434 PHYSIOLOGY 



part in the determination of sensation than in the determination of 

 movement, and that there may, and indeed must, be a reverberation of 

 impulses through these ascending and descending tracts, so that it 

 must be difficult to dissociate the various elements in the extremely 

 complex neural events which are involved, say, in the simplest kind of 

 conscious sensation. 



As we trace out the evolution of the brain we find an ever-increasing 

 subordination of the lower to the higher centres, so that in man himself 

 many reactions which in the lower animals are carried out by the spinal 

 cord alone, involve the educated co-operation of the cerebral hemi- 

 spheres. With this increased control there is a corresponding increase 

 in the development of long paths. In the brain of a fish, for 

 instance, the cerebral hemispheres are connected only with the fore- 

 brain ; a little higher up there are connections between the hemi- 

 spheres and the mid-brain as well. The chief long tracts are those 

 which run between the thalamus, the mid-brain or the hind-brain, and 

 the spinal cord. With the huge development of the cerebral hemi- 

 spheres in man there is also development of long paths, the pyramidal 

 tracts, from the hemispheres down to all the motor mechanisms of the 

 cord, and of tracts which connect all parts of the cortex with the grey 

 matter of the pons and indirectly with the cerebellum. The tracts 

 which in the lower animals were of supreme importance in deter- 

 mining subordination of lower to higher centres, of immediate reactions 

 to those determined by the organs of foresight, dwindle therefore 

 in importance. Those tracts, such as the thalamo-spinal, tecto- 

 spinal, vestibulo-spinal, which form the main mass of the white 

 matter of the brain stem in lower types of vertebrates, become 

 reduced to a few scattered fibres in the brain of man and are insig- 

 nificant as compared with the great cerebro-bulbar and cerebro-spinal 

 tracts. 



ASCENDING TRACTS 



THE TRACTS OF THE FILLET. The fibres which enter the spinal 

 cord by the posterior roots pass into the posterior columns and along 

 these to the dorsal column nuclei, the nucleus gracilis and the nucleus 

 cuneatus, where they end by arborisations among the cells composing 

 these nuclei. From these nuclei the axons of the cells pass in various 

 directions, the chief mass of them forming the deep arcuate fibres. 

 These emerge from the inner side of the nuclei and pass through the 

 raphe to the other side of the medulla, where they turn Up and form 

 the definite collection of longitudinal fibres, lying dorsally to the 

 pyramids, which is known as the main tract of the fillet, or, often, 

 the mesial fillet. As these fibres traverse the pons they are joined 

 at the outer side by a number of bundles which are derived from the 



