554 



THE NEKVOUS SYSTEM. 



Later still, the development of the great sensory and motor tracts contributes 

 largely to the dimensions of the basal lamina. 



As the two basal laminae (one on each side of the median plane) increase in 

 thickness the epithelial cells in the intervening floor-plate become stretched and 

 lengthened (Fig. 483), so that a definite septum or raphe is formed between the 

 two halves of the rhombencephalon. 



The fate of the extreme posterior edge of the alar lamina is very interesting. 

 The nervus acusticus is inserted into this edge in the region of the recessus 

 lateralis, and from it masses of neuroblasts develop to form receptive nuclei for 

 the two parts of this nerve, these being the cochlear and vestibular. These are the 

 nucleus cochlearis and nucleus vestibularis respectively. Sensory fasciculi, bringing 

 impulses from muscles, skin and related structures in all parts of the body, make 

 their way into the superior part of the vestibular nucleus, and it grows and forms 

 a large thickening of the posterior edge above the recessus lateralis. Eventually, as 

 it extends medially (Fig. 484), it reaches and invades the roof-plate and fuses with 

 the corresponding rudiment of the other side. Thus a semilunar band, the 

 primitive cerebellum, is formed, arching across the posterior aspect of the meten- 

 cephalon. The part of the dorsal edge which lies below the vestibular nucleus 

 becomes bent over (forwards) to form what is known as the rhombic lip (Fig. 483). 

 It is destined to be transformed into a series of masses of gray matter, the chief 



function of which is to emit fibres to carry 

 impulses into the cerebellum. But most of 

 these fibres pass not so much to the part of 

 the cerebellum derived from their own side 

 as to that of the opposite side. Thus, from 

 above downwards, the thickened margin of 

 the fossa rhomboidea on each side develops 

 into the following structures: cerebellum, 

 the rest of the vestibular nucleus, the 

 cochlear nucleus, the nuclei pontis (and 

 arcuate nuclei), the olivary nucleus, the 



nucleu8 acilis a <* th nucleus CUneatUB, 



FASC.SOL. 



VAGUS 



HYPOGLOSSAL 



FIG. 485. TRANSVERSE SECTION OP THE HUMAN 



EMBRYO AT A LATER STAGK THAN THOSE At an early stage of development most of the 

 SHOWN IN FIG. 483. (After His.) neuroblasts that form the rudiments of thf 



nuclei pontis, nuclei arcuati, and nucleus 



olivaris inferior begin a process of migration, the course of which is deter- 

 mined by the source and direction of the afferent tracts passing into each nucleus 

 Such migrations are of common occurrence throughout the brain, and attempts t( 

 explain them have given rise to much discussion. The attractive force which appear; 

 to lead certain nerve-cells away from the place where they originally developed ha; 

 been called neurobiotaxis by Ariens-Kappers. But the solution of the problems o 

 these migrations is quite a simple one. If we take the case of a nerve-cell (A~), a 

 an early stage of development, which collects afferent impulses through its dendrite 

 from the cell B, and emits an efferent impulse through its axon to the cell C: a 

 the whole nervous system is very small at the stage under consideration, the thre 

 cells necessarily will be comparatively close the one to the other a fact which ma 

 be represented by the positions of the letters thus : 



BAC. 



In the course of subsequent growth it must inevitably happen that the points 

 and C will become removed further and further apart. If we suppose that th 

 cell B remains constant, the cell A will be faced with two alternatives if it is t 

 continue to link together the elements B and C: either its dendrites or its axo 

 must elongate. Now the axon is specially modified in structure for conducti 

 impulses for long distances, and the dendrites are not so specialised. Therefo 

 it invariably happens that it is the axon that becomes lengthened. In other wore 

 the cell-body A, considered in its relations to C, appears to migrate towards t 

 direction B from which its chief supply of afferent impulses comes. This may 1 

 represented thus : Tt A * f 



