210 
NERVOUS SYSTEM — GANGLIA. 
become decicledly concentrated and forms enlargements or ganglia 
in various i)arts of the hotly, which are connected together by nervous 
cords and give rise to innumerable delicate nerve fibrils, whose 
ramifications extend to all parts of the animal. 
Tiie Ganglia are the most important part of the nervous system, 
anil are constituted by concentrated masses of whitish, or somewhat 
pigmented, nerve tissue, which arise during embryonal development 
by the thickening and subse<tueut invagination of the epiblast or 
integument. They are composed of ganglion cells of variable size, 
each of which may be furnished with some minutely delicate processes 
called Dendrites, and one or more strong fibrillar prolongations, the 
cells being known as Unipolar, Bipolar or Multipolar, in accordance 
with the number of these important nerve fibres emanating from them ; 
I'Tg. 41G. Fig. 417. 
Fig. IK). — Axis cylinder of the visceral ganglion cellsof Unio pic torn ni {V,.) X 500(after Rawilz). 
Fig. U7. — Axis cylinder of the cerebral ganglion c^\\so( A noi{o?it a anatina (L.)x500(after Rawitz). 
these various prolongations are furnished with a central core or axis 
cylinder, and serve to connect the constituent cells of the ganglia with 
each other and with other parts of the nervous system. 
The ganglion cells, however, differ in their arrangement fi'om that 
olitaining in the Vertebrates, as they are di.sposed radially around 
the central portion or nucleus, which is constituted by the aggregation 
of their filirillar prolongations, and also gives origin to the more 
homogeneous nerve fibrils. Small spherical cells exist only in the 
nioi'e anterior lobes of the cei’ebral ganglia or its connections and 
are assumed to be the seat of the more specialized sensory faculties. 
Dui’ing rest the ganglion cells store nj) nei’vous energy and also 
chromatophilous substances which become used up during their 
