350 SPECIAL HISTOLOGY. 



and we must, in this question, for the present, abstain from all conclu- 

 sions drawn from analogy. 



[Of the chemical nature of the sheath of the nerve-fibres but 

 little is known. When boiled for about five minutes in concentra- 

 ted acetic acid, it is not dissolved, scarcely changed. It resists 

 boiling in caustic soda for a short time without altering, except that it 

 swells up a little ; longer boiling dissolves it. It remains unchanged in 

 water, alcohol or ether, even when boiled. Pettenkoffer's test for 

 bilin (sugar and sulphuric acid) which reddens protein and elein, but 

 according to Schulze, neither collagenous substances nor elastic tissue, 

 does not color the nerve-sheath. Yet its action cannot be ascertained 

 with the most desirable accuracy, since the nerve-medulla is simultane- 

 ously reddened. Sulphuric acid and potassa render the nerve-sheath 

 yellow (xantho-proteinic acid), producing the same effect as they do, ac- 

 cording to Paulsen, on elastic tissue. Hence it seems as if the nerve- 

 sheath agrees in composition with elastic tissue, except that it is evidently 

 less insoluble in alkalies. From Kolliker's Micr. Anat. II. 1. DaC.] 



In order to see the medullary sheath or nerve-pulp in its normal con- 

 dition, a nerve of an animal just killed, without any addition, must be 

 quickly brought under the microscope ; in which case some isolated fibres 

 will always be seen quite unchanged, although, as the nerve dries, they 

 are very rapidly altered. Besides this method, I would also recommend 

 the examination of the nerves in the transparent parts of animals, either 

 alive or just killed (nictitating membrane, mucous membrane of the 

 Frog, tail of Tadpole, c.), the observing of them on warmed pieces of 

 glass (Stark.), and after treatment with chromic acid, which frequently 

 preserves, particularly the cerebral fibres, quite uninjured. The nerve- 

 pulp or medulla is obviously a viscid, fluid, extensible, glutinous sub- 

 stance, to be compared in point of consistence with thick oil of turpen- 

 tine, and which under pressure assumes all possible figures, appearing 

 in the form of globules, filaments, and membranous masses, of very dif- 

 ferent aspects, with pale or dark borders, and opaque or clear. In 

 chemical composition it consists principally of fatty matter. 



The central filament of the nerve-fibres, which was perhaps seen as 

 enrly as by Fontana, and with which we have become better acquainted 

 under the name of " primitive band" given to it by Remak, or " cylinder- 

 axis" as it has been termed by Rosenthal and Purkinje, is indisputably 

 the most difficult of investigation, and the least known portion of the 

 nerves. There is no microscopist who has not frequently seen this axis- 

 fibre, but it may, without fear of contradiction, also be asserted, that 

 there is none, not even excepting Remak himself, its discoverer, who can 

 boast that he has studied and learned its relations in every particular. 

 For this reason, but few, as Hannover and J. Muller, are unconditionally 



