12 
W. J. Dakin 
The Neuropil. This is the central part of the ganglion and has 
received the names of »Punktsubstanz« and »Marksubstanz«. Its true 
nature has also been for some time a matter of doubt. Leydig stated 
it consisted of a network and a tangle of the finest fibres, Solbrig ref erred 
to it as a granular mass penetrated by a fine system of fibres, and Haller 
described it as consisting to a largo extent of the processes of ganglion 
cells, which brauch forming a polyhedral meshed net. It is unnecessary 
however here to go into the history of the subject. The neuropil consists 
chiefly of a tangled mass of the finest nervo fibrillae, which arise by the 
breaking up of ganglion celi processes or the nerve fibres from peripheral 
nerves, or from lateral processes of axones and it is difficult to say whether 
these fibres form a true net or a tangled mass of blind ending fibrils. 
It is penetrated in various directions by thicker nerve fibres passing 
direct through the ganglion, from one nerve to another or by axones 
which can be traced from ganglion cells in the cortex and which run direct 
into the peripheral nerves. Apathy and Bethe (1903 pag. 42) consider 
the fibrillae in the neuropil to form a closed net by anastomosing. Retzius 
(1905) and others maintain that there is no direct union, and Freiden- 
felt (1897) States that methylene blue preparations show that the »Mark- 
substanz« is in no way a real net and that every fibril leaving a centrai 
ganglion comes direct from a celi without the Interpolation of a nerve 
net. The latter part of his statement is however incorrect and whilst 
it is extremely difficult to determine whether the closely packed deli- 
cate fibrillae of the neuropil really anastomose or simply rest one upon 
the other I believe real nets are formed to a certain extent. One reason 
for this statement is the great difficulty in finding blind endings which 
ought to be more readily visible if the neuropil is not a closed 
network. 
The neuropil occupies as akeady pointed out the centrai part of 
the ganglion and lobes, with the exception of the dorso-central lobes 
which are almost entirely composed of ganglion cells, and their axones. 
The neuropil is not found in the connectives or peripheral nerves, and 
longitudinal sections through the latter show that quite dose to the gang- 
lion, the nerves contain only the parallel running fibres. 
It is impossible to trace ali the axones of the ganglion cells, through 
the visceral ganglion of Pecten. Many of them break up and are lost 
very soon in the neuropil. Others which pass directly into peripheral 
nerves or connectives are lost because of the difficulty of foUowing an 
individuai fibre through sections of the neuropil. Only those fibres, 
whose course has been definitely foUowed are described below, and it 
