrain. ; 
Fingers and Toes. As we have already pointed 
out, the mammals have normally five fingers for each 
hand or fore foot, and five toes for each hind foot. In 
many different ways these digits have been changed 
and adapted to their special work, yet the fundamental 
plan of structure has remained unaltered. Our own 
hands are the most perfect animal tools in existence, 
capable of manifold and delicate movements, but they 
do not differ as much from the fore feet of cats, bats, 
or even whales as we might at first suppose. The chief 
point of superiority in the human hand is found in the 
position and scope of movement of the thumb. In 
some of the higher monkeys the thumb is opposed to 
the fingers very much as in man, but in all cases the 
