•212 



NERVOUS SYSTEM. 



[sect. 109. 



The envelope or sheath of the nerve- tubes (limiting membrane, 

 Valentin), is an extremely delicate, yielding but elastic, homo- 

 geneous, and transparent membrane, which is invisible in perfectly 

 unaltered nerve-fibres, except in a few places, but comes readily 

 into view on the employment of proper re-agents, at least in the 

 thicker fibres of the nerves and central organs, and, in its che- 

 mical characters, agrees in all essential points with the sarcolemma 

 of the muscular fibres. This envelope has not hitherto been 

 demonstrated upon the finest fibrils of the peripheral and central 

 nervous system, and, accordingly, it is still uncertain whether these 

 fibres possess sheaths or not. 



Within the homogeneous sheath lies the nerve-medulla (medul- 

 lary sheath, Rosenthal and Purkinje; white substance, Schwann) 

 (figs. 90, 3, b), in the form of a cylindrical tube, closely and accu- 

 rately surrounding the central fibres. It is, in the fresh nerve- 

 fibre, perfectly homogeneous, viscid like a thick oil, transparent 

 and clear, or white and shining, according to the light by which it 

 may be viewed, and evidently causes the peculiar lustre of the 

 nerves. Under the influence of cold, of water, most acids, and 

 Fig . 9i. many other re-agents, the nerve- 



medulla is quickly and very con- 

 stantly altered ; this alteration is 

 principally duetoa coagulation, which 

 proceeds from without inwards, and 

 affects sometimes the whole medulla, 

 sometimes only its outermost layers. 

 In the latter case, the nerve-tubes 

 appear with double contours (figs. 

 90, 91), or with medullary sheath 

 coagulated to a more or less extent 

 externally, but still fluid internally; 

 in the former, the fibres have appa- 

 rently quite grumous and dark con- 

 tents (fig. 91). The coagulated 

 nerve-medulla rarely appears homo- 

 geneous, but mostly grumous, gra- 

 nular, as if composed of irregular 

 masses of various sizes, and, on the 

 addition of acetic acid, frequently as 

 if formed of small isolated or re- 

 ticularly united rods. By pressure, also, the nerve-medulla alters 

 very readily. It either flows out at the extremities of the tubes, 



Ilumin nerve-tubes, magnified 350 times. 

 Three of them are fine, one of which is vari- 

 cose, one of middling thickness and with ji 

 simple contour, and three thick, two of which 

 are double contoured and one with grumous 

 contents. 



