18 



NATURAL HISTORY. 



motion among the vertebrae ; while in the body of the 

 animal the motion between them is slight. 



9. In the snake tribe of Vertebrates the vertebras are 

 very numerous, and the motion between them is as free 

 as in the tails of quadrupeds. Some species have over 

 three hundred, while in man there are only twenty-four. 



10. The skeletons of the different kinds of animals that 

 I have mentioned differ from each other in many respects. 

 For example, the fish has nothing in its skeleton that is 

 like the bones of the extremities in man, and that of the 

 serpent is composed merely of vertebrae, with very short 



ribs. There are some fishes that have 

 no ribs. In the turtle, as you see in Fig. 

 6, the ribs spread out into broad plates, 

 which, joined together, make its upper 

 covering, termed the carapace. 



11. While the differences are of ex- 

 treme variety, the skeletons of all these 

 animals agree in one thing in having a 

 vertebral column. They are, therefore, 

 classed together as vertebrate animals. 



12. Connected with this grand char- 

 acteristic of this division of the animal 

 kingdom there is another, viz., the ar- 

 rangement of the great central organs 

 of the nervous system. These are in- 

 closed in the skull and vertebral column. 

 The brain is in the skull, and the verte- 

 bras contain the spinal marrow, which 

 extends from the brain through the 

 length of the body. Each vertebra has 

 a round opening through it, as you see 

 in Fig. 2. When, therefore, all the ver- 

 tebras are joined together, there is a 

 tube-like passage through the column. 

 In this lies the spinal marrow, or cord, 



rf Ma d n Spi " as it is often called. In Fig. 7 you have 



