816 THE NERVE SYSTEM 



inner surface of the neurilemma. and embedded as it were in the mvelin. usually midway between 

 two nodes, lies the oval-shaped nucleus of the neurilemma. 



Myelinic axones are usually from 4 to 10 microns in diameter; the extremes range from 2 to 

 20 microns. 



Myelinic axones without a neurilemma constitute the \vhite substance of the brain and 

 spinal cord, as well as the optic nerves. They differ from the axones just described in t\v>i 

 particulars the neurilemma is absent and there are no nodes interrupting the continuity of the 

 myelin sheath. A network of neuroglia replaces the neurilemma as a supporting tissue. 



Amyelinic axones with a neurilemma (Retnak's fibres; sympathetic nerve fibres] constitute 

 the majority of the sympathetic axones and the axones of the olfactory nerves. The mvelin 

 sheath is absent and the axone is invested, more or less completely, by a nucleated cellular sheath 

 or neurilemma. 



Amyelinic axones without a neurilemma are naked axones, most numerous. in the central 

 ganglia. Most axones of longer course are devoid of any sheath in the cytoproximal and pre- 

 terminal portions, whatever investment they may receive in the intermediate portion. 



NERVE CELL NIDI OR NUCLEI. 



Nerve cells are more or less definitely grouped in the gray substance of the brain 

 and cord to form what are conventionally termed "nuclei." Inasmuch as the 

 term nucleus has long been given to the vesicular body in the interior of all cells, 

 ambiguity would be avoided by designating such nerve cell groups by the term 

 nidi (plural of nidus, "a nest"). 



"NERVE FIBRES" AND NERVES. 



Prior to the general adoption of the neurone concept it was customary to desig- 

 nate the conducting elements of the nerve system by the term nerve fibres in dis- 

 tinction from the nerve cells. As has been pointed out above, the distinction 

 no longer holds, but the designation "nerve fibre" is still retained in anatomic 

 vocabulary and recurs so frequently in common parlance that, even with the new 

 conception which has been formed of the architecture of the nerve system, the 

 term cannot yet be entirely discarded in favor of "axone," although it probably 

 will eventually. 



Nerves are round or flattened bundles of axones which serve to bring the central 

 axis into relation with the periphery and other tissues of the body. The nerves 

 of the body are subdivided into two great classes the cerebrospinal, which are 

 attached to the cerebrospinal axis, and the sympathetic or ganglionic nerves, which 

 are attached to the ganglia of the sympathetic. The cerebrospinal nerves con- 

 sist of numerous nerve fibres (myelinic axones) collected together into small or 

 large bundles or fasciculi and enclosed in a membranous sheath. 



Structure of Nerves. In structure the common membranous investment, or sheath of the 

 whole nerve, which is called the epineurium, as well as the septa given off from it, and which 

 separate the fasciculi, consists of connective tissue, composed of white and yellow elastic fibres. 

 the latter existing in great abundance. The tubular sheath of the smaller fasciculi composing 

 the nerve trunk, called the perineurium, consists of a fine, smooth, transparent membrane, 

 which may be easily separated, in the form of a tube, from the fibres it encloses; in structure 

 it consists of connective tissue which has a distinctly lamellar arrangement, being composed of 

 several lamellae, separated from each other by spaces containing lymph. The nerve fibres are 

 held together and supported within the fasciculus by delicate connective tissue called the endo- 

 neurium (sheath of Henle). It is continuous with septa which pass inward from the innermost 

 layer of the perineurium, and consists of a ground substance in which are embedded fine bundles 

 of fibrous connective tissue which run for the most part longitudinally. It serves to support the 

 capillary vessels, which are arranged so as to form a network with elongated meshes. The 

 cerebrospinal nerves consist almost exclusively of myelinic axones, the amyelinic axones existing 

 in very small proportions. 



