THE MOTIONS OF ANIMALS 73 



The plan of the skeleton in the vertebrates is the same 

 throughout the whole great group, which includes the 

 fishes, amphibians, reptiles, birds, and mammals. The 

 differences lie in the varying development of the differ- 

 ent parts and in the modification in the size and form of 

 these parts. The plan includes a central axial portion, 

 the spinal column, made up of a series of elements called 

 vertebra. In a completely developed skeleton, as that of 

 a cat (fig. 41), the groups of vertebrae are the cervical, 

 i.e., those of the neck; dorsal, those to which the ribs 

 are attached; lumbar, just posterior to the dorsal; sacral, 

 the group to which the ilium is bound, and caudal or 

 tail vertebra. In the higher vertebrates the vertebra; 

 of the sacral group are fused into one bone, the sacrum; 

 while the caudal vertebrae, posterior to the sacrum, are 

 fused into one or a few bones in animals without a tail and 

 in them named the coccyx. The bones of the spinal 

 column are firmly bound together, constituting a somewhat 

 flexible but very firm axis, to which the head and limbs 

 are attached and from which the main pulls on these parts 

 can be exerted. The head skeleton consists of a central 

 group of bones in front of the axial skeleton, being an 

 extension of it, and around it are placed the other bones 

 of the head. To the axial skeleton are attached two pairs 

 of limbs; an anterior pair, joined to the axis by a group 

 of bones known as the shoulder girdle, and a posterior 

 pair joined to the axis by the pelvic girdle. The leg of 

 the cat, already described, may be taken as a represent- 

 ative limb. 



Now if we examine a series of vertebrate skeletons, 

 representative of the different groups from the lowest 

 forms of fishes through to the highest mammals, we can 

 see that notwithstanding the different forms and sizes of 

 the various animals and their different ways of locomotion 

 — swimming, flying, crawling, running, walking, and leap- 



