The spinal column is peculiarly adapted, with its strong ligaments, its cartilage cushions, its perfect flexibility and flexuousness, to withstand jars and shocks. Yet the spine is the door by which disease enters the organism. Concussion of forces, the energy from the environment encountering the bodily resistance, is of no serious effect upon the organism--of no permanent or irreparable effect--unless it affects the spine and brings about vertebral subluxation, disturbance of the normal alignment between vertebrae, and thereby interrupts the perfect healing and controlling influence exerted by the vital part of the segment, the central nerve portion. When a concussion of forces _does_ produce subluxations, does disturb the perfect poise and balance of that center of structure of the body, its consequences affect an entire body segment, producing, or tending to produce, disturbances through the entire segment. Disease is the indirect consequence of the contact of man with his environment and is _natural_ but not _normal_. The spinal column is a _center_ or a series of centers for disease. In this column will be found the primary cause--the introductory element--by which disease first makes its appearance in a previously healthy body. Comparative Anatomy The study of Comparative Anatomy is necessary to a complete understanding of the human organism. We may trace in the simplest forms of animal life the beginnings and foreshadowings of the same plan of organization. We may follow it through the ascending scale and watch its complexity develop, and by viewing each step in the process we may come fully to realize that the original plan has been preserved throughout, though often in such form that by study of the single species we should fail to recognize it. We lack space for complete consideration of this subject and shall merely suggest certain facts and phases. No clear analogy can be drawn until we reach the worm, with its rudimentary spinal column and nerves. Roughly speaking, dissection of one spinal segment with its nerves and their controlled area--if this were possible--would separate from the rest a fairly regular _layer_ similar to all the other layers. This is the primitive segmentation. It is shown much more clearly in the fish but the segments have begun to curve with their periphery directed slightly caudad and some have already shown a preponderating growth over other segments and a change of shape from the original symmetry. The reptiles and birds show still more complicated segmentation. It is notable that in these lower animals the purely reflex portion of the nervous system is highly developed while the volitional and sensory portions, the cerebral hemispheres, are yet rudimentary. In birds, particularly, the cerebellum is very highly developed because its function of co-ordination of muscles for the maintainence of equilibrium is required in a high degree for flying.