NERVE -CELLS. 129 



minutes' exposure to sunlight the preparations may 

 be examined. It will be found that the sheath of 

 each funiculus or nervous bundle is covered by large 

 epithelioid marking, and if the preparation be very 

 successful, two, three, or even more layers deep of 

 such markings may be counted by examining with 

 a high power and carefully adjusting the microscope. 



In addition to this it will generally be found that 

 the medullated nerve-fibres themselves are marked 

 transversely at each of the nodes of Ranvier by a 

 dark line, or rather ring, surrounding the axis- 

 cylinder. These markings appear to be owing to the 

 existence of a substance here, between the segments 

 of the nerves, which, like the intercellular substance 

 elsewhere, has an affinity for the metal. Finally, at 

 many of the nodes of feanvier, particularly if the 

 immersion in the silver solution had been rather 

 prolonged, the latter will have penetrated to and 

 become reduced in the substance of the axis cylinder, 

 which therefore will here have the appearance of a 

 dark line or band piercing the ring of interseg- 

 mental substance, the tw T o together having the sem- 

 blance, under a moderate power, of a little black 

 cross upon the nerve at those points. Sometimes 

 in such preparations the axis-cylinder, where stained 

 at the nodes by the reduced silver, presents a cross- 

 striated appearance, but the meaning of this is not 

 known. 



Nerve- and ganglion- cells ; structure of the 

 nerve-centres. To study the exact form and ap- 

 pearance of the tissue elements of the nervous 

 centres they must be as much as possible isolated 

 and examined in teased preparations, although their 

 position and local relations are best made out in 

 sections of the several organs in which they occur. 

 For the present we will confine ottrselves to a descrip- 

 tion of the best methods for isolating the nerve-cells. 

 For this purpose the tissue is treated similarly from 

 whatever part of the brain or spinal cord the piece 

 to be examined is removed ; but, as a typical example, 



