523 
This method for the differential staining of axons we have called 
the pyridine-silver technique. It is a modification of the Casat stain 
which itself stains axons with the greatest clearness in the gray sub- 
stance of the central nervous system and in the peripheral ganglia. 
The Casat method does not, however, clearly differentiate non-medullated 
fibers from connective tissue and neuroglia, and fails to bring these 
axons out with any clearness in the cerebrospinal nerves and in the 

Fig. 1. From a cross section of the trunk of the human vagus nerve some 
distance below the nodose ganglion. Pyridine-silver. X 1020. 
fiber tracts of the central nervous system. By the use of pyridine 
it is Impossible to secure a selective staining of the non-medullated 
fibers, making these appear black while all other parts of the tissue 
are light yellow or colorless. 
The steps of this technique are as follows: The animal is exsanguin- 
ated and the desired tissue promptly removed and placed in absolute 
