CONNECTIVE TISSUE OF NERVOUS SYSTEM 



767 



the central portion of the nerve fibre or its so-called "axis-cylinder." It is of course the essen- 

 tial portion of the fibre. As noted above in describing their development, nerve fibres are classified 

 according to the character of the sheaths. Those which possess sheaths of myelin, a peculiar 

 form of fat, are known as medullated fibres, and those in which the sheaths are merely mem- 

 branes of condensed fibrous tissue, void of myehn, are non-medullated fibres. A medullated 

 fibre also possesses a fibrous membrane outside its myeUn sheath, known as the neurilemma or 

 sheath of Schwann. The neurilemma is of the same origin and general structure as the sheath 

 of the non-medullated fibre, and both possess nuclei scattered along them. Medullated fibres, 

 at more or less regular intervals, show constrictions at which the myehn sheath ceases, but 

 over which the neurilemma continues. These constrictions are the nodes of Ranvier. The mye- 

 lin is in the form of an emulsion, whose fat droplets are supported in a fine fibrous reticulum 

 (neurokeratin), while the neurilemma without serves to hold it in place. The neurilemma pos- 

 sesses from one to three or four sheath nuclei between adjacent nodes of Ranvier. 



There is no sharp line of separation between medullated and non-medullated fibres, for in 

 any locahty there may be found axones in all degrees of meduUation. Most of the fibres 



Fig. 610. — Diagram of Transverse Section of Spinal Cord with Roots of Spinal Nerve 

 AND Neighbouring Ganglia Attached, Illustrating Simplest Forms of Neurone 

 Chains. 



Fasciculus cuneatus 



Cephalic branch of spinal ganglion neurone 



belonging to the sympathetic system (processes of sympathetic neurones) are non-medullated, 

 but both partially medullated and completely medullated sj'mpathetic fibres may be found. 

 (See fig. 609.) The myehn sheaths of completely medullated sympathetic fibres are always 

 thinner and less well developed than those of medullated cranio-spinal fibres. Most of the 

 fibres belonging to the cranio-spinal nerves and to the central nervous system are medullated, 

 but among the fibres belonging to either there are to be found numerous non-medullated fibres. 

 As indicated in fig. 609, nodes of Ranvier are absent in the medullated fibres of the central 

 system. 



In all the higher vertebrates, the myehn sheath always begins on the axone a short distance 

 from its parent ceU-body. The neurilemma of the medullated and the fibrous membrane of 

 the non-medullated fibre are each faintly continuous with the fibrous connective tissue sur- 

 rounding it, and, in the cranio-spinal and sympathetic ganglia, in which each ceU-body of the 

 neurone has a fibrous capsule about it, the fibrous membrane or the neurilemma, as the case 

 may be, is directly continuous into the capsule of the cell-body. Upon approaching its final 

 termination, in other tissues or upon the dendrites or cell-body of other neurones, the nerve 

 fibre always loses its sheath, the telodendria of the axone always being bare when placed in 

 contact with the other element. In losing the sheath, the mj-ehn sheath, if present, always 

 ceases and the fibrous membrane becomes continuous with the tissue investing the receiving 

 element, whether the capsule of the ganglion cell, the sarcolemma of the skeletal muscle fibre, 

 the corium of the skin, or the connective-tissue capsule of the encapsulated terminal corpuscle, 



The connective tissue of the nervous system is of two main varieties — white fibrous connec- 

 tive tissue and neuroglia. White fibrous tissue alone supports and binds together the peripheral 

 system, and it is the chief supporting tissue of the central system. As connective tissues, these 

 two varieties are quite similar in structure, each consisting of fine fibrillse, either dispersed or in 

 bundles, among which are distributed the nuclei of the parent syncytium. In both tissues 

 nuclei are frequently found possessing varying amounts of cytoplasm which has not yet been 

 transformed into the essential fibrils. 



In addition to its enveloping membranes, the three meninges, which are of white fibrous 

 tissue, the white fibrous tissue supporting the central system within is quite abundant. It is all 



