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 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 XYIII. 



THE NERVOUS SYSTEM. 



Nervous Matter exists in the form of cells, fibres, or 

 tubes. In the cellular state it is grayish, and accumulated 



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 



Fio. 132. Nerve-cells from Human Brain: A, associ- kinds of fibres, like 

 ated with nerve-tnbes aud blood-vessels; B, multi- . , 



polar nucleated cells. in structure, but 



