The articles here presented are modern and unhackneyed. Selected primarily as models for teaching the methods of exposition employed in the explanation of mechanisms, processes, and ideas, they are nevertheless sufficiently representative of certain tendencies in science to be of intrinsic value. Indeed, each author is a recognized authority.
ain stem, the movements of the head on the atlas had to be limited to mere nodding or side-to-side bending. The movements which are so necessary to us, that of turning our heads so that we can sweep our eyes along the whole stretch of the skyline from right to left, and from left to right, were rendered impossible. This defect was also overcome in a simple manner. The joints between the first and second vertebrae--the atlas and axis--were so modified that a turning movement could take place between them instead of between the atlas and skull. When we turn or rotate our heads, the atlas, carrying the skull upon it, swings or turns on the axis. When we search for the manner in which this has been accomplished, we see again that Nature has made use of the simplest means at her disposal. When we examine a vertebra in the course of construction within an unborn animal, we see that it is really made up by the union of four parts (see Fig. 4): a central block which becomes the "body" or supporting part; a right and
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