DEVELOPMENT OF NERVOUS SYSTEM 



761 



the myelin retains the form of an emulsion, and as it increases in amount it incloses the adjacent 

 fibrUs T\-hich serve as a framework supporting the droplets of the emulsion in its meshes. Thus 

 supported, the increasing myelin does not inclose the adjacent nuclei and endoplasm of the 

 original syncytium. Probably because of the fibrous support of the myehn thus obtained, 

 meduUating fibres may be often seen presenting the beaded appearance shown in fig. 603, G, 

 instead of an even distribution of the emulsion after it has become continuous along the axone. 

 The "beads" probably represent the uneven beginning of the accumulation indicated in B 

 of this figure. Increasing further, the myeUn becomes a cylinder of even thickness, the adjacent 

 nuclei being pressed away against its surface and the adjacent fibrils also condensed upon it. 



There is good reason to beheve that the fibrous portion of the sheath, the primitive sheath 

 or neurilemma, of the medullated axone arises as a condensation of the fibrils of the surrounding 

 stroma during development, that the sheath cells represent certain of the nearest nuclei in- 

 corporated from the original syncytium, and that the so-caUed neuro-keratin of the myehn 

 represents the fibrous framework of the myehn inclosed by it during its accumulation upon the 



Fig. 604. — Showing Some of the Varieties of the Cell-bodies of the Neurones 

 OF THE Human Nervous System, including the Dendrites and Small Portions of 



THE AXONES. AXOXE ShEATHS NOT INCLUDED. 



A. From spinal ganghon. B. From ventral horn of spinal cord. C PjTamidal cell from cere- 

 bral cortex. D. Purkinje cell from cerebellar cortex. E. Golgi cell of type II from spinal 

 cord. E. Fusiform cell from cerebral cortex. G. Sympathetic, a, axone; d, dendrites; c, 

 collateral branches; ad, apical dendrites; hd, basal dendrites; c, central process; p, peripheral 

 process. 



axone. The theory that the myehn arises as a difTerentiated portion of the axone and the theory 

 that it is formed by the neurilemma have been advanced. That it is accumulated from the 

 immediately surrounding fluid of the stroma and adheres to the axone, added droplets coalescing 

 there, in preference to other tissue elements because of some physical or chemical peculiarity 

 of the axone, is more probably correct. 



As the medullary sheath approaches completeness, constrictions may be observed at more 

 or less regular intervals at which the myehn emulsion is absent. There are the nodes of 

 Ranvier. The process by which they arise is not clearly understood. While the fibre is growing 

 in length, new myehn is added at the nodes. The internodal segments of the sheath increase 

 in length with age, and each segment may possess from one to several sheath nuclei. 



In adolescence, fibres whose medullary sheaths are in various stages of completeness may 

 be found both in nerve bundles in the central system and in the cranio-spinal nerves, and in 

 both, the sheaths of some axones certainly never acquire myehn. Also, in the adult, fibres 

 whose medullary sheaths present the beaded appearance may be observed, probably repre- 

 senting cases of arrested accumulation of myehn. According to Westphal there is a slight in- 

 crease in the thickness of the sheath with age. Larger axones acquire thicker sheaths of myehn 

 than smaller ones. Some fibres of the sympathetic system are medullated but in such the 

 myelin sheath is relatively thinner than in the cranio-spinal system. Beaded sheaths are 

 frequent in sympathetic rami, though non-meduUated fibres are most abundant. 



