384 PHYSIOLOGY 



cord alone, involve the educated co-operation of the cerebral hemispheres. 

 With this increased control there is a corresponding increase in the develop- 

 ment of long paths. In the brain of a fish, for instance, the cerebral hemi- 

 spheres are connected only with the fore-brain ; a little higher in the scale 

 there are connections between the hemispheres 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 hemispheres 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 determining 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 insignificant 

 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 join the spino-thalamic fibres 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 central continuation of fibres connected with 

 those derived from the cochlear nerve. This part is known as the lateral 

 fillet. The cells of the accessory and lateral nuclei of the cochlear nerve 

 send their axons by the trapezium to the superior olivary nucleus and 

 other small masses of grey matter on the other side. In these nuclei the 

 fibres for the most part terminate, but a fresh relay of neurons carries 

 on the impulses and forms the main part of the lateral fillet. These pass 

 up, getting more dorsal as they ascend, and finally terminate in the inferior 

 corpora quadrigemina. The mesial fillet, which we can regard as a con- 

 tinuation of certain spinal tracts upwards, is reinforced throughout the 

 whole extent of the medulla and pons by fibres originating from the masses 

 of grey matter in which the sensory cranial nerves terminate. Certain 

 of these fibres may form a distinct tract in the formatio reticularis, known 

 as the central or thalamic tract of the cranial nerves. Another similar 

 tract in the formatio reticularis is derived from the central terminations 



