166 
COMPARATIVE ZOOLOGY. 
and also the tail, for locomotion and prehension, keeping 
a horizontal attitude; while the Apes, half erect, as if 
they were half-quadruped, half-biped, go shambling along, 
touching the ground with the knuckles of one hand and 
then of the other. In descending the scale, from the 
most anthropoid Ape to the true Quadruped, we find the 
centre of gravity placed increasingly higher up—that is, 
farther forward. Birds and Men are tbe only true bipeds ; 
the former standing on their toes, the latter on the soles 
of the feet. Terrestrial Birds walk and run ; while Birds 
of flight usually hop. The Ostrich can for a time outrun 
the Arabian Horse; and the speed of the Cassowary ex¬ 
ceeds that of the swiftest Greyhound. 
CHAPTER, XVIII. 
THE NERVOUS SYSTEM. 
Nervous Matter exists in the form of cells, fibres, or 
tubes. In the cellular state it is grayish, and accumulated 
b in masses, called 
ganglia , or centres , 
which alone origi- 
© 
nate nervous force; 
the fibrous and tu¬ 
bular kinds are gen¬ 
erally white, and 
arranged in bun¬ 
dles, called nerves , 
which serve only as 
conductors. Most 
nerves contain two 
kinds of fibres, like 
in structure, but 
Fig. 132. — Nerve-cells from Human Brain: A, associ¬ 
ated with nerve-tubes and blood-vessels; B, multi¬ 
polar nucleated cells. 
