THE ENDOSKELETON 131 



r.nd cause much local differentiation in the varying degrees of 

 development of these processes. Although topographically 

 :-elated to the trunk, and classed with trunk muscles in works 

 on anatomy, they belong morphologically to the limbs, and 

 when these latter are small, as in salamanders, the muscles are 

 small also, seldom extending to the vertebral column, and 

 thus exercise little or no modifying influence upon it. 



Other modifications of the vertebral column are due to 

 the movement of the body as a whole and to the separate and 

 more or less specialized motions of the head and tail. Thus, 

 to perform the crawling movement of salamanders and most 

 reptiles, where a sinuous motion of the body axis forms the 

 principal mode of locomotion, there must be a large amount 

 of flexibility, especially in regard to lateral movements, be- 

 tween the separate vertebrae; and thus the amphiccelous form 

 of intervertebral articulation, the restricted motion of which 

 proves sufficient for fishes, becomes converted into true ball- 

 and-socket joints by the ossification (or chondrification) of 

 the ball of notochord contained in the cavities between each 

 pair of adjacent cups, and by its anchylosis to one of the con- 

 tiguous vertebrae. This forms the ball; the unmodified cup- 

 shaped end of the other vertebra serves as a socket. The 

 anchylosis of the notochordal balls may take place with either 

 the preceding or the succeeding vertebra; in the former case 

 each vertebra of the series will have the cup at its anterior, 

 and the ball at its posterior end, forming the type known as 

 precocious, while in the latter case the reverse condition is the 

 result, such vertebrae being designated as opisthoccelous. 



Both of these conditions are common among amphibians 

 and reptiles, but with the attainment of limbs sufficiently stout 

 to entirely sustain the weight of the body such a flexibility of 

 the vertebral column is not only unnecessary but becomes a 

 positive detriment, and thus in mammals the vertebrae become 

 accelous, that is, the articulations are reduced to mere flat 

 contact surfaces, and the notochordal balls are transformed 

 into the intervertebral cartilages that serve as cushions. In 

 the cervical vertebrae of many mammals the opisthoccelous 



