IOI2 



HUMAN ANATOMY. 



Developing intercostal net~ve of pig 

 embryo of lo mm. ; tip of nerve is composed 

 of fibrils surrounded by sheath-cells. X 360. 

 {Bardeen.) 



<i: 







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

 surrounded by membranous septa, they correspond to the axis-cyiinders of the individual nerve- 

 fibres, enclosed by the 7ieHrilemvia and its cells. The endonetiriiim appears comparatively late 



and, like the neurilemma, is a product of the mesoblast. 

 Later, condensations of the mesoblast around the definite 

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

 provide the perineurium and the epineurimn 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 termi}idl plexuses are established during the development of functional 

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



The medullary sheath is a comparatively late acquisition, since it does not appear until 

 about the fourth month of foetal 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 medullated fibres within the 



cerebro-spinal axis are devoid of this sheath, and, further. Fig. 860. 



that the myelin sometimes appears before the neurilemma 

 ( Kolster, Bardeen). While it 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 (Wlassak), or from the 

 apparently fluid substance that after a time surrounds 

 the axis-cylinder (Bardeen). Regarding the formation of 

 \h& framework supporting the droplets of myelin, Hardesty* 

 inclines to the view that certain sheath cells, which appear 

 during medullation, 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 origin of the afferent 

 (sensory) neurones, whose cell-bodies are situated within 

 the spinal and other ganglia, is entirely different from that 

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

 of the spinal nerves, the development of the ganglia pro- 

 ceeds from a group of ectoblastic cells that form a ridge, the 

 ganglion-crest, on the margin of either lip of the still open 

 neural tube (Fig. 860), just where the general ectoblast 

 passes into that lining the groove. On approximation of 

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

 into a wedge-shaped mass that completes the closure of the 

 neural tube and constitutes a centre of proliferation from 

 which the cells migrate outward over the dorso-lateral wall 

 of the tube. The proliferation is not imiform but most 

 marked at points that correspond to the mesoblastic 

 somites, in consequence of which a series of segmentally 

 arranged cell-aggregations appears on each side of the 

 neural tube. These collections are the anlages of the 

 spinal ganglia. Within them certain cells soon become fusiforrn and, assimiing the r61e ot 

 neuroblasts, send out a process from either end. One process — the axone— grows centrally, 

 while the other — the dendrite — extends peripherally and becomes the chief part of a sensory 

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



B 









^1/' 



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'«V5 



Transverse sections of dorsal region 

 of human embryos, showing early differ- 

 entiation of spinal ganglion ; A, B, neural 

 tube still open ; C, Z>, tube closed ; a, 

 ganglion-ridges ; b, fused ridges ; c, out- 

 growth to form ganglion ; d, ectoblast. 

 X 230. (Lenhoss^k.) 



^Amer. Journal of Anatomy, vol. iv., 1905. 



