NERVOUS TISSUES. 



267 



1C4 



A chain of ganglions is situated on each side, near the vertebral 

 foramina, through which the cerebro-spinal nerves issue. These 

 ganglions radiate many nerves, connecting them one with another 

 and with the cerebro-spinal nerves, and ramifying in a plexiform 

 way upon the viscera and coats of the blood-vessels : they con- 

 stitute the 'sympathetic' or 'ganglionic' system in Vertebrates. 



In the cerebro-spinal nerves the primitive fibre consists of a 

 transparent elastic homogeneous tubular 

 membrane (neurilemma), fig. 1 64, a ; its 

 contents are pulpy, homogeneous in the 

 living or recently dead state, and may be 

 pressed out of the sheath ; when treated 

 with water, as in fig. 164, a, or with alco- 

 hol, they condense into a white layer, 

 giving that colour to the tube : within the 

 white substance Remak defines a ' flattened 

 band,' and Purkinje an ' axis-cylinder.' 

 When treated with ether, oil-globules co- 

 alesce in the interior, and accumulate around 

 the exterior of the tube, fig. 164, b. 



The delicacy of the neurilemma, and 

 mobility of its contents, lead, in many cases, 

 to partial dilatations of the tube, of a ' vari- 

 cose ' character, probably due to post-mortem 

 influences : in the living or natural state, the primitive nerve- 

 tube or fibre appears to be perfectly cylindrical. 



The following are results of Todd's admeasurements of their 

 diameter, in the different vertebrate classes : — 

 Fishes (Eel) YoTa of an inch. 

 Reptiles (Frog) -^-^ to ^ 7 of an inch. 

 Birds 2-oVo t° 3 0*0 °f an i ncn « 

 Mammals Yihr to nVo °f an inch. 1 



Primitive nerve-fibres do not divide or branch; they are 

 associated together, in simple juxtaposition, supported by fine 

 layers of areolar tissue, which condense at the periphery into a 

 common sheath, to which the term ' neurilemma ' is commonly, 

 but not properly, given : it answers to the sheath which surrounds 

 a muscle, similarly binding the constituent fibres of the nerve 

 together, and supporting their nutrient capillaries. These are the 

 smallest in the body ; they run chiefly parallel with the nerve- 

 fibres, forming oblong meshes, completed at intervals by cross- 

 vessels. Sometimes the nerve-fibres have a wavy course within 

 the general sheath, fig. 165. In a few instances they have been 



1 ccv. p. 593. 



Nerve tubes altered by re-agents. 

 ccv. 



