TISSUES, ORGANS, AND SYSTEMS. 



95 



28. 



Nervous Tissue. — The essential elements of this tissue are of 

 two kinds, the nerve-fibres and nerve-cells {ganglion, globules). 

 The primitive fibres or tubules of the nerves have either a dis- 

 tinct medulla or they have none. The former consist of three 

 parts ; of a structureless delicate membrane, the sheath of the 

 primitive tubules; of a central, soft but elastic fibre, the cent nil 

 or axis-band [axis -cylinder, Purkinje; primitive band of Ilemak); 

 and of a viscid white laj^er placed between them, the medullary 

 sheath. In the tubules without medulla, which in man occur 

 only in certain peripheral ex- 

 pansions (retina, olfactory or- 

 gan, cornea, Pacinian corpus- 

 cles), the structureless coat 

 contains nothing but a homo- 

 geneous or finely granular, clear 

 substance, which appears to be 

 identical with the central band 

 of the other tubules, and at 

 any rate may be considered 

 analogous to it, so that the 

 medullary layer may be sup- 

 posed to be absent in these. 

 The primitive nerve tubules of 

 both kinds, especially of the 

 former, occur of very different 

 dimensions, and may thence be divided into fine ones of 0-0005 

 — 0-002'", those of a medium size of '002—0 004'", and thick 

 ones of 0-004 — 001'". Their course is either isolated, so that 

 one tubule runs from the centre to the periphery ; or they 

 divide, especially in their terminal expansions, into a greater or 

 smaller number of branches ; or, lastly, they form actual anas- 

 tomoses and networks. Besides this, many nerve-tubules are 

 connected with nerve-cells, so that they either arise from them 

 or are interrupted in their course by interposed nerve-cells. 



These nerve-cells, or as they are called in the 



gang 



lia, 



Fig. 37. Tubular nerve-fibres of man, x 350. Four of tbem fine, two of them 

 being varicose, one of a medium thickness with a simple contour, and four thick ones ; 

 two having double contours, and two with granular contents. 



