472 TEXT-BOOK OF EMBRYOLOGY. 



of the geniculate bodies and the diminution of the mid-brain in importance 

 already alluded to (p. 467). (2) The centrifugal connections consisting of (a) 

 the pyramids passing from the precentral area of each hemisphere to various 

 lower efferent neurones, or neurones affecting the latter, and forming part of the 

 internal capsule and pes pedunculi ; (b) fibers from various parts of the hemis- 

 phere, forming the greater part of the rest of the internal capsule and pes, and 

 terminating principally in the pontile nuclei whence a continuation of this 

 system (the fibers of the middle peduncle), passes to the cerebellar hemisphere. 

 The great increase in size of the cerebellar hemispheres, of the contained 

 nuclei dentati, and probably of the superior cerebellar peduncles are further 

 effects of this new connection, which has already been alluded to (see Cere- 

 bellum, p. 466), (Fig. 409.) 



Another important effect of the development of the pallium is the assump- 

 tion by man of the upright position, due both to the specialization of the 

 hand to execute pallial coordinations and its consequent release from locomo- 

 tion, and also to the overhanging of the eyes by the enlarged cranium. The 

 great increase of cerebellar connections may be partly due to the new 

 problems of equilibrium connected with the upright position. 



GENERAL DEVELOPMENT OF THE HUMAN NERVOUS SYSTEM DURING 



THE FIRST MONTH. 



One of the earliest stages in the development of the human nervous system 

 is shown in the 2 mm. embryo of about two weeks (Fig. 410). This shows 

 the stage of the open neural groove. The appearance of a transverse section 

 of the neural plate, groove and folds, in other forms, is shown in Figs. 411 

 and 412. 



The neural folds now become more and more elevated and finally meet, thus 

 forming the neural tube as previously described (p. 451). The fusion of the 

 neural folds begins in the middle region and thence extends cranially and cau- 



junctivum (superior cerebellar peduncle); brack, pon., brachium pontis (middle cerebellar 

 peduncle); b. q. i., brachium quadrigeminum inferias (a link in the cochlear pathway) ; c. g. I., 

 lateral or external geniculate body; c.g.m., medial or internal geniculate body; c.qitad., cor- 

 pora quadrigemina; f.cort.-sp., cortico-spinal fasciculus (pyramidal tract);/, c. p.-f. frontal 

 cortico-pontile fasciculus (from frontal lobe); f.c.-p.t., temporal cortico-pontile fasciculus 

 (from temporal lobe); /. c.-p.o., occipital cortico-pontile fasciculus (from occipital lobe); 

 f.cun., fasciculus cuneatus (column of Burdach); f.grac., fasciculus gracilis (column of 

 Goll) ; /. s.-t., tract from cord to mid-brain roof and thalamus (sometimes included in Gowers* 

 tract); f.sp.-c.d., dorsal spino-cerebellar fasciculus (tract of Flechsig); f.sp.-c.v., ventral 

 spino-cerebellar fasciculus (tract of Gowers, location of cells in cord uncertain); lem. lat., 

 lateral lemniscus or lateral fillet; lemniscus 1 med., medial lemniscus or fillet (the part to the 

 thalamus is mainly a neopallial acquisition); n.coch., cochlear nerve; n. cun., (terminal) 

 nucleus of the column of Burdach; n.grac., nucleus of the column of Goll; n.dent., nucleus 

 dentatus; n. opt., optic nerve; n.r., nucleus ruber (red nucleus); pes ped., pes peduncu'.i 

 (crusta); pulv. thai., pulvinar thalami; Pyr., pyramid; rod. ant. y ventral spinal root; rod. post,. 

 dorsal spinal root; rod. opt., optic radiation (from lateral geniculate body, and pulvinar ( ?), 

 to calcarine region) ; somaes., bundles from thalamus to postcentral region of neopallium; 

 s p. gang., spinal ganglion; thai., thalamus. 



