The investigation which forms the subject matter of this paper was the outcome of three observations: (i) the great variation in size of the funiculi comprising peripheral nerves ; observations on 80 specimens have revealed diameters ranging from 0.04 to 3 m m ; (ii) the variation in tKickness of the perineurial sheath with the size of the bundle; (iii) the hernia tion of the funicular contents which occurs through a rent in the perineurium; this suggests that there is a pressure inside the bundle which is resisted by the perineurium.
As a result of these observations it was decided to investigate more precisely the relationship existing between the size of a bundle and the thickness of its perinenrial sheath.
MATERIAL AND METHODS
The material on which the observations were made was that used f o r an earlier study of the cross-sectional area of peripheral nerve trunks occupied by nerve fibers (Sunderland and Bradley, '49). I n all, 20 specimens each of the radial, median, ulnar, sciatic and popliteal nerves were examined but the measurements recorded in this paper are from 11 specimens of the median, 4 of the ulnar, 2 of the radial and 1 of the sciatic nerve and its popliteal divisions. These, however, demonstrate all of the features presented by the other specimens.