428 THE TISSUES. 



while it is non-medullated, pale,- or even consists of the axis-fibre 

 alone, in another part. The division into coarse and fine fibres is 

 therefore more important, doubtless, in a histological than in a phy- 

 siological point of view. The axis-fibre alone is a constant structure. 



Pale or non-medullated fibres naturally occur in the following 

 situations : 1. Those of the Pac^i^nian bodies ; 2. The nucleated 

 pale fibres in the terminations of the olfactory nerves ; 3. The per- 

 fectly transparent non-nucleated fibres in the cornea; 4. The pale 

 processes of the nerve-cells in the central organs and ganglia. It 

 will appear that all the nerve-fibres of the embryo are in the con- 

 dition of the pale fibres now under consideration. We, however, 

 find them in different stages of development in the adult. In the 

 olfactory nerve they remain altogether in the embryonic stage, the 

 contents being much less consistent than an axis-fibre. In the 

 ohionian bodies their contents in all respects represent an axis-fibre ; 

 and the processes of the nerve-cells often exactly resemble an axis- 

 fibre, though they are frequently of a softer consistence, correspond- 

 ing with the contents of the nerve-cell. In the invertebrata, only 

 the pale nerve-fibres are found. 



The preceding are the only forms of fibres found in the cerebro- 

 spinal nervous system. 



But is there not still another variety of nerve-fibres (gray fibres) 

 peculiar to the sympathetic or gangiionic nerves? 



So far as any peculiar appearance under the microscope is con- 

 cerned, the reply may be decidedly in the negative ; though Bidder 

 and Volkmann maintained that these fibres are smaller than those 

 in the cerebro-spinal nerves, and also in other respects different. 



The fact is, the cerebro spinal nerves contain dark-bordered tubes 

 of all sizes, from the finest to the largest; those derived from the 

 sensitive roots of the spinal nerves being generally finer than those 

 from the motor roots. But the branches of the sympathetic also 

 contain the same varieties of nerve-fibres, the only perceptible dif- 

 ference being that the proportion of the finer tubes, J-Q^-Q^ to g^uu ^ 

 an inch in diameter, is greater in the sympathetic nerves. Some of 

 these fine fibres are also known to originate in the ganglia of the 

 sympathetic, but in a manner similar to the origin of the fine fibres 

 of the cerebro-spinal nerves in the cord and the encephalon. We 

 also find fine fibres, precisely like the so-called sympathetic fibres, 

 constituting the distal termination of the coarsest nerve-fibres ; and 

 that all the coarsest double-bordered nerve-fibres are, at a particular 



