PARTS DERIVED FROM THE FORE-BRAIN 



903 



molecular ground substance. In a flat-wise section of the brain stem the outline 

 of this nucleus resembles an inverted L or the tip of a boat hook ; the axones from 

 the cells of this nucleus (of Edinger and Westphal) supply the Ciliary muscle and 

 Sphincter iridis (pupillary motion). The main nucleus, composed of several 

 sub-groups, lies caudolaterad of the preceding, and is composed of larger cell 

 elements. 



The root fibre bundles from this nuclear group pass ventrad, breaking through 

 the medial longitudinal fasciculus, separating like the strands of a horse's tail by 

 the interference of the red nucleus, to become gathered into more compact bun- 

 dles between the mesal edge of the substantia nigra and intercrural region, and 

 emerging by eight to twelve fascicles which compose the trunk of the oculomotor 



nerve. 



Antero-external nucleus. 

 Antero-internal nucleus. 



Nucleus of Edinger and 

 Westphal. 



Third ventricle. 



Central nucleus. 



A nterior dorsal nucleus. 

 A nterior ventral nucleus. 

 X. 





M. 



Posterior ventral 



nucleus. 

 -Posterior dorsal nucleus. 



^-Crossed fibres. 



H_ Nucleus of origin of 

 trochlear nerve. 



Decussation of trochlear 

 nerve. 



Trochlear nerve. 



FIG. 663. Showing the different groups of cells, which constitute, according to Perlia, the nucleus of origin 



of the oculomotor nerve. (Testut.) 



The origin of each nerve is not limited to the nuclei of its side ; a part is decussated 

 and the decussated origin is related to the innervation of the Internal rectus. By 

 means of association neurones in the medial longitudinal fasciculus the oculo- 

 motor and abducens nuclei of one side are brought into relation, affording an 

 organic basis for the synergism existing between the Internal and External recti 

 muscles in the conjugated lateral eye movements. 



The paradox of the facial nerve supplying muscles under the reflex dominion of 

 the retina (Orbicularis oculi) instead of the oculomotor may be explained by the 

 assumed existence of fibres emerging from the oculomotor nucleus, entering the 

 medial longitudinal fasciculus and joining the root of the facial. 



Parts Derived from the Fore-brain. 



The fore-brain or prosencephalon includes those portions of the brain which are 

 derived from the cephalic one of the three primary brain vesicles. It includes, 

 according to prevailing schemas, a thalamic portion (the thalamencephalon or 

 diencephalon) and the telencephalon. The two divisions constitute a structural 

 continuity and exhibit a mutual dependency so close that the arbitrary distinction 



