20 NERVOUS SYSTEM. 



changed, their anatomical elements appear in the form of 

 simple fibres with strongly accentuated borders. The diam- 

 eter of these fibres is from ^fa to -p^Vo" of an inch. 1 To 

 observe the fibres in this way, it is necessary to take a nerve 

 from an animal just killed and examine it without delay. 

 In. a very short time the borders become darker and the 

 fibre assumes an entirely different appearance. By the use 

 of certain reagents, it can be demonstrated that a medullated 

 nerve-fibre is composed of three distinct portions ; viz., a 

 homogeneous sheath, a semi-fluid matter contained in the 

 sheath, and a delicate central band. 



The tubular sheath of the nerve-fibres is a somewhat elas- 

 tic, homogeneous membrane, never striated or fibrillated, 

 and presenting generally oval nuclei, with their long diam- 

 eter in the direction of the tube. This is sometimes called 

 the neurilemma, a name, however, which is more generally 

 applied to another membrane. It is sometimes spoken of, 

 also, as the " limiting membrane of Valentin," or " the sheath 

 of Schwann." In its chemical and general properties, this 

 membrane resembles the sarcolemma, though it is less elas- 

 tic and resisting. It exists in all the medullated nerve-fibres, 

 large and small, except those in the white portions of the 

 encephalon and spinal cord. It is not certain that it does not 

 exist in the small, non-medullated fibres, though its presence 

 here has never been satisfactorily demonstrated. 2 As we 

 before remarked, the tubular membrane cannot be seen in 

 the perfectly fresh nerves ; and even after they have become 

 changed by desiccation, its demonstration requires the use of 

 reagents. In the ordinary medullated fibres, however, it 

 may be isolated by boiling the nerve in absolute alcohol and 

 then in acetic acid, or by treating it with cold caustic soda. 

 By then boiling the nerve for an instant in the caustic soda, 

 fragments of the tube may be isolated, when they resemble 

 the membrane forming the canals of the kidney. Another 



1 LITTRE ET ROBIN, Didionnaire de mededne, Paris, 1865, Article, Nerveux. 

 9 KOLLIKER, Element cPhistologie humaine, Paris, 1868, p. 315. 



