ON NERVINE SECRETION 87 



backward flow (Fig. 21), thus securing the integrity of the 

 nerve cells, cerebral and ganglion ic, against mechanical 

 pressure and violence, besides aiding in procuring the 

 outlet, or the excretion, of the materials secreted by, and 

 produced in, the nerve cells with their waste products. 



The "white substance of Schwann" and its sheath, with 

 the associated " nodes of Ranvier," thus act in the triple 

 capacities of insulating, supporting, and protecting 

 agencies. The occurrence of pain, it might here be 

 remarked, as a nerve phenomenon as well as a symptom 

 of disease, is bound up with the maintenance of the 

 integrity of these structures, and consequently pain may 

 thus be expected to be felt where they are naturally thin, 



FIG. 21. NERVE-FIBRES STAINED WITH NITRATE OF SILVER TO SHOW 

 FROMMANN'S MARKINGS IN THE AXIS-CYLINDER. (Ranvier.) 



A, Fibre showing a node, a, with the constricting ring. The axis-cylinder has 

 become shifted, and the part which was opposite the node and which is stained 

 by the silver, is now below it ; r, conical enlargement of the axis-cylinder. 



B, Isolated axis-cylinder. 



as at the distal terminations of the peripheral nerve fibrils 

 in the cutaneous envelope of the body, where the " white 

 substance of Schwann " is said to be usually absent or 

 sparsely distributed, and in the substance of tissues and 

 organs, where sensory nerve fibrils more or less likewise 

 terminate. It may also be expected to be felt wherever 

 the continuity of these protective tissues is interfered with 

 by pressure, or injured by traumatic influences : pain, 

 moreover, in itself may be regarded as a molecular 

 disturbance of the material of the axis cylinders of the 

 nerve fibrils involved, initiated, and realised, or felt, at the 

 spot, or conveyed outwards to the distal terminations of 

 the involved fibrils from the initiating central, or proximal, 

 nerve fibre indirectly or reflexly, so to speak, from the 

 lower centre with which it is related. 



