The visceral ganglion of Pecten, etc. 
7 
The microscopie structure of the yisceral ganglion. 
Kawitz is the only investigator who has concerned himself with 
the histology of this ganglion. In the last few years however our know- 
ledge of the structure of nervo cells and the associated non-nervous cells 
has increased at a rapid pace, thanks to the great developments in mi- 
croscopie technique and it is not surprising that the older descriptions 
need some modification. The lamelHbranchs seem to have proved un- 
favourable examples for research on the structure of the ganglion cells 
(though the gastropoda have received considerable attention), and there 
is but httle literature on the ganglia of this group. 
Freidenfelt (1897, 1905) and Bochenek (1905) have discussed 
certain points in the structure of the ganglia of Anodon, and List (1902) 
refers though not in detail to the microscopie structure of the ganglia in 
the Mytilidae. An investigation of the microscopical structure was not 
originally premeditated but it became evident from the first sections that 
several of Rawitz' observations like those on the macroscopic structure, 
were incorrect and hence more special methods were adopted and I have 
endeavoured to trace as much of the minute histology as possible in a 
general work on the ganglion. The visceral ganglion is composed of a 
cortex (often of several layers) of ganglion cells enclosing a centrai mass 
which has received the names of »Punktsubstanz« (Leydig 1857), »Mark- 
substanz« (DiETL 1877), and Neuropil (His, Bethe). The whole is en- 
closed by a sheath which in its staining properties resembles connective 
tissue. Its extensions can be traced easily after the use of Mallory's 
Connective tissue stain (see Zeit. Wiss. Microsc. Voi. 18 1901). This 
ganglion sheath consists of a homogeneous ground substance which stains 
amore intense colour than the connective tissue ground substance, though 
similar to it and contains numerous neuroglia ceUs and fibres. The 
sheath is not so well developed in Pecten as in some other lamelH- 
branchs of which Mya truncata is a good example. It is continued over 
the nerves and in the case of the cerebro-visceral connectives sends 
numerous lamellate processes into them. Rawitz (1887) stated that there 
were two sheaths, an outer and an inner. The iatter corresponds to the 
one I have mentioned above; the other is probably only due to the ad- 
j acent connective tissue in which the ganglion lies buried. From the 
ganghon sheath extensions pass into certain parts of the ganglion. The 
sheath itself penetrates deeply between the dorso-ventral lobes and bet- 
ween these and the ventro-central lobe, but the extensions penetrate 
between groups of ganglion cells. This is additional to the neuroglial 
