NERVOUS SYSTEM — CONNECTIVES AND COMMISSURES. 
211 
activity. Exercise or the transmission x>f nervous stimuli is always 
accompanied by enlargement of the nerve cells and their nuclei, and 
exhaustion is shown by their shrivelling and the presence of diffuse 
chromatin therein. 
Ganglia may be of a primary, secondarj% or accessory character. 
The primary ganglia are the large compact and paired nerve masses, 
which have their moities placed at opposite sides of the body, but 
connected together by commissures, which always cross the median 
longitudinal line : they comprise the cerebral or supra-oesophageal 
ganglia and the ganglia directly connected therewith by paired 
connectives ; the secondary or commissural ganglia are the lesser 
nerve masses which are more or less closely associated together to 
form the compound visceral ganglia ; and the accessory nerve masses 
are the small additional ganglia developed on the courses of the 
various nerves to subserve some special function. 
The Connectives are important nerve trunks richly furnished with 
ganglion cells along their whole course ; they are always longitudinally 
directed and do not cross the median line, but connect together the 
ganglia at the same side of the body. 
The three connectives which primitively 
originate from the right cerebral ganglion 
are the cerebro-pedal, which connects 
the right cerebral and the right pedal 
ganglia ; the cerebro - pleural, which 
joins the right cerebral and the right 
Fig. 418. — Section through cerebro- 
pleuro - pedal connective of Unio 
pictortivt showing the ganglion 
celjs interspersed amongst the nerve 
substance, X 175 (after Rawitz). 
pleural ganglia ; and the cerebro-buccal, which unites the right cerebral 
with the right buccal or stomato-gastric ganglion, when the latter are 
recognizably present ; the left cerebral ganglion is similarly joined to 
the left moities of the other ganglia, and in addition the right and left 
pedal ganglia are also united with the right and left pleural ganglia 
by the pleuro-pedal connectives. 
The Commissures are transverse nerve cords, usually with few 
ganglion cells, which cross the median line of the body more or less 
directly and connect together the moities of the same ganglionic 
centre placed at opposite sides of the median line ; thus the cerebral 
commissure joins the right and left moities of the cerebral ganglia 
above the oesophagus, while the remaining ganglia, the pedal, the 
visceral, and the buccal or stomato-gastric, are similarly connected 
beneath it. 
