322 HUMAN PHYSIOLOGY 



sheath, but acquire this structure later on, and nerve fibres 

 of different functions acquire it at different periods. The 

 development of the medullary sheath can be readily investi- 

 gated, for the medullated nerve fibres are white, while the 

 fibres not containing this sheath appear gray. It may be 

 assumed that the function of the nerve fibre is only com- 

 pletely present when the nerve sheath has been formed. 



In the spinal cord the medullary sheaths of the anterior 

 and posterior roots are first formed, i.e. the tracts serving 

 for reflex actions. After this, the sheaths of the antero- 

 ground, the lateral ground bundle, and of Burdach's column, 

 i.e. bundles which contain fibres chiefly for the indirect 

 reflex tracts. Then the sheaths of the long sensory tracts 

 leading to the brain are formed, and finally the sheath of the 

 long motor tracts leading from the brain. From the succes- 

 sive developments of the medullary sheaths it is evident 

 that, in the spinal cord, the simpler reflexes appear first; next 

 the more complicated and radiated ; and finally the paths 

 for the stimuli causing sensations and voluntary movements. 



In the corona radiata, the centripetal fibres for the sensory 

 areas of the cerebral cortex are developed before the corre- 

 sponding centrifugal fibres ; hence the conditions for the 

 formation of sensations are perfected before those for the 

 formation of voluntary movements. Some of the fibres for 

 the sensory areas develop after birth (see page 329). 



In the medulla oblongata, however, there appear at an 

 early date groups of cells whose axis-cylinder processes 

 course down the anterior and lateral columns of the cord 

 (hence centrifugal fibres) ; these fibres are already medul- 

 lated when the sensory roots of the medulla have no medul- 

 lary sheaths. These cells and fibres are, therefore, well 

 developed and function at the time when the posterior roots 

 still appear embryonic. This indicates that the action of 

 centres is automatic and not reflex. The sensory nerves, 

 when fully developed, stimulate and eventually regulate the 

 centres which, prior to this, were already active. It must 

 be remembered that the medulla contains the important 



