Johnston, Parts of the Neurone. 615 



olfactory fibers are relatively fine. The stimuli which the olfactory 

 cells receive are commonly weak, often subliminal. These fine 

 fibers are met by the largest and coarsest dendrites known in the 

 central nervous system. May the great volume of the mitral 

 neurone provide in some way for the reception of very weak 

 impulses ? May it serve to sum up the impulses received at the 

 same time from several fibers — a function that might be called 

 synchronous summation .? Whether this particular suggestion 

 shall prove to have any value is of no moment. It seems to the 

 writer that some significance must attach to the great volume of the 

 mitral neurones; and to that of the Purkinje neurones; as also 

 the minute size, coupled with the great number, of the granule 

 cells of the cerebellum. The writer is wiUing — ^perhaps more 

 wiUing than most persons — to grant much to the factors of varia- 

 tion and chance in the determination of the actual form and 

 structure of the elements of the nervous system in a given animal, 

 but here are structures among the most constant and uniform 

 throughout the vertebrate series. The chief feature in that 

 uniformity is that of great volume and this can scarcely be ascribed 

 to accident. The Purkinje cells and the large cells of the ves- 

 tibular centers seem to be concerned with the preservation of 

 muscular tone. This requires weak rhythmical stimulation and 

 the suggestion presents itself that the large size of these elements 

 may in some way be important for the discharge of rhythmical 

 impulses. The preservation of tone and equihbrium in the lower 

 fishes have been assigned by some to certain neurones of gigantic 

 size, the Miillerian cells and fibers. 



A complete consideration of this subject must take into account 

 the cahber of peripheral sensory fibers. Upon this I have little 

 to contribute here. In the lamprey the fineness of visceral sensory 

 fibers which is a fairly constant character in vertebrates reaches 

 an extreme. The cutaneous fibers vary considerably in caliber, 

 while the neuromast fibers are, next to the motor fibers, the 

 thickest in the body. Two significant facts are to be noticed: 

 first, that the fibers supplying the taste organs in the pharynx (or 

 water tube) are coarser than the general visceral fibers, and second, 

 that the fibers of the velar nerve are much finer than the average 

 fibers of the trigeminus going to the skin. The terminal ramifica- 

 tions of the general visceral and cutaneous fibers are very rich, 

 the visceral certainly not less rich than the cutaneous. Since this 



