THE PROSENCEPHALON 843 



The two fasciculi constitute the principal association pathways of the brain-stem, and, true 

 to their nature as such, they are among the first of its pathways to acquire meduUation. In the 

 mesencephalon they become two of its most conspicuous tracts, and their course, in most inti- 

 mate association with the nuclei of origin of the nerves supplying the eye muscles, suggests 

 what is probably one of their most important functions, viz., that of associating these nuclei 

 with each other and of bearing to them fibres from the nuclei of the other cranial nerves neces- 

 sary for the co-ordinate action of the muscles of the optic apparatus associated with the functions 

 of these other nerves. 



Fibres from each medial longitudinal fasciculus terminate either by collaterals or terminal 

 arborisations about the cells of the motor nuclei of all the cranial nerves, and each nucleus prob- 

 ably contributes fibres to it. It also receives fibres from the nuclei of termination of the sensory 

 nerves especially the vestibular. Thus it contains fibres coursing in both directions, and, 

 while it is continually losing fibres by termination, it is being continually recruited and so 

 maintains a practically uniform bulk. Thus, a given lesion never results in its total degenera- 

 tion. Many of the fibres coursing in it arise from the opposite side of the mid-line. A special 

 contribution of fibres of this kind is received by way of the fountain decussation from the 

 nucleus of the superior coUiculus of the opposite side. As noted above, it is in part continu- 

 ous into the spinal cord as the ventral fasciculus proprius. It receives some fibres by way of 

 the posterior commissure of the prosencephalon from a small nucleus common to it and the 

 posterior commissure situated in the superior extension of the central grey substance of the 

 mesencephalon. Van Gehuchten and Edinger describe for it a special nucleus of the medial 

 longitudinal fasciculus situated beyond this commissure in the hypothalamic rsgion. This 

 nucleus may be explained as an accumulation of the gray substance of the reticular forma- 

 tion below and as receiving impulses from the structures of the prosencephalon which are dis- 

 tributed by its axones to the structures below by way of the medial longitudinal fasciculus. 



Scattered in the posterior part of the posterior perforated substance, near the superior 

 border of the pons, is a small group of ceU-bodies forming the inter -peduncular nucleus (inter- 

 peduncular ganghon of von Gudden). Fibres arising in the habenular nucleus of the diencepha- 

 lon curve posteriorly, forming the fasciculus retrojlexus of Meynert, and terminate about its 

 cells. Fibres arising from its cells course dorsalward and terminate about association neurones 

 in the ventral periphery of the central grey substance. It is concerned with olfactory impulses. 



SUMMARY OF THE MESENCEPHALON 



1. Quadrigeminate bodies: 



(c) Inferior coUiculi, their nuclei and brachia. 

 (b) Superior coUiculi, their nuclei and brachia. 



2. Peduncles of the cerebrum 



3. Aqueduct of the cerebrum. 



4. Central grey substance. 



5. Substantia nigra. 



6. Decussation of superior cerebellar peduncles and the red nuclei. 



7. Medial lemniscus, lateral lemniscus and nucleus of lateral lemniscus. 



8. Mesencephalic nucleus and root of masticator nerve. 



9. Trochlear nerve and its nucleus. 



10. Oculomotor nerve and its nucleus. 



11. Mesencephalo-spinal and rubro-spinal tracts. 



12. Medial longitudinal fasciculus, its nucleus, the nucleus of the posterior commissure. 



13. The fountain decussation. 



14. Interpeduncular nucleus. 



As frequently realized in the above, the structures of the mesencephalon are both overlapped 

 by, and are of necessity functionally continuous with, the structures of the next and most ante- 

 rior division of the encephalon, the prosencephalon. 



2. THE PROSENCEPHALON 



The prosencephalon or fore-brain includes those portions of the encephalon 

 derived from the walls of the anterior of the three embryonic brain-vesicles. In 

 its adult architecture it consists of — (1) the diencephalon (interbrain) , comprising 

 the thalamencephalon or the thalami and the structures derived from and 

 immediately adjacent to them, and, in addition, the mammillary portion of the 

 hypothalamic region; (2) the telencephalon (end -brain), comprising the optic 

 portion of the hypothalamic region and the cerebral hemispheres proper. The 

 last mentioned consist of the entire cerebral cortex or superficial mantle of grey 

 substance, including the rhinencephalon, and also the basal gangha or buried 

 nuclei (corpus striatum), together with the tracts of white substance connecting 

 and associating the different regions of the hemispheres with each other and with 

 the structures of the other divisions of the central nervous system. 



EXTERNAL FEATURES OF THE PROSENCEPHALON 



A. THE DIENCEPHALON.— The hasal surface of this division of the brain 

 consists of only the mammillary portion of the hypothalamic region (fig. 668). 



