184 Proceedings of Royal Society of Edinburgh. [sess. 
of potash, osmic acid, and nitrate of silver. The interaction of 
bichromate of potash with nitrate of silver results in the formation 
of chromate of silver, which is precipitated, in the form of a dense 
blackish matter on the surfaces of nerve cells and on the processes 
connected with these. In this way numerous fine fibrils are 
shown, most of which escaped detection by the older methods, and 
a large number of new and important facts have thus been brought 
to light. These facts are now described in almost every text-book, 
but their significance and importance have not yet been appreciated 
even by physiologists, and still less by those who are interested 
generally in physiological science, more especially in its bearings 
on psychological questions. This must be my excuse for bringing 
a matter forward in this address which has lately occupied much 
of my attention. 
A nerve cell, with its prolongations or processes, may now be 
regarded as a nervous unit, to which Waldeyer has given the 
name of neuron. The cell is a mass of protoplasm, showing usu- 
ally in its interior numerous fine fibrils, which enter it by each 
process, and cross and re-cross in all directions, so as to form a fine 
and irregularly-meslied network. From the cell arise numerous 
processes or poles, to use the old term. With the exception of 
the bipolar cells in the ganglia on the posterior roots of the spinal 
nerves, and a few others, all nerve cells show numerous processes, 
or they are multipolar. These processes also appear to be fibrillar 
in their texture, and they are of two kinds : — (1) Protoplasmic 
processes, or dendrites , which, by dividing and subdividing, form 
a very fine set of twigs, like the twigs on the branclilets of a tree 
as seen against a winter sky. This dendritic appearance is called 
an arborization. (2) A thicker process, of variable length, which 
is now known to be the axis-cylinder of a nerve fibre. In the 
cerebral nervous organs these axis-cylinder processes may be very 
short, amounting to only one or two millimetres, but axis-cylinders 
may extend from cells in the anterior cornua of grey matter in the 
cord to muscles in the limbs, and they may then reach a length of 
many millimetres. The axis-cylinder process also terminates in a 
smaller kind of arborisation, by which it comes into relation with 
(1) a motor apparatus, such as the contractile fibres of a muscle ; 
(2) a secretory apparatus, such as the cells in the acini of a 
