77//-; PRQSENCEPHALON 



their course, in most in! imate association with the nuclei of orisrin of the nerves 

 supplying the eye muscles, suggests what is probably tlicir most ini])ortant func- 

 tion, viz., that of associating these nuclei with each other anil of bearing to them 

 til ires from the nuclei of 1 lie oi her cranial nerves necessary for the co-ordinate action 

 of the muscles of (lie 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. 

 ..ml each nucleus probably contributes fibres to it. It also receives fibres from the 

 nuclei of termination of the sensory nerves. Thus it contains fibres coursing in both 

 directions, and. while it is continually losing fibres by termination, it is being con- 

 tinually recruited and so maintains a practically uniform bulk. Thus, a given le.-ion 

 never results in its total degeneration. 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 re- 

 ceived by way of the fountain deeussation from the nucleus of the superior colliculus 

 of the opposite side. As noted above, it is in part continuous into the spinal cord. 

 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 meseneephalon. Van Gehuch- 

 ten and K< linger describe for it a special nucleus situated beyond this commissure 

 in the hypot halamic region. This nucleus may be explained as receiving impulses 

 from the structures of the prosencephalon which are distributed to the structures 

 below by way of the fasciculus longitudinal!* medialis. 



As frequently realized in the above, the structures of the meseneephalon are both 

 overlapped by, and are of necessity functionally continuous with, the structures of 

 the next and most anterior division of the encephalon, the prosencephalon. 



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 t he t halami 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 ganglia or buried nuclei (corpus striatum), to- 

 gether 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 system. 



KXTKUXAL FEATURES OF THE PROSENCEPHALON 



The diencephalon. The basal surface of this division of the brain consists of only 

 the mammillary portion of the hypothalamic region. This comprises (1) the cor- 

 pora mammiliaria (albicantia), the two rounded projections situated in the ante- 

 rior part of the interpeduncular fossa, and (2) the posterior perforated substance or 

 the small triangle of grey substance forming the floor of the posterior part of the 

 third ventricle, and which represents numerous openings for the passage of branches 

 of the posterior cerebral arteries (fig. OK)). The hypothalamic portions of the cere 

 bral peduncles might be included. The structures of the optic or remaining portion 

 of the hypot halamus belong to the telencephalon. 



The </<;;;/ surjuci' of the diencephalon is completely overlapped and hidden by 

 the telencephalon. and covered by the intervening ingrowth of the cerebral meninges. 

 the tela chorioidea of the third ventricle (velum interpositum). These removed, 

 it is seen that the t halami are by far the most conspicuous objects of the diencephalon. 

 They, together with the parts developed in connection with them, are distinguished 

 as the thalamencephalon. The thalamencephalon consists of (1) the thnlmni: 

 (2) the nictdthdldi/itix or geniculate bodies; and (3) the i iiitlmltinnts, comprising the 



