THE TELENCEPHALON. 



nerves, may traverse the entire thickness of the brain and the length of the spinal 

 cord. They arise from the pyramidal cells of the Rolandic cortex, follow the corona 

 radiata into the internal capsule, within which they occupy approximately the front 

 half of the posterior limb, those destined for the cervical nerves lying in advance of 

 those for the trunk and leg nerves. Within the peduncle, the cortico-spinal tract 

 appropriates approximately the middle third of the crusta, having the pontine paths 

 to its outer side. The further course of these fibres leads through the ventral part 

 of the pons and of the medulla, until near the lower limit of the last-named division 

 of the brain-stem, the greater part of the pyramidal strands take part in the motor 

 decussation and thence descend within the lateral pyramidal tract to their appropriate 

 levels where they end in relation with the radicular cells of the anterior horn (page 

 1043). The fibres which do not cross in the pyramidal decussation exchange their 

 lateral position for a median one and continue within the cord as the direct pyramidal 

 tract at the side of the median longitudinal fissure. Before gaining their final levels 

 within the cord, these fibres also cross, by way of the anterior white commissure, to 

 end around the root-cells of the opposite side. 



DEVELOPMENT OF THE PARTS DERIVED FROM THE FORE-BRAIN. 



It has been pointed out in the general sketch of the development of the brain (page 1060), 

 that the fore-brain very early undergoes subdivision into two secondary cerebral vesicles, the 

 anterior of which is the lelencephalon, or end-brain, and the posterior the diencephalon. Each 

 of these secondary vesicles 



FIG. 1027. 



Diencephalon Tegmentum 



Thalame 



sph 



Hypothalam 

 Future lateral 

 ventricle 



Pallium 



Posterior limit 



of telenceph- 



alon 



Rhinenceph- 



alon 



Corpus striat 



Geniculate ganglion of facial 

 Vestibular ganglion 



Membranous labyrinth 



Mid-brain 



III nerve 



ammillary 

 body 



Pons 



gives rise on each side to 

 two general regions, an 

 upper and a lower, which 

 in 'the .telencephalon are 

 the hemisphtzrium and the 

 pars optica hypothalami 

 and in the diencephalon 

 are respectively the thala- 

 mencephalon and the pars 

 mamillaris hypothalami. 

 These two parts of the 

 hypothalamic region to- 

 gether constitute the hypo- 

 thalamus, which includes 

 the portion of the lateral 

 wall of the fore-brain lying 

 below the level of the fo- 

 ramen of Monro and cor- 

 responds to the ventral or 

 basal lamina of the neural- 

 tube (Fig. 914). Thistract 

 gives rise to the structures 

 situated along the floor of 

 the third ventricle the 

 mammillary bodies, the 

 tuber cinereum, the in- 



fundibulum and the posterior lobe of the pituitary body, the optic chiasm and the optic tracts. 

 The anterior wall and the roof of the fore-brain always remain thin. This is especially true 

 of the roof, which, with the exception of its hindmost part where the posterior commissure 

 is formed, does not lead to the development of nervous tissue but remains thin, being 

 later represented by the attenuated epithelial layer which constitutes the morphological roof 

 of the third ventricle. The anterior wall of the fore-brain is the thin median partition 

 known as the lamina terminalis, which, whilst giving rise to the rudimentary sheets of gray 

 matter found within the lamina cinerea and the septum lucidum, is to a large extent concerned 

 in the production of the great commissure, the corpus callosum. 



The hemisphferium, one on each side, comprises by far the greater portion of the end-brain 

 and represents an enormous expansion of the dorsal or alar lamina of the neural tube. Very early it 

 exhibits a differentiation into : (a) the pallium, (b) the rhinencephalon and (c) the corpus striatinn. 



The Pallium. Of the three parts of the hemisphaerium, in man the pallium soon becomes the 

 most conspicuous, since from the walls of this rapidly expanding hemispherical pouch is derived 

 the great sheet of cortical gray substance which invests the cerebral hemisphere. For a time 

 enclosing a large caviiy with thin walls, the pallium later becomes consolidated by the 



mm 



(Ex 



Reconstruction of brain of human embryo of four and one-half weeks (10.2 

 .), inner surface of the fore-brain and mid-brain exposed by mesial section, 

 iterior of same brain is shown in Fig. 1141.) X 12. Drawn Irom His model. 



