CHEMICAL SIGNS OF IRRITABILITY 31 



at a maximum only about o . 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 2.3Xio~ 7 to 

 4 . 7 X io~ 7 g. CO 2 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 

 6.7Xio~ 7 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 . 6X io~ 7 g. of carbon dioxide, 

 while the optic nerve gives somewhat more, namely, 2 . 6 to 

 5X io~ 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. 



