THE SPINAL COED AS A CONDUCTOK 357 



(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, represents 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 calls 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, cerebellum, or optic thalamus, 

 and must serve as afferent channels for unconscious as well as for conscious 

 reactions. The quality of an afferent impulse can be defined only by its 

 origin, or by its effect on consciousness, and much discussion has arisen 

 as to the exact path of the various cutaneous and muscular sensations in 

 the cord. 



It is evident that an impulse might travel to the cortex by way of the two 

 cerebellar tracts through the cerebellum, or by way of the posterior columns 

 through the intermediation of the bulbar nuclei, or by the spino-thalamic 

 fibres, or by a series of relays from one segment of the cord to another 

 through grey and white matter alternately. It is supposed that all of the 

 ascending tracts may convey afferent impulses from the posterior spinal 

 roots to the brain, although evidence as to the part taken by each tract is 

 very conflicting. The following account represents the views which may 

 be regarded as the most probable (Page May) (Fig. 176) : Pain impulses, 

 on entering the cord by the posterior roots, cross to the other side at once, 

 and then pass up, chiefly in the antero-lateral column, by the spino-thalamic 

 fibres as far as the optic thalamus. Sensations of heat and cold take a 

 very similar course. Hence they are generally affected by lesions of the 

 cord in the same way as pain sensations. Impulses of touch and pressure, 

 after entering the cord, pass up in the posterior column of the same side for 

 four or five segments, then cross gradually and pass up in the opposite anterior 

 column. Impulses serving muscular sensibility, including the impulses 

 from joints and tendons, take two courses. Those which do not reach 

 consciousness and are involved in the involuntary guidance of muscular 

 movements, run up chiefly in the anterior and posterior cerebellar tracts 

 of the same side. Those which furnish the material for conscious sensations 

 and give information as to the position of the limbs, &c., are entirely homo- 

 lateral, and travel up in the posterior columns of the same side of the cord. 

 All impulses which reach the brain cross finally to the optic thalamus and 

 thence to the cerebral cortex of the opposite side. 



