708 A TEXTBOOK OF PHYSIOLOGY 



The Cranial Nuclei. The nucleus of the fourth nerve lies close to 

 the middle line in the lower part of the mid-brain. The fibres arising 

 from it pass dorsally outwards, decussating in their course just above 

 the aqueduct of Sylvius. The fourth nerve is effector in function, 

 supplying motor fibres to the superior oblique muscle of the eye. 

 This muscle, acting in conjunction with the inferior rectus, enables 

 the eye to look directly downwards. It is to be noted that the left 

 muscle is supplied by fibres from the right side of the mid-brain. 



The nucleus of the third nerve lies in the upper part of the mid-brain 

 in a central position just ventrally on either side to the aqueduct of 

 Sylvius. Its fibres pass ventrally outwards, and go to supply all the 

 muscles of the eyeball except the superior oblique and the external 

 rectus. An intimate communication is established with the fourth 

 and sixth cranial nuclei by means of the posterior longitudinal bundle. 

 It is suggested that some fibres from the sixth nuclei run in this bundle 

 to the third nuclei, and enter into the formation of the third nerves. 

 This is particularly suggested for the fibres supplying the internal 

 recti the antagonizers of the external recti muscles supplied by the 

 sixth nerve. 



The Inferior Corpora Quadrigemina are masses of white matter, 

 in the centre of which are contained nerve-cells, around which end 

 fibres of the lateral fillet. These fibres are connected with the auditory 

 nuclei; the inferior corpora quadrigemina are to be regarded as cell- 

 stations in connection with hearing. 



The Superior Corpora Quadrigemina are likewise made up of white 

 matter and groups of nerve-cells. They are to be regarded as impor- 

 tant cell-stations in connection with vision, in particular in connection 

 with the regulation of eye movements. Around the cells end fibres 

 from the optic tract, and also from the lateral fillet. From its cells 

 arise fibres which pass to ths nuclei of the third nerve, and downwards 

 in the posterior longitudinal bundle probably to the sixth and 

 seventh nerve. 



The Geniculate Bodies. In close association with the quadri- 

 geminal bodies are the geniculate bodies, of which there are two pairs 

 the external and internal. They are small, elevated masses of 

 nerve-fibres and nerve-cells. Around the cells of the external bodies 

 end fibres from the optic tract, while from its cells arise fibres which 

 pass as the optic radiation to the occipital part of the cerebral cortex 

 concerned in vision. The external bodies are therefore important 

 cell-stations in connection with sight. 



The internal bodies are concerned with hearing. Around the cells 

 terminate the fibres of the lateral fillet from the auditory nuclei. The 

 cells give rise to the auditory radiations, which pass to the tem- 

 poral region of the cortex. The two internal geniculate bodies are 

 connected with each other by v. Gudden's commissure, which runs 

 in the posterior part of the optic tract. 



Passing upwards, the brain-stem enters into association on either 

 side with three masses of grey matter the optic thalamus and the 



