THE NATURAL HISTORY OF MAN 47 



the anterior part of the spinal cord and thread their way 

 among the embryonic cells till they reach the proper muscle, 

 where they break up into fine fibrils which become embedded in 

 its fibers. Nerve fibers, therefore, are merely the very elon- 

 gated processes of nerve cells. How these fibers are directed 

 to their proper terminations among the myriads of cells, 

 whether by chemical attraction or other force, and so seldom 

 go astray, has never been determined. 



Before the nerves function they become surrounded by a 

 translucent covering, called the myelin sheath, which is as- 

 sumed at different periods during development, in the various 

 parts of the spinal cord and brain. In general the myelin 

 sheaths are assumed first by the peripheral sensory and motor 

 nerves, thereby completing the reflex mechanism which in man 

 occurs at about the sixth month of prenatal life. Following 

 this the different groups of cells in the spinal cord are asso- 

 ciated by the development of the myelin sheaths on the con- 

 necting nerves, thus arranging for association of the reflex 

 actions. Still later the nerves connecting the spinal cord with 

 the cerebellum myelinate, thus completing the mechanism for 

 coordination of movements. Then the afferent nerves, which 

 connect the spinal cord with the brain, assume their myelin 

 sheaths, thus preparing the pathway for sensory impressions. 

 Not until after birth, however, are the efferent tracts connect- 

 ing the cerebrum and spinal cord myelinated, thus placing the 

 cerebrum in control of the parts below and completing the 

 motor pathway. This late completion of the motor tracts in 

 man explains the great helplessness of the human infant at 

 birth, a condition which is in striking contrast to that in many 

 of the lower animals. It is believed that the assumption of the 

 myelin sheaths in the various association tracts of the cerebrum 

 continues during the period of growth and perhaps until forty 

 years of age. As no brain cells are ever formed after birth, 

 the increasing myelination of the nerve fibers is the chief struc- 



