794 



THE POPULAR SCIENCE MONTHLY. 



as those of the horse, the pig, and of the ruminants, the ends of the 

 toes are applied to the ground, and are covered with larger hoofs, 

 which surround the toe, and the cushion is nearly or quite dispensed 

 with. These animals are especially distinguished by the fact that their 

 metapodial keels extend entirely round the end of the bone, dividing 



the front, as well as the end and back, 

 into two parts. This structure would 

 seem to be a result of the greater force 

 of the impact resulting from use of the 

 legs, experienced by the end and front 

 of the bone, which receives the blows. 



I cite one more instance of the effect 

 of use on the structure of the skeleton, 

 and this time from the vertebral col- 

 umn. In a certain order of Batrachia 

 "- from the Permian formation (the Kha- 

 chitomi), the bodies of the vertebras are 

 curiously segmented. Instead of a solid 

 slightly modified from a cylinder, as in 

 most vertebrates, we have three pieces. 

 One of these is of nearly the form of a 

 segment cut from an apple or orange 

 having a crescentic outline and wedge- 

 shaped section. The sharp edge is con- 

 cave and is directed upward. On one 

 side of each of the horns of this cres- 

 cent a rhombic piece is applied, which, 

 widening upward, supports the separate 

 arch of the vertebra. These three pieces 

 when together leave a central vacancy, 

 which becomes, when all the vertebras 

 are placed end to end, a canal or tube. 

 This was occupied by what is known as 

 the chorda dorsalis, which is the cen- 

 tral axis of the body of the simplest 

 vertebrates, and is present in the early 

 embryonic stages of all. In the growth 

 of a reptile or mammal this flexible 

 chorda is replaced by the bony verte- 

 bral bodies. The osseous material ap- 

 pears in the membranous sheath which 

 covers it, and, gradually encroaching 

 on it, first cuts it into segments and 

 then fills it entirely. In the rhachito- 

 mous batrachia this process is not com- 

 pleted, as the chorda remains more or 



a 

 .2 



03 

 +h 

 P 

 <X> 



a 



to 



SI 



a 



o 



.a 



as 

 3 

 C3 



J3 



a 

 .o 







o 



fcu 



I 



z 



3 



o 





O 

 &h 



o 



S5 

 



J 

 O 



K 



Id 

 Eh 



K 

 M 



o 

 Eh 



k 

 < 



\ 



in 



2 



