398 



PHYSIOLOGY 



CHAP. 



fourth nerves). According to Bernheimer's diagram (Fig. 186) 

 the nucleus of the levator palpebrae and that of the superior 

 rectus are connected with the corresponding muscles by direct 

 or homolateral fibres ; the nucleus for the internal rectus and that 

 for the inferior oblique send to their corresponding muscles both 

 direct and crossed fibres ; while the fibres of the nucleus of the 

 inferior rectus and according to Panigrossi of the superior oblique 

 as well all cross. The nucleus of the sixth or abducens nerve, 

 which is distinct from the nuclei of the third and fourth nerves, 

 sends out, according to Bergmann's investigations, only direct 

 fibres. 



Intimately connected with the mass of the bilateral nuclei of 



the third and fourth nerves 

 (which Bernheimer terms 

 the principal nuclei) are 

 three other small nuclei 

 (which he calls accessory}, 

 one unpaired, median, with 

 large cells (M} } the other 

 two symmetrical, with small 

 cells (mm). The motor fibres 

 that run out from the acces- 

 sory nuclei are slender and 

 myelinate late ; they run 

 medialwards, and unite with 

 the oculomotor without de- 

 cussation. According to 

 Bernheimer, the principal 

 nuclei innervate the ex- 

 V ternal ocular muscles, and 



FIG. 186. Diagram of the nuclei of the oculomotor the aCCCSSOrV nuclei the in- 

 nerves. (Partly after Bernheimer.) Nip, lateral - * . . , 



principal nuclei ; Na, mesial accessory nuclei. Other ternal mUSClCS, I.e. the 



explanations in text, sphincter of the iris and the 



ciliary muscle. Both the direct and the crossed fibres of the roots 

 remain separate within the mid-brain, and only unite in the trunk 

 of the oculomotor nerve shortly before leaving it. 



Bernheimer determined the localisation of the different nuclei 

 that innervate the external and internal ocular muscles in 

 monkeys with Nissl's method of studying the central lesions that 

 occur after the removal of the muscles (1897), as well as by the 

 isolated electrical stimulation of the different parts of the nucleus 

 (1899). 



The anatomical foundation of the bilateral association and co- 

 ordination of the eye -movements depends on the reciprocal 

 relations of the different nuclei of origin, as well as their relations 

 with the optic nerve and the cerebral cortex. 



The nuclei of origin of the motor nerves to the eye are 



