CARDIAC INHIBITION IN DECAPOD CRUSTACEA 147 



Fig. 3. Cardiac ganglion of Paiiiilinis inteniiptiis, portion of trunk containing 

 third and fourth large cells and junction of dorsal nerve (methylene blue stain). 

 Observe thick inhibitor (/) and thinner accelerators (a) entering from upper right. 

 A number of fine fibers surround cell somata and run along initial segment of 

 axon (arrows). Tangled mass of fine fibers in center of figure is considered 

 "neuropile". Calibration, about 100 //. 



The extrinsic cardioregulatory fibers originate in the thoracic gangha, 

 probably in the first three segments. In Panulirus argus they leave the ventral 

 thoracic ganglion mass in three pairs of nerves. The most anterior (segmental 

 nerve 1) carries the inhibitor fiber, the two more posterior nerves (segmental 

 nerves 2 and 3) carry one accelerator fiber each. All run dorsally through the 

 lateral body musculature and enter the pericardium to unite in the lateral 

 pericardial plexus, a portion of the pericardial organ of Alexandrowicz 

 (1953). From each plexus a single dorsal nerve runs to the heart. This may 

 contain several fibers near its origin, but only three, one from each of the 

 original nerves, appear to enter the heart and reach the cardiac ganglion 

 (Maynard, 1953). 



Alexandrowicz (1932) described two systems of extrinsic fibers; a large- 

 fibered System 1 (described above) and a small-fibered System II. The present 

 observations suggest that the small-fibered Systein II may be a portion of 

 the pericardial organ which does not actually enter the cardiac ganghon. 

 All of the following physiological evidence indicates that only the larger 

 System I fibers (one inhibitor, two accelerators) are involved in the present 

 investigations. 



In Panulirus, the fibers of the dorsal nerve enter the ganglion in the region 



