566 NERVOUS SYSTEM. 



in diameter ; others have only the medullary covering and the axial band ; and others 

 present the axial band alone. Most of these anatomical elements have essentially the 

 same physiological conducting properties ; the variations in their structure depending 

 upon differences in their anatomical relations. In view of these facts, it will be con- 

 venient to adopt some anatomical classification of the fibres. 



In the most simple classification of the nerve-fibres, they are divided into two groups ; 

 one embracing those fibres which have the conducting element alone, and the other pre- 

 senting this element surrounded by certain accessory structures. In the course of the 

 nerves, the simple fibres are the exception, and the other variety is the rule ; but, as the 

 nerves are followed to their terminations in muscles or sensitive parts, or are traced to 

 their origin in the nerve-centres, we find that they lose one or another of their adven- 

 titious elements. These two varieties we shall term : 1. The medullated fibres, and 2. 

 The simple, or non-medullated fibres. 



Medullated Nerve-fibres These fibres are so called by French and German writers 

 because, in addition to the axis-cylinder, or conducting element, they contain, enclosed 

 in a tubular sheath, a soft substance called the medulla. This substance is strongly 

 refractive and gives the nerves a peculiar appearance under the microscope, from which 

 they are sometimes called the dark-bordered nerve-fibres. As the whole substance of 

 the fibre is enclosed in a tubular membrane, these are frequently spoken of as nerve- 

 tubes. 



If the nerves be examined while perfectly fresh and unchanged, their anatomical ele- 

 ments appear in the form of simple fibres with strongly-accentuated borders. The diame- 

 ter of these fibres is from -^^ to yyV^- of an inch. 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 elastic, homogeneous membrane, 

 never striated or fibrillated, and presenting generally oval nuclei, with their long diame- 

 ter in the direction of the tube. This is sometimes called the neurilemma, a name, how- 

 ever, 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, although it is 

 less elastic 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, although its presence here has 

 never been satisfactorily demonstrated. 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 medul- 

 lated 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 method is to treat the nerve 

 with fuming nitric acid, afterward adding a solution of caustic potash. The fatty sub- 

 stance is then discharged in small drops, the central band is dissolved, and the empty 

 sheath is seen, swollen and tinged with yellow. 



The medullary substance fills the tube and surrounds the central band. This is called 

 by various names, as myeline, white substance of Schwann, medullary sheath, nervous 

 medulla, etc. It does not exist either at the origin of the nerves in the gray substance 

 of the nerve-centres or at the peripheral termination of the nerves, and it is probably not 

 an essential conducting element. "When the nerves are perfectly fresh, this substance is 

 transparent, homogeneous, and strongly refracting, like oil ; but, as the nerves become 



