CHEMICAL SIGNS OF IRRITABILITY 31 
at a maximum only about 0.03 per cent of the wet weight 
of the tissue. 
Comparison of carbon dioxide output of nerve fibers and 
nerve ganglia.—From the table already presented it is 
clear that the living nerve trunk containing no nerve 
cells gives off carbon dioxide at a rapid rate. It is inter- 
esting to see whether nerve tissues containing ganglion 
cells produce more or less carbon dioxide per gram per 
hour than the nerve fibers. For this purpose we studied 
the ganglionated nerve cord on the back of the heart 
of the king crab (Limulus polyphemus). ‘This is an 
elongated automatic ganglion which has been shown 
to be the direct cause of the heart-beat. It was isolated 
carefully from the heart, the operation taking but a few 
minutes, placed in the biometer, and its carbon dioxide 
output measured. It was found to give 2.3X1077 to 
4.7X1077 g. CO, per centigram per ten minutes at 22.8° 
to 23° C. The rate was somewhat lower in the larger 
individuals, which were usually females. This amount 
of carbon dioxide is very small when compared with the 
output of the claw nerve of the spider crab, which with- 
out stimulation gives off from an equal weight of tissue 
6.71077 g. If, however, the comparison be made 
with the claw nerve or with the optic nerve of Limulus 
itself{—the same animal as that from which the ganglion 
was taken—the rate in the ganglion is found to be about 
the same as that in the fibers. The claw nerve of 
Limulus gives only about 2.61077 g. of carbon dioxide, 
while the optic nerve gives somewhat more, namely, 2.6 to 
5X10~7 g., depending on what portion of the optic nerve 
is taken (see p. 76). Limulus is a very sluggish, slow- 
moving animal, whereas the spider crab is more active. 
