436 PHYSIOLOGY 



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 mitter 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 rein- 

 forced 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 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 position 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. 

 These pass round on each side and can be seen to make connection 

 with the back part of the thalamus, the external geniculate body, 

 and the superior corpus quadrigeminum. Part of the tract, which is 

 sometimes called the mesial root, passes into the internal geniculate 

 body. This part of the tract has probably nothing to do with vision 

 and forms a commissure running in the optic chiasma connecting the 

 internal geniculate bodies of the two sides. The course of the optic 

 fibres is shown in the diagram (Fig. 199). In man and in some other 

 mammals, e.g. dog, monkey, the nerve fibres decussate incompletely 

 in the chiasma. The uncrossed bundle is derived from the outer half 

 of the retina of the same side, whereas the crossed bundle is derived 



