THE ENDOSKELETON 



133 



porpoises, and the dugongs or sea-cows, which, having de- 

 scended from terrestrial ancestors, have become secondarily 

 adapted to an aquatic life, form a remarkable corroboration 

 of the statement that a movable neck is incompatible with a 

 natatory habit, since in these the seven cervical vertebrae, typi- 

 cal of the Mammalia, have become greatly flattened antero- 

 posteriorly, and are either fitted so closely together that but 

 little motion is possible between them, or are even anchylosed 

 into a single piece, thus not only reducing the length of the 

 neck region and approximating the head to the shoulders, 

 but depriving it of motion, two important piscine charac- 

 teristics. 



Not only are the intervertebral articulations in the cervical 

 region extremely flexible in general, but that of the first with 

 the skull and the first with the second become especially modi- 

 fied, changes which often profoundly affect the shape of these 

 vertebrae. The first of these articulations is a double modified 

 ball-and-socket joint, the protuberances, or occipital condyles, 

 occurring upon the occipital region of the skull along the 

 lateral edges of the foramen magnum, and fitting into saucer- 

 shaped depressions on the anterior face of the first vertebra, 

 or atlas. In Amphibia and Mammalia these condyles are wide 

 apart and distant from one another, while in reptiles and birdsr 

 they coalesce in the mid-ventral line and form what appears 

 to be a single median condyle, the two components being 

 usually indicated by a median groove. In all cases the motion 

 between the skull and the atlas is in one plane only and 

 imparts to the head the bowing motion. 



The turning from side to side is effected by the articulation 

 of the first vertebra with the second, and is due to a curious 

 modification by which the body of the first vertebra remains 

 disconnected from its own neural arch and anchyloses with 

 that of the second vertebra, the axis, forming its pivot-shaped 

 odontoid process, around which the ring-shaped atlas may 

 rotate. This typical relation of the first two vertebrae, occur- 

 ring in reptiles, birds and mammals, is modified in amphibians 

 through a secondary inclusion of the elements of the atlas 



