84 BIOLOGICAL PHYSICS 



slow, or rapid, according to the condition of the body in 

 relation to rest and movement, and according to the 

 activity of the sanguineous circulation and the mental 

 organism (Figs. 18, 19). 



This circulation is molecular and capillary, and is 

 effected by the usual forces operative under such circum- 

 stances, plus the influences due to vitality, in each of 

 which respects it may be compared to the " ascent of the 

 sap " and kindred movements in plant life. 



The substance composing the axis cylinder of nerve 

 fibril (Figs. 8, n), or what we may call the true 

 nervine substance, is of consistency sufficient to prevent 



FIG. 19. PART OF A SECTION OF ONE OF THE FUNICULI OF THE SCIATIC 

 NERVE OF MAN. Magnified (after Key and Retzius). 



P, Perineurium, consisting of a number of closely arranged lamellae. En, processes 

 from the perineurium, passing into the interior of the funiculus, and becoming 

 continuous with the endoneurium, or delicate connective tissue between the nerve- 

 fibres. The connective tissue fibrils of the endoneurium are seen cut across as fine 

 points, often appearing to ensheath the nerve-fibres with a circle of minute dots 

 (fibril-sheath of Key and Retzius). Numerous nuclei of connective-tissue cells are 

 imbedded in the endoneurium ; v, section of a blood-vessel. 



rapid movement, but is yet sufficient to allow of 

 considerable molecular and mechanical displacement, and 

 hence circulation ; as, for example, when a nerve trunk is 

 pressed upon for some time its nerve fluids, or its intra- 

 fibrillar contents, become discontinuous to such an extent 

 as to prevent the passage of the usual nerve force 

 currents until their continuity is again restored, which 

 usually requires an appreciable time. The axis cylinder, or 

 true nervine substance, is surrounded by a coating, or con- 

 taining wall, of neurokeratine, which, in turn, is overlaid by 

 a continuous covering of the " white substance of Schwann" 

 enclosed by its containing, or primitive, sheath, also 

 composed of neurokeratine (see figure 8, page 73). The 

 continuity of this insulating and protective covering, or tube, 

 of " the white substance of Schwann," or medullary sub- 



