NERVE TISSUE 



141 



B 



nerve fibres of the central nervous system as described by most 

 observers. Cajal, as already mentioned (p. 139), describes these 

 fibres as having a neurilemma. Their structure is similar to the 

 above-described structure of a medullated nerve fibre with a neuri- 

 lemma, except for the absence of the latter 

 sheath. 



As to the physiological significance of the 

 structural elements of the neurone, we have little 

 absolute knowledge but certain fairly well- sz 

 grounded theories. 



That portion of the neurone which surrounds 

 the nucleus — the cell body — is, as already stated. " 

 the genetic centre of the neurone, the nucleus as 

 in other cells being probably concerned in the sp 

 general cell metabolism. From the behavior of 

 the processes when cut off from the cell body it /^, 

 is evident that the latter is the trophic or nutritive 

 centre of the neurone. 



It is probable that in most or perhaps all p^ 

 neurones the usual direction of conduction along 

 the axone is cellulifugal, i.e., from cell body to gs 

 terminal arborization. The dendrites and cell 

 body would receive and probably integrate the 

 various nerve impulses received by the neurone. 

 In other words, the dendrites and cell body 

 receive, the axone transmits and the axone 

 branches and terminal arborizations distribute. 

 This general direction of conduction indicates a 

 certain polarity of the neurone. WhUe the 

 cerebro-spinal ganglion cell is obviously polar- 



— 



sg 



le - 



ss 



Fig. 86. — Scheme of Structure of Medullated Per- 

 ipheral Nerve Fibre of a Fish (Nemileff). A, Cross 

 section; B, longitudinal section; on left, fibre is shown 

 as stained intra vitam with methylene blue; on right, 

 myelin is shown black as in osmic acid staining, with 

 the incisures of Schmidt indicated; sz, cells of sheath 

 of Schwann; n, their nuclei; .J.r, sheath of Schwann; 

 sp, processes of the cells of sheath of Schwann or the 

 myelin sheath network; le, larger trabeculje of proto- 

 plasmic framework of medullary sheath arranged 

 obliquely to a.xis-cylinder and forming the so-called 

 "funnels"; leo, clear streaks in libres treated with 

 osmic acid, corresponding to le, incisures of Schmidt; 

 mo, myelin blackened with osmic acid; ax, axis- 

 cylinder; pa, periaxial space around axis-cyhnder; gs, 

 "coagulum sheath," granules probably representing 

 coagulated fluid in periaxial space; pf, peripheral, 

 non-fibrillar, part of axis-cylinder; /, neurofibrils of 

 axis-cylinder; r, ring-like thickening of Schwann's 

 sheath at node of Ranvier: 0, cavity in r. 



n 



sz 



ax 



ss 



sz 



sp 



Pi 

 g^ 



f 



