Comparative Anatomy of the Spinal Cord

This chapter reviews the comparative anatomy of the spinal cord with particular attention to the lower forms, and gives some general remarks on the embryonic cord.

Abstract

This chapter reviews the comparative anatomy of the spinal cord with particular attention to the lower forms. The spinal cord is generally considered as the lowest level and the most simply organized part of the central nervous system. Of all the parts of the central nervous system, the spinal cord or medulla spinalis preserves the early embryonic tube-like shape most clearly, and although secondary form changes occur in some groups, it may be said that this organ is generally of a cylindric appearance. The length of the spinal cord varies considerably. During the course of evolution, a relatively simply spinal nervous mechanism has been gradually overshadowed and superseded by a more complex secondary system. The gray matter of the spinal cord shows a progressive elaboration and an increasing segregation of separate cell masses in the series of vertebrates, but the primitive configuration: a central core of gray surrounded by an outer zone of white matter is principally maintained throughout the subphylum. The chapter gives some general remarks on the embryonic cord.