760 



THE NERVOUS SYSTEM 



the mesenchymal tissue being carried in by the ingrowing blood-vessels. After the mixture of 

 the nuclei resulting from this fusion of the syncytia from the two sources, nuclei of mesodermal 

 origin cannot be distinguished from those of ectodermal origin. Further, axones outgrowing 

 from the embryonic gangha and neural tube carry with them adhering portions of the ectodermal 

 syncytium into the sm-rounding mesenchymal (fig. 603). 



As development proceeds further, each syncytium becomes resolved into a reticulum of 

 granular endoplasmic processes, containing the nuclei, with transparent exoplasm occupying 

 its meshes. Fibrils soon form in the exoplasm and from these develop the connective-tissue 

 fibres, whether neurogha in the central nervous system or mesenchymal fibrous tissue both without 

 and within it. Certain of these fibrils of course surround the axones imbedded among them 

 and from condensations of such fibrils are derived the fibrous sheaths of the axones, the sheath 

 nuclei being acquired from the adjacent nuclei of the original syncytium. These sheaths 

 become more dense or pronounced as the axones extend and the fibrous tissue increases with 

 growth, but there are always present fine marginal fibrils by which the sheaths grade into the 

 looser fibrous tissue about them. It is generally believed that the tissue giving rise to these 



Fig. 603. — Drawings Illustrating the Origin of the Axone and the Development of 



THE Medullary Sheaths. 



A, ventral portion of transverse section of an embryonic spinal cord involving portion of 

 periphery of future ventral horn and part of the mesenchymal (mesodermal) syncytium out- 

 side the external hmiting membrane of the cord. B, later stage of ventral root (peripheral) 

 axone with myehn droplets adhering to it and fibrillated stroma surrounding it. C, stage 

 in which mj^ehn droplets, supported by fibrils of stroma, have increased and accumulated 

 to form a practically continuous myehn or medullary sheath. D, final stage with medullary 

 sheath of even thickness, showing a node, and showing a neurilemma, sheath nucleus and 

 fibrous framework of the myehn ("neurokeratin") derived from the fibrils of the original 

 stroma. 



Myelin 



external limiting ^_^,(^^j^ 

 membrane -\~■^r^ 



V 



Mesenchymal \, 

 syncytium ( 



^- Neurilemma 



\. \ Medullary 

 \\\r- Sheath 



\\\«u-' — Sheath cell 



axone sheaths is of mesodermal origin. However, in amphibian larvae, Harrison has shown that 

 some sheath nuclei at least are derived from the nuclei of the ectodermal syncytium of the 

 ganghon crest, and Neal has noted in elasmobranchs the fact that nuclei migrate from the ven- 

 tral waU of the neural tube along with the axones growing out to form the ventral roots of the 

 spinal nerves. Whether all or any of these nuclei are originally ectodermal, and, if so, whether 

 such ectodermal tissue gives rise to all axone sheaths, especially in the higher animals, are 

 questionable contentions. 



Axones possessing only fibrous sheaths comprise the non-meduUated nerve fibres. The 

 majority of the sympathetic fibres are of this variety, and Ranson has found numerous non- 

 medullated fibres present in the spinal nerves. The generally accepted form of non-meduUated 

 sympathetic fibres may be seen in fig. 009, C. 



Medullatcd fibres are those which possess an investing coat of fat or myelin in addition to 

 the fibrous sheath. Most of the fibres in the central nervous system and most of those belong- 

 ing to the cranio-spinal nerves proper acquire myehn sheaths. Myelin begins to appear upon 

 axones shortly after the beginning development in the syncytium of the fibrils of the fibrous 

 connective tissue, and thus after the beginnings of what will become the fibrous sheaths. The 

 fibrous portions of the sheaths in the central nervous system develop less rapidly and are far 

 more scant than those of tlie meduUated fibres of the peripheral nerves. Probably because 

 of this, it ha,s been claimed tliat myelin begins to appear on the axones of the central system 

 before the appearance of tlie fibrous sheath. In man, the first appearance of myehn occurs at 

 about the fourth month, but inychnisation is not completed till after birth. The cranio-spinal 

 nerves contain completely medullated fibres before the central system does. 



Myehn first appears as small droplets adhering to the axone at irregular intervals. These 

 droplets increase in size and number and gradually accumulate to form a practically continuous 

 sheath of fat immediately investing the axone. They probably result from the coalescence of 

 finer dronlets floating in the surrounding fibrillated stroma. However, collecting upon the axone, 



