CHAPTER IV

VERTEBRATES:  BACKBONE AND BRAIN

In tracing man’s ancestry from fish upward we ought properly to describe three or four fish, an amphibian, a reptile, and then take up the series of mammalian ancestors.  But we have not sufficient time for so extended a study, and a simpler method may answer our purpose fairly well.  Let us fix our attention on the few organs which still show the capacity of marked development, and follow each one of these rapidly in its upward course.

We must remember that there are changes in the vegetative organs.  The digestive and excretory systems improve.  But this improvement is not for the sake of these vegetative functions.  Brain and muscle demand vastly more fuel, and produce vastly more waste which must be removed.  At almost the close of the series the reproductive system undergoes a modification which is almost revolutionary in its results.  But we shall find that this modification is necessitated by the smaller amount of material which can be spared for this function; not by its increasing importance, still less its dominance for its own worth.  The vertebrate is like an old Roman; everything is subordinated to mental and physical power.  He is the world conqueror.

The important changes from fish upward affect the following organs:  1.  The skeleton.  A light, solid framework must be developed for the body. 2.  The appendages start as fins, and end as the legs and arms of man. 3.  The circulatory and respiratory systems developed so as to carry with the utmost rapidity and certainty fuel and oxygen to the muscular and nervous high-pressure engines.  Or, to change the figure, they are the roads along which supplies and munitions can be carried to the army suddenly mobilized at any point on the frontier. 4.  Above all, the brain, especially the cerebrum, the crown and goal of vertebrate structure.  The improvement is now practically altogether in the animal organs of locomotion and thought.  Still, among these animal organs, the lower systems will lead in point of time.  The brain must to a certain extent wait for the skeleton.

1.  The skeleton.  The axial skeleton consists, in the lowest fish, of the notochord, a cylindrical unsegmented rod of cartilage running nearly the length of the body.  This is surrounded by a sheath of connective tissue, at first merely membranous, later becoming cartilaginous or gristly.  Pieces of cartilage extend upward over the spinal marrow, and downward around the great aortic artery, forming the neural and haemal arches.  These unite with the masses of cartilage surrounding the notochord to form cartilaginous vertebrae, which may be stiffened by an infiltration of carbonate of lime.  The vertebral column of sharks has reached this stage.  Then the cartilaginous vertebrae ossify and form a true backbone.  I have described the process as if it were very simple.  But only the student of