THE SPINAL CORD AS A CONDUCTOR :;:,: 



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. 

 Both of these branches give off collaterals in the whole of their course, most 

 numerous near the 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 commissure to grey matter on other side. 



(2) Fibres ending in grey matter of posterior horns. 



(3) Fibres ending round cells of Clarke's column. 



(4) Fibres to lateral horn. 



Since the motor nerves arise from the anterior horn-cells, the first set, 

 the ' sensori-motor ' collaterals, represent the shortest possible spinal reflex 

 path. The second group may also represent a spinal reflex path with two 

 relays of cells, and therefore greater choice of response and longer reaction 

 time. The third set puts into action the cerebellar tracts which arise from 

 the cells of Clarke's column, and therefore call into play a much more com- 

 plicated mechanism, the limits of whose action it would be difficult to define. 

 The collaterals to the lateral horn probably represent the afferent tracts of 

 the various visceral and vaso-motor reflexes which we shall study later. 



We find no special tracts devoted to those impulses which affect con- 

 sciousness as sensations. All tracts going towards the cerebral hemispheres 

 are interrupted by cell relays, in the medulla or cerebellum, and must serve as 

 afferent channels for unconscious as well as for conscious reactions, 

 quality of an afferent impulse can only be defined by its origin, or by i 

 effect on consciousness, and much discussion has arisen as to the 

 path of the various cutaneous and muscular sensations in the cord. 



It is evident that an impulse may travel to the cortex by way o 

 cerebellar tracts through the cerebellum, or by way of the posten 

 through the intermediation of the bulbar nuclei, or by a series of relays 



