386 PHYSIOLOGY 



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 continuation 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 termina- 

 tions of the fifth nerve, and is known as the trigemino-thalamic tract. All 

 these fibres pass up in the tegmentum of the mid-brain and finally end, partly 

 in the grey matter of the subthalamic region and partly in the grey matter of 

 the thalamus itself. To the thalamus are also continued a few fibres from the 

 lateral fillet. By this means the head ganglion of the fore-brain is in a posi- 

 tion to receive, so to speak, samples of the afferent impressions derived from 

 every sense-organ of the body. 



THE VISUAL PATHS. Two classes of afferent impressions which arrive at 

 the optic thalamus are probably of equal importance to all the other afferent 

 impressions taken together. These are impulses derived from the organs of 

 vision and of smell. The greater part of the fibres composing the optic nerves 

 arise as axons of the ganglion-cells of the retinae. Passing backwards, the 

 nerves of the two sides join in the optic chiasma, which is closely attached 

 to the floor of the third ventricle. After joining in the chiasma the optic 

 nerves are apparently continued round the crura cerebri as the optic tracts. 



