402 PHYSIOLOGY 



of Marie in the angle of the anterior fissure. Others pass up with 

 Helweg's bundle partly to end in the olivary body, partly to run on 

 with the mesial fillet towards the thalamic region. 



The white matter of the cord can thus be regarded as made up of 

 short and of long tracts, which maintain direct connection between 

 the following parts of the central nervous system : 



(1) Different levels of the cord itself by means of the proprio- 

 spinal fibres. 



(2) Hind-brain and spinal cord, by the anterior and posterior 

 cerebellar tracts, the posterior columns, and the spino-olivary fibres 

 among the ascending tracts, and the vestibulo-spinal and olive-spinal 

 among the descending tracts. 



(3) The mid-brain and cord connections are represented by the 

 spino-tectal tracts in the lateral columns as a direct ascending path, and 

 by the rubro-spinal tract which furnishes a direct efferent connection 

 between mid- brain and cord. 



(4) The fore-brain, viz. the thalamus, receives only a few scattered 

 fibres spino-thalamic, which run chiefly in the lateral and anterior 

 columns. It has no direct efferent path to the cord. 



(5) The cerebral cortex, the master tissue of the body, receives 

 no fibres directly from the cord or periphery of the body, but by the 

 pyramidal tracts is able to influence directly the activities of the 

 motor mechanisms at every level of the cord. These fibres, so far as 

 is known, exist only in mammals, and show a great increase in relative 

 extent when traced from lower to higher types. While in the rabbit 

 the pyramidal tract is hardly perceptible, in the monkey it is the best 

 marked of all the tracts, and in man is still more highly developed. 

 This relative increase, which is probably associated with the 

 shunting of more and more of the reactions of the body from the 

 region of the unconditioned reflexes to that of the educatable re- 

 action, is shown not merely by the tract occupying a larger proportion 

 of the transverse area of the cord, but by its fibres being more densely 

 set within that area. 



THE PATHS OF IMPULSES IN THE CORD 



The greater part of the white matter is thus concerned in trans- 

 mitting impulses to nerve-cells in the brain, and from the brain 

 towards the cord. The complex reactions determined by these 

 impulses are in many cases as unconscious and automatic as those we 

 have studied in the spinal cord, even though they may involve the 

 activity of the cerebral cortex itself. Others, however, influence 

 consciousness, so that their afferent side appears in consciousness as 

 sensations of various qualities, and their efferent side as the result 

 of volition, i.e. as willed or emotional movements. 



