(DONALDSON, 1895, p. 154). The fact that it was maintained. in the growing fiber was also noted (DONALDSON, 1901, p. 180; 1901, A, p. 326); and later DUNN ('00, '02) verified the relation by the study of the medullated fibers in the sciatic nerve of the frog. It seemed desirable, however, to extend the observations on this point, and in 1901, Mr. HOKE took up the question and determined the relative area of the axis and sheath in cross sections of fibers from the nerves of 27 species of vertebrates representing the five great classes.1

Technique. The animals were killed with chloroform. The nerve, usually from the brachial plexus, laid bare and partially fixed in situ with osmic acid (1% sol.). After half an hour the nerve was removed on a piece of cardboard to prevent shrinking, and replaced in a 1% solution of osmic acid for 24 hours. Then imbedded in paraffin by the usual method. The sections were cut 3.5 thick and mounted in colophonium.

The measurements of the large fibers were made under the magnification of 340 diameters, or, in some cases, 265 diameters. The very small fibers were measured with the 1-12 oil immersion.

The Effect of Osmic Acid Treatment upon the Size of Medullated Peripheral Nerves.-This reaction was studied because of its obvious bearing upon the observations here presented.

BOLL ('76) states that 1% osmic acid causes a swelling of the sheath to almost double its normal size. This, however, was the result of putting a fiber which had been teased out while fresh, in a drop of the acid and examining it after a short time. Prolonged immersion in osmic, he says, in the same place, is followed by a shrinkage of the fiber. These statements lose value by reason of the fact that teasing out the fresh fibers stretches them and thus alters their reaction to the reagent.

BOVERI (85) Concludes that the normal form and structure of the medullary sheath is very little modified by the osmic acid

1 Mr. HOKE's account of his work was accepted as a thesis for the degree of Master of Science in the department of Neurology of the University of Chicago in 1902. The data in that thesis form the basis of the present paper.

treatment, and this view is shared by a number of other observers who have studied this question. The method employed in this investigation was practically the same as that used by BOVERI.

Speaking generally, it may be said that mistreatment tends to cause a swelling of the medullated fiber (in 1% osmic acid) in which the sheath becomes somewhat more swollen than the axis. The most reliable measurements, therefore, are those made on nerves which have suffered the least mechanical damage.

To determine whether our method of treatment produced permanent alteration in the size of the nerves used, a series of observations was made upon the eighth, ninth and tenth spinal nerves running free along the dorsal wall of the body cavity of the frog, and also on the sciatic nerves of the white rat.

The nerves were laid bare and a bristle bent in the form of the letter U, exerting a tension of .25 to .40 grams, according to the size of the nerve, was rapidly tied to either end of the nerve. In this way about one centimeter of nerve was included between the ends of the bristle. The nerve thus prepared was then removed and before putting it in any fluid its diameter was carefully measured under the microscope. It was then placed for twenty-four hours in a shallow cell containing osmic acid. The subsequent treatment was exactly similar to that given under the paragraph on technique. Bristle and attached nerve were finally mounted in colophonium and the diameter again carefully measured.

The results of these observations are presented in Table I for the Frog and Table II for the Rat.

The column on the left gives the diameter in μ of the fresh nerves with the bristle attached. On the right is given the diameters in while the nerves are in colophonium, after complete treatment by the osmic acid method as described above. In Table I, the middle column introduces the diameter of the nerves after having been in osmic acid twenty-four hours. The final number at the foot of each column gives the square of the average rad ius. This number, when multiplied by would

give in sq. the total area of the nerves measured. As indicated, the nerves of the frog have increased in area 2.4%, those of the rat, .57%.

The measurements upon the rat were the last made, after some skill had been acquired in this manipulation, and are probably the more reliable.

TABLE I.

To show the effect of the technique upon the eighth, ninth and tenth spinal nerves of the Frog.

Seven specimens.

To show the effect of the technique upon the sciatic nerve of the White Rat.

Nine specimens.

The results of these observations seem to justify the conclusion that the osmic acid treatment, followed by the preparation for examination, produces but little change from the normal diameter (or area) of peripheral nerve fibers.

Conditions Determining the Choice of the Fibers to be Measured.-There were used for measurement only those fibers which had been cut at right angles to their long axis and which stood vertically. It is easy to see that any departure of a fiber from the vertical would make the measurements for the thickness of the sheath too large, and those for the axis correspondingly too small. From measurement were excluded those fibers in which the medullary sheath appeared double, as occasionally occurs when the fiber has been cut through an enlarged cleft of LANTERMANN.

The rare instances in which the section passes through an internodal nucleus, or just above or below a node--where the relative area of the medullary sheath is very greatly increasedwere easily avoided. A fiber in which the sheath was wrinkled, or which departed much from the circular form, was not measured. Where the section of the nerve fiber was suitable in other respects-and at the same time was slightly oval-two diameters were taken and the mean taken as the value to be used. Among the small fibers those that were stained gray and not black were classed as immature, and were not measured. It is among the small fibers that the greatest normal range in the relative development of the medullary sheath appears, and it is here too, that the greatest difficulties in making exact measurements are met, any departure from the vertical being especially disturbing.

The very small medullated fibers which appear in the rami communicantes were not studied in this investigation.

Method of Measurement.-On sections of fibers thus prepared and thus selected, the diameter of the entire fiber was first measured and then the diameter of the axis. In order to give an idea of this procedure we may take as an example the first group of ten measurements, the final results of which appear in Table VI after Specimen I. In the first column of Table

III (see below) is given under A + S, (axis + sheath), the total diameter of each fiber, while under A (axis) is given the diame ter of the axis. The two columns to the right give the corre The measurements were first put down in

sponding radii.

terms of the divisions of the ocular micrometer-the subdivis

ions being estimated to tenths of these units.

TABLE III.

Diameters and Radii of entire fibers and axes, in divisions of

To transform these measurements into we multiply in

each instance by the value of one division of the ocular micro

meter which in this case was 3.06 u.

As the radii alone will (See Table IV.)

It is on the basis of these radii that we calculate the areas of the fibers and of their axes, and for this we employ the formula 2 giving the value of 3.14.

As the square of the average radius for the group is not equivalent to the average of the squares of the radii, and as the