BouGHTON, Oculomotor Nerve. 159 



The fact that the weight- (sohd hne, Fig. i) and age- (broken hne, 

 Fig. 2) curves do not differ very much is readily explained by a 

 glance at the growth curve (solid line, Fig. 2), which does not 

 show unusual individual variations. The growth curve (solid 

 line, Fig. 2) and the "fibers-days " curve (broken line, Fig. 2) show 

 about the same degree of fluctuation. Moreover, segment for 

 segment, they are approximately parallel (with the single exception 

 of the second from the last segment), so that any variation in the 

 number of fibers is matched by a corresponding increase or decrease 

 in the body-weight. This shows very clearly the intimate relation 

 between the number of fibers and the body-weight. This depend- 

 ence of the number of medullated fibers upon the w^eight of the 

 animal (irrespective of the age) gives us a suggestion as to one of 

 the differences existing between large and small individuals of the 

 same species. It should be stated, however, that the above rela- 

 tion holds good only when all the rats compared are reared under 

 the same conditions, for one rat (body-weight, 172 grams; age, 

 unknown), not represented in the curves, but entered last in 

 Table I, and raised under less favorable conditions than the 

 others, had a body-weight much lower than normal, but the num- 

 ber of fibers usually found in rats weighing more than 200 grams. 

 3. The younger fibers which grow into the nerve, for the most 

 part, do not attain the size of the older ones, although all continue 

 to increase in size during the life of the animal. This is shown by 

 the fact that the "large" fibers can always be readily distinguished 

 from the " small " fibers. The constancy in the number of " large" 

 fibers indicates, probably, that they constitute a special group 

 which medullates early, while the "small" fibers have grown in 

 subsequently. The sudden increase from 764 "large" fibers to 

 880 between eleven days and fifteen days probably indicates that 

 all the fibers really belonging to this group had not become medul- 

 lated at the earlier date. Table II shows the increase in the 

 diameters of the "large" and "small" fibers. The first column 

 shows the average diameter of the "large" fibers. The figures 

 were obtained by taking the average of five "large" fibers that 

 seemed to be of about the average size. The second column 

 shows the average diameter of the largest five fibers in the sections 

 measured. The fourth column shows the diameters of the best 

 developed of the "small" fibers. It is shown in this table that 

 the largest of the "small" fibers does not, during two years, attain 



