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.
Those land animals which walk on all fours approach still closer but their arrangement is much more readily comprehensible than in man. As the animal stands on all fours with head extended, a gigantic cleaver slicing out each vertebra and pair of nerves and slicing straight toward the base of support might be said to divide the body approximately according to the structural and functional arrangement in segments. Yet no segment so separated would exactly correspond to the nerve distribution; there would be enlargement of some organs with extension into the zone previously occupied by their neighbors; enlargement here and atrophy there; invagination of one organ by another and overlapping and intermingling of parts. Even the relation between the spinal cord segments and the vertebrae has departed much from the primitive so that the growth of the vertebrae has exceeded that of the cord and the cord terminates opposite the Lumbar region instead of at the end of the Sacral canal. It may here be remarked that in the human embryo the cord at first occupies the entire length of the neural canal formed within the vertebrae; that in the adult it terminates opposite the lower border of the body of the first Lumbar vertebra and that the nerves, still retaining their original foramina of exit and their relation to the somatic segments, pass downward within the canal to their respective openings and collectively form a brush like mass called “cauda equina.”