464 ELEMENTARY PHYSIOLOGY less. 



themselves divide) to this point of entry each nerve-fibre ex- 

 tends ensheathed as 07ie continnons imdivided axis-ajlindcr 

 in a long succession of internodal segments. At the point 

 of entry into the cord the perineurium passes into the pia 

 mater and the general connective tissue framework of the 

 cord. The neurilemma and the nodes disappear. Often 

 the axis-cylinder can be traced towards the anterior horn of 

 the grey matter, invested only by a sheath of medidla 

 which gradually becomes thinner and thinner until at 

 length it disappears, and the fibre, thus reduced, 

 passes into one of the processes of one of the 

 large nerve cells, which lie in the anterior cornu of the 

 grey matter (see p. 467). 



The axis-cylinder of a motor nerve-fibre, therefore, is in 

 fact an extremely fine and long process of a nerve cell, 

 which passes at its peripheral end into one or more muscle 

 fibres ; in other words, the nerve cell and the muscle cells 

 are the central and peripheral end-organs of the nerve-fibre. 



With one or two exceptions, sensory (afferent) nerve- 

 fibres are not distinguishable by any structural character 

 from motor nerve-fibres. Wherever special-sense or- 

 ganules (p. 345) exist, the sensory fibres are connected 

 with them by means of their axis-cylinder from which the 

 neurilemma and medulla have di.sappeared. If, as before, 

 we follow the sensory nerve-fibres back towards the 

 spinal cord, we find that they pass through the ganglion 

 on one of the po.sterior roots, and then enter the substance 

 of the cord, passing towards the posterior cornu. Like 

 the motor nerve-fibres, they lose their noded neurilemma 

 as they enter the cord, so that in this case also it is again 

 the axis-cylinder which provides the actually continuous 

 connection between the sense organ and the central 

 nervous system. 



The neurilemma, with its nucleus, and the medulla may 

 be regarded as a covering which provides for the protec- 

 tion and nourishment of each successive length of the 

 essentially important axis-cylinder. 



