356 PHYSIOLOGY 



ing tracts, and the vestibulo-spinal and olivo-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 the spino-thalamic fibres 

 which, though scattered, are of considerable importance. They run chiefly in 

 the lateral and anterior columns. Its efferent fibres cannot be traced below 

 the lower cervical region. 



(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 reflex to that of the educatable reaction, 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 transmitting 

 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 con- 

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

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



The posterior spinal (sensory) roots at their entrance into the cord divide 

 into two bundles. The smaller of the two, situated more laterally and 

 consisting of fine fibres, enters opposite the tip of the posterior horn and turns 

 up at once in Lissauer's tract, a bundle of fine longitudinal fibres close to the 

 periphery of the cord. The fibres seem to pass into and end in the substance 

 of Rolando. The larger median bundle of coarse fibres passes into the pos- 

 tero-external column. Here each fibre divides into a descending and an 

 ascending branch, the former running in the comma tract, the latter in the 

 posterior columns up as far as the gracile and cuneate nuclei of the medulla. 

 I lot h of these branches give off collaterals in the whole of their course, most 

 numrrous nrjir tin- point of entry of the nerve. These collaterals may be 

 divided into four sets according to their destination: 



(1) Fibres ending round cells of anterior horn on same side or crossing by 

 posterior commi-inv to -iw matter on other side. 



