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 witli 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 the 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 and 

 fibres. In the cellular state it is grayish, and accumu- 

 lated in masses, 

 called ganglia, or 

 centres, which 

 alone originate 

 nervous force; 

 the fibres are gen- 

 erally white, and 

 arranged in bun- 

 dles, called nerves, 

 which serve only as 

 conductors. Most 

 nerves contain two 



'i<j. 132. Nerve-cells from Human Brain: A, associ- kinds of fibres, like 

 ated with nerve-tubes and blood-vessels; B, multi- . 

 polar nucleated cells. Highly magnified. m Structure, Dllt 



* See Appendix. 



