1012 



HI -.MAX ANATOMY. 



FIG. 859. 



nerve < 



embryo oi in mm.; tip of nerv. 



of fibrils surrounili-il by sheath-cells. 



(Harder*.) 



FlG. 860. 



-- 



continues .\ml the bundles of fibrillae become progressively smaller and more compact until, 

 surrounded In membranous septa, they correspond to the axis-ey/imters of the individual nerve- 

 fibres, enclosed by the neurilemma and its (ells. The endoneurium appears comparatively late 



and, like the neuriK-mma, is a product of the mesoblast. 

 1-ater, condensations of the mesoblast around the definite 

 bundles of nerve-fibres and about the entire nerve-trunk 

 provide the perincurium and the epineurium respectively. 

 During its course to the periphery the young nerve gives 

 rise to numerous branches, the points of outgrowth being 

 indicated by a preparatory increase of the peripheral cells 

 which often form a tubular projection into which the nerve- 

 fibrillae grow. The proximal plexuses (such as the 

 brachial or lumbar) are formed during the outgrowth of 

 the nerves from the'region of the central nervous system ; 

 the coarser distal plexuses arise during the extension of 

 the branches to the various parts for which they are 

 destined ; whilst the finer terminal plexuses are established during the development of functional 

 unity between the nerve-fibres and the structures to which they are distributed. 



'1 he medullary sheath is a comparatively late acquisition, since it does not appear until 

 about the fourth month of I'u-tal life. Within the central nervous system the tracts of nerve- 

 fibres obtain their medullary coat at different times > some not until after birth), a variation that 

 is of much service in enabling the anatomist to trace the course of the individual paths of con- 

 duction. The origin and method of formation of the medullary substance has been, and in fact 

 still is, a subject of discussion. It is, however, certain that its production is not dependent 

 upon the neurilemma, since the niedullated fibres within the 

 o-spinal axis are devoid of this sheath, and, further, 

 that the myelin sometimes appears before the neurilemma 

 ( Kol.ster, Hardeeii). \Vhileit is doubtful whether the myelin 

 is directly formed from the outer part of the axis-cylinder, 

 as suggested by Kolliker, it is probable that this structure 

 exerts some influence resulting in the deposit of the myelin- 

 droplets either from the blood iWIassak), or from the 

 apparently fluid substance that after a time surrounds 

 the axis-cylinder ( I'.ard.-. -n \. Regarding the formation of 

 \\\^ framework supporting the droplets of myelin, Han!' 

 inclines to the view that certain sheath cells, which appear 

 during mednllation, are probably concerned. From the 

 foregoing account it is evident that the axis-cylinder is 

 derived from the ectoblast and the neurilemma from the 

 ectoblast ; the origin of the medullary sheath is still unde- 

 termined, but most probably is also ectoblastic. 



Development of the Ganglia. The < irigin t >f the afferent 



(sensory) neurones, whose cell-bodies are situate. 1 within 



the spinal and other ganglia, is entirely dim-rent from that 



of the efferent ( motor) ones above described. In the 



of the spinal nerves, the di-vlopment of the ganglia pro- 



oup of ectoblastic cells that form a ridge, the 



ganglion-erest, <>n the margin of either lip of the still open 



neural ti sr), just where the general ectoblast 



M into that lining \\ "i approximation of 



the lips of the latter, the cells of the ganglion-crests fuse 



into I Mpe.l mass that completes tin- closure of the 



1 tub.- ami constitutes a centre of proliferation from 



which the cells migr.it.' outward over the dorso -lateral wall 



of the tul-. 111. prohter.ition is not uniform but most 



marked at points that correspond to the mesoblastic 



somites, in consequence of which a series of segmentally 



ins appears on . .. li side of the 



!-( tions are the ai the 



spinal ganglia. Within them certain cells soon become fusiform and, assuming the role of 



TJeuroM.ists, send out ftp) .ill either end. One process the axone grows Centrally, 



while the other the <|.- n .!rite extend- peripherally and becomes the chief part of ft sensory 



nerve-fibre. The subsequent growth of the neurone is not symmetrical, but to one side, and so 



Transverse sections of dorsal r ^ion 

 of human i-mlnwis, shnum- .-.ul\ iliffcr 

 eiitiali.ni t "-piiKil K.iiiKlion ; A. //, neural 

 tut- -til! opni; < '. /;. tub,- closed ; <7, 

 ganglion-ridges; A, fused mini's; <-, out- 

 Riouth to form '-Million ; rf, ectoblast. 

 X 230. (Ltnkos*- 



1 Amer. Journal of Anatomy, vol. jv., 1905. 



