332 Journal of Comparative Neurology. 



are as a rule supplied by much smaller nerve fibers than are the 

 large and highly functional organs of the same fish. 



Still another case is furnished by the terminal buds 

 (gustatory organs) of the outer skin and barblets of some 

 fishes. The distribution and innervation of these organs I 

 have worked out in some detail in the case of Ameiurus 

 melas, as already reported (Herrick, 'oi). These organs 

 are similar in structure to the taste buds in the mouth and 

 like them they are innervated by communis nerves, so that 

 the nerves of the two sets of sense organs can be directly com- 

 pared. It is characteristic of communis nerves generally that 

 their diameter is less than that of other types of cerebro spinal 

 nerves and the medullary sheaths are thin. The communis 

 nerves which supply the taste buds of the mouth cavity are not 

 exceptions to this rule, but those which supply the very large 

 gustatory organs of the outer skin of the siluroids are consid- 

 erably larger and are provided with much more dense medullary 

 sheaths than is usual for fibers belonging to this system. The 

 more highly developed terminal organs and greater functional 

 importance doubtless have called forth a change in the character 

 of the nerve fibers. That these cutaneous sense organs of the 

 siluroid fishes are in fact highly functional as a gustatory appa- 

 ratus I can definitely afifirm on the basis of experiments now 

 nearly ready for publication. 



Miss Dunn concludes that in the case which she has exam- 

 ined there is no direct correlation between the diameter of the 

 nerve fibers and the length of the fibers. I would make this 

 conclusion general and add to it that there is, in some cases at 

 least, a correlation between the diameter of the fiber and the 

 functional importance of the fiber, or the physiological impor- 

 tance of its terminal organ as compared with other organs of 

 the same system. The qualification stated, "of the same 

 system," is important. In 1899 I formulated (p. 173 of the 

 Menidia paper) the following definition of the functional system 

 of nerves: "Each system may be defined as the sum of all 

 fibers in the body which possess certain physiological and 

 morphological characters in common, so that they may react in 



