T H E LENGTH O F THE ORGAN O F CORTI I N MAN

MARY HARDY Otologzcnl RescarcA Laboratory, Tl2e Johns Hoplmis Uiniwrst tg

FIVE FIGURES

INTRODUCTION

The length of the organ of Corti is the distance from its basal to its apical end as measured along the curves of the cochlear spiral. Direct measurement of this dimension of the organ of Corti is difficult because of its location within clense bone, its delicate structure and its shape. The litera- ture contains records for but nine hnman ears1 Retzius (1884), by direct measureniciit with a micrometer, found the length of the menibrana and papilla basilaris in five ears to be as f o l l o ~ s : a 7-month fetus, 34.0 mm. ; an 8-year-old child, 32.0 mm. ; a 24-year-old man, 33.3 mm., the same for the left and the right ear ; a 30-year-old man, 34.0 mm. The average for the three adult ears, 33.5 mm., is the figure generally quoted as the length of the organ of Corti. The other four measurements were made before the publication of Retzius’ great work, and, as cited by him, are : Hensen (1865), 33.5 mni. ; Waldeyer (1873), 28.0 mm. for one ear and 31.0 mm. for another ear ; Krause (1876) , 33.0 mm. Retzins did not regard the variations iii length as significant.

This paper reports measurements of the length of the organ of Coi*ti iii sixty-eight human cochleae. The measnre- ments were made by an indirect method, graphic reconstruc- tion based on serial sections. By this method, not only is

Sieheiiaiaiiii (1890) and othcrs haye made iiiaiiy corrosion preparations of the cochlea. Since the organ of Corti is iiot at a coiistaiit dis tance from any surface of the l~oi iy cochlea, its length caiiiiot be determined f rom the dimeiisioiis of the casts.

291

292 MARY HARDY

the total length of the organ of Corti determined but also the length of each turn and half-turn and the exact number of turns in the spiral. Hence the method makes possible a con- sideration of variations in proportionate lengths of the several regions as well as of variations in total length. The data for the sixty-eight ears are analyzed 1) with respect to the varia- tions in total and regional lengths and the correlations of these measurements with variations in the number of turns, and 2) with respect to the variations that are associated with age, sex, race (White and American Negro) and side of body.

MBTERIAL AND METHOD

The material was selected primarily for a study of the relation of variations in the number of cochlear ganglion cells to acuity of hearing. With respect to length of the organ of Corti, therefore, the sixty-eight ears represent a random sampling of the laboratory’s collection of temporal bone sections. The sampling, however, is not random with respect to age. Two-thirds of the ears are from individuals over 40 years of age, only five are from children under 10. No fetal ears have been measured. The number of cases o r individuals in the group is forty-six: both ears of twenty-two and one ear of twenty-four individuals are used. As to race, thirty-eight of the ears are from Whites and thirty are from American Negroes. The sex distribution is not even, since only fourteen of the sixty-eight ears are from females.

The histologic technique was the same for all of the material. A full account of the method of preparation is given by Crowe et al. ( ’34). Briefly, it is formalin fixation, nitric acid decalcification, celloidin embedding, serial sectioning at a thickness of 24r.l and staining of each fifth section with Ehrlich’s hematoxylin and eosin. All cochleae used in this study were cut in the so-called vertical plane of the temporal bone. The basilar membrane is firmly attached to the dense bony capsule of the cochlea and to the modiolus: therefore, although the organ of Corti swells and shrinks in other dimen- sions during histologic preparation, there is no reason to believe that its length is appreciably altered.

TJEXGTTI O F THE ORGAN O F CORTI I S MAR’ 293

From the serial sections of each cochlea a graphic recoii- structioii of the line of contact of the lieads of tlie pillar cells of the organ of Corti was made to scale, after tlie procedure described by Guild ( ’21) for guinea pig cochleae. The general method has been utilized in this 1aboratorjT for several years to chart the location of cochlear lesions. The following description of the method of reconstruction is from the paper by Crowe (it al. (’34) :

The graph is plotted on a large sheet of ruled paper on which each horizontal space is regarded as representing one of the serial sections, a i d is numbered accodingly. In verti- cal series tliere are five sections in which the line of contact betwcen the heads of the outer and the inner pillar cells is cut tangentially. The mid-points of the spaces corresponding to these five sections are joiiied by a continuous line made up of four semicircles. The semicircles represent the ‘axis of tlie organ of Corti’ of the lower apical, upper middle, lower middle and upper basal turns.

Thc graphic representation of tlie lower basal turn consists of two s e p e n t s : 1) a 90” circular arc eoiinecting the eiid of the semicircle for the upper basal turn and a point represent- ing the greatest ‘bulge’ on the lower basal turn, and 2 ) an arc with a longer radius connecting this latter point and the point representing the extreme basal end.

The location of these two points is determined as follows : The section with the organ of Corti cut radially is at the level of the greatest ‘bulge,’ which is usually somewhat posterior to the mid-modiolar region. In this section the actual distance betwcen the pillar cell junctions i ii tlie lowcr middle and lowet, basal turns is measurcd with a niicrorneter ocular. The ‘bulge’ point of the lower basal turn is thus located 011 the ruled paper. The position for the point representing the extreme basal end cannot be determiiied by direct measurements, since in vertical scries other turns of the cochlea are iiot present in the section containing the basal end. Therefore the level of the extreiiie basal riid with respect to the tangents of the lower middle and lower apical turns was determined in a group of cochleae cut in the horizontal plane, and the average of these observations used for locating the h s a l end point on the gi-aplis of the cochleae cut in the vertical plane. The necessary points for the upper apical turn are detcrmined by direct measurements of the sections.

294 ,MARY HAKDY

Fig. 1 The lengths of the different turns of each organ of Corti are determined from graphic reconstructions such as this. The boundaries between half -turns are established by the method of construction of the graph, described in the text.

LENGTH O F THE ORGAN O F CORTI I N M A N 295

A typical graph, constructed by this method, is shown in figure 1: the half-turn regions are, in this paper, designated and bounded as indicated in the figure. The scale of the graph, or the magnification, is equal to the width of one space of the ruled paper divided by the thickness of one section. The grafih is equivalent to an enlargement of an ortho-projection of the line of the heads of the pillar cells onto a plane surface at right angles to the modiolar axis of the cochlea. Two errors are inherent in the method. First, ortho-projection makes no allowance f o r the differences in distance of the two ends of each turn from the plane onto which the image is projected. Secondly, in the cochlea itself the changes in curvature are gradual while in the graph they are abrupt. The effect of the first error is to make all the derived measurements less than the actual, but only, on the average, by about 4 of 1%. The effect of the second error is unknown, but certainly the magnitude of the error is small. Therefore no attempt has been made to introduce correction factors into the calculations.

The figures given in this paper f o r the lengths of the several parts of each organ of Corti are the direct measurements of the corresponding parts of the graphic reconstruction, divided by the magnification factor. It is believed that the measure- ments thus obtained^are within the limits of error of any method, direct or indirect, by which the length of the organ of Corti can be determined.

OBSERVATIONS AND DISCUSSION

Material as a whole. The data for the sixty-eight ears are recorded in table 1. The average total length of the organ of Corti in this material is 31.52 mni. The range of variation in total length is a little over 10 mm. The longest organ of Corti in the group, ear no. 48, is 35.46 mm., or about 40% longer than the shortest one, no. 44, which is 25.26 mm. in length. The variations in length of the organ of Corti are comparable to the normal variations in size of other parts of the body. All nine of the earlier measurements, cited in

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LENGTH O F THE ORGAN OF CORTI I N MBN 297

R L R R L R R

L R L R L R L L R R L R

L R L R L R R R R L R L R R

NO. AGE IN YEARS

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58 59 l 1

63

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66 67 68 69 l 1

71 78 85

36 37 38 39 40 41 42

43 44 45 46 47 48 49 50 51 52 53 54

55 56 57 58 59 ti0 61 62 63 64 ti5 66 67 68

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TABLE 1 4 o n t i n u c i i

Lower basal

11.28 10.86 10.10 11.73 11.07 10.81 11.46

9.75 9.16

11.16 11.19 11.42 11.25 10.60 11.26 12.19 12.08 11.41 11.74

11.25 11.26 10.57 10.74 11.10 10.80 10.89 11.77 i1.96 9.60

10.93 11.03 11.31 11.18 12.19 9.05

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LENGTH OF T H E ORGAN O F DORTI, LN MILLIMETERS

Upper basal

7.50 7.22 7.01 8.26 7.61 7.54 7.80

G.22 5.81 7.61 7.80 8.44 8.52 7.54 7.54 7.65 8.07 7.39 7.62

7.45 7.46 6.79 6.94 7.20 7.30 7.69 7.28 8.37 6.67 7.63 7.24 7.39 6.81

\ 8.66 5.81

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5.13 5.39 5.47 5.69 5.60 5.50 5.77

4.83 4.07 5.15 5.50 5.G6 5.58 5.47 4.79 4.98 5.52 5.43 5.47

5.16 5.73 4.98 4.98 5.30 5.40 5.24 5.33 5.32 5.13 5.73 ,i.14 5.13 5.04 6.22 3.73 5.24

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4.18 4.41 4.26 4.34 4.15 3.92 4.34

3.24 2.87 4.03 4.43 4.37 4.52 4.11 4.15 4.22 4.15 4.15 4.09

4.30 4.28 3.69 3.86 4.22 4.15 4.26 4.24 4.18 3.85 4.20 3.88 3.85 3.77 5.32 2.71

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Lower apical

3.56 3.39 3.69 2.83 3.84 3.62 3.64

3.13 2.68 2.75 2.71 3.73 3.54 3.58 2.87 3.58 3.20 3.45 3.54

3.77 3.77 2.94 3.03 2.94 3.05 3.51 3.26 3.17 2.71 2.83 3.32 3.47 3.02 3.90 1.13 3.17

Upper apical

1.76 1.02 1.36

0.14 1.42 1.66

0.48 0.67 0.43

1.42 2.04

0.19 0.79 1.05 0.48 0.48

1.15 1.38 0.80 0.98

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Total

33.41 32.29 . 31.89 32.85 32.41 32.81 34.67

27.65 25.26 31.13 31.63 35.04 35.45 31.30 30.80 33.41 34.07 32.31 32.94

33.08 33.88 29.77 30.53 30.76 30.70 32.76 33.41 34.15 27.96 31.32 30.90 31.92 30.42 35.45 25.26 31.52 - __

R., right; L, left. W, white; C, colored, or American Negro.

298 MARY HARDY

the first paragraph of this paper, fall well within the limits of the range of variation in total length for this material. Two are shorter than the average for the sixty-eight ears and seven are longer.

The lengths of the half-turns decrease more or less regularly from basal to apical end, the average lengths being, respec- tively: lower basal, 10.83 mm.; upper basal, 7.40 mm.; lower middle, 5.24 mm. ; upper middle, 4.04 mm. ; lower apical, 3.17 mm. ; upper apical, 0.84 mm. When these measurements arc assembled in terms of whole turns-basal, middle and apical- the average lengths are, respectively, 18.23 mm., 9.28 mm. and 4.01 mm. Each turn is about half as long as the preceding one; the approximate percentage figures are 58, 29 and 13, respectively. Roughly, therefore, the human organ of Corti is a spiral with a radius that decreases a t such a rate as to make each successive turn half the length of the preceding one.

The range of variation in length of the half-turns differs, proportionally, for the several regions. The variability in length, or the range of variation compared to the actual length, increases from the basal to the apical end. Expressing the variability as the ratio of the maximum to the minimum measurements for the entire material, the figures for the respective half-turns are : lower basal, 1.36 ; upper basal, 1.39 ; lower middle, 1.67; upper middle, 1.96; lower apical, 3.45; upper apical (omitting cases with no upper apical segment), 14.78. It is interesting that the part of the cochlea which is the last to develop both phylogeiietically and ontogenetically is the least stable with respect to length of the organ of Corti.

Biluterul symmetry. The lengths, both total and regional, of the organ of Corti in the two ears of the same individual differ but slightly as compared to the total range of variation of this material. Pairs may be distinguished from single ears in table 1 by the ditto malks which denote the second ear of an individual. In only one instance are the total lengths practically the same for both sides (nos. 59 and 60). The average difference in length between the two organs of Corti of one individual is only 1.07 mm. In 59% of the tmenty- two pairs the difference in length is less than 1 mm., in 23%

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300 MARY HBRDY

it falls between 1 and 2 mm. and in 13.5% between 2 and 3 mm., and in one case, or 4.5%, the difference is 3.37 mm. (nos. 64 and 65). Bilateral symmetry is found, as one ~vould expect, in the dimensions of this sensory organ as in other paired organs of the body.

Age . The number of ears from infants and children is, in this material, far too small to warrant any definite conclusion as t o whether or not there is postnatal growth of the cochlea. The measurements made show a slight trend toward increase in length of the organ of Corti with advancing age. Both total and regional lengths average somewhat less for the ears from individuals in the first two decades of life than fo r those in the older groups (table 2). The difference seems to be distributed evenly over all turns of the cochlea. There is no indication that the organ of Corti grows from the basal or the apical end. The measurements for each ear (table 1) show that there is a marked degree of overlapping. For example, a 10-week-old infant is found to have an organ of Corti considerably longer than the average for adults. These facts with respect to a membranous part of the cochlea arc in good agreement with the reports other investigators have made of form and dimensions of the bony cochlea, based on direct measurements of skeletal material or on corrosion uasts.

The belief that the general shape of the cochlea is fixed by the time of birth is corroborated by a study of the material as arranged in table 3. Although the number of cases for each decade is small, there is a striking difference in dis- tribution by number of turns of the cochlea as compared to distribution by total length in millimeters. The number of cochleae with only two and a half turns is, in the group over 40 years of age, the same as the number with about two and seven-eighths turns. However, the distribution according to measured length (shorter than 30 mm., between 30 and 33 mm. and over 33 mm.) does shorn an age difference. In the first two decades the percentage with a short organ of Corti is much greater than at any age over 40 years. In

L E N G T H O F THE ORGAN O F CORTI IN MAN 301

__--

4 1 4 3 2 0 6 2

18 2

reverse, the percentage with a long organ of Corti is greater at the older ages. These figures are suggestive, but are not considered conclusive evidence for postnatal growth of the cochlea. To determine the age at which the cochlea ceases to grow wonld require study of a large number of eases at each of a series of closely spaced age intervals.

Sex. Of the sixty-eight cochleae studied fifty-four are from males and fourteen from females. The averages of the measurements, arranged by sex, are presented in table 4. Each group has approximately the same number of right and of left ears and of White and Negro individuals. The average

__

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TABLE 3

T o show the distribution of sez, race, side of body, number of turns and total length o f the organ. of Corti, with respect t o age

_ _ _ 9%

1 20 4 57 2 100 6 75

10 50 6 50 8 57

~-

ACE IN YEARS

___

0- 9 10-19 20-29 3 0-3 9 40-49 5 0 5 9 60-85

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1 20 0 . . 0 .. 0 . . 8 40 4 33 4 28

NUMBER OF EARS NUMBER O F EARS AND PER CENT OF TOTAL

3 2 2 5 0 2

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total length of the organ of Corti is greater in the male by 0.83 mm., or about 3%. This difference is distributed pro- portionally throughout the spiral, since the figures for per cent of total length show that the half-turn lengths are relatively the same for both sexes. A further division of the material into right and left ears shows that the right organ of Corti of males is, on the average, longer than the right organ of Corti of females, and that the left of males is longer than the left of females. The conclusion seems warranted that as a rule the organ of Corti is slightly longer in the male than in the female.

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LENGTH O P THE ORGAN O F CORTI I N M.4N 303

Rnce. The averages of the total length and of the regional lengths of the organ of Corti are, for the two races repre- sented in this material, practically identical (table 4). The distribution of the material as to race is : thirty-eight White and thirty Negro. The distribution as to sex and as to side of body, right 01- left, is also about equal in both racial groups. Of tlie thii-ty-eight White cochleae, thirty-two are male and six female, twenty-one a re right and seventeen lcft. I n the Negro group there a re twenty-two male and eight female, fifteen right and fifteen left. Analysis of the data, both as to actual aiid as to proportional measurements, shows that the factor of race, JYhite or American Negro, does not iiifluence the length of the organ of Corti.

S i d r of body . Analysis of thc material, grouped as to right or left side of body, shows that the average total leiigth of the orgaii of Corti of thc right ear is slightly greater than of the lcft (table 4) . For the entirc rriaterial the average length of the right organs of Corti is 31.83 mm., and of the lefts it is 31.16 mm.; the difference is a little over 25%. Proportioiially the t x o sides a re tlic same for all parts of the spiral except the upper apical region, which includes 011 the right side 0.9% more of the average total lciigth than on the left side. Fo r the twenty-two iiidividnals with both cars measured, the right organ of Corti is the longer in sevcnteen, 01- 77%; for this group the average length of the twenty-lwo rights is 32.27 mm. and of the twenty-two lefts is 31.53 mm., or a difference of about 2.3%. Of the twenty-four single ears measured, four- teen a re rights aiid ten a rc lefts. The average total length is 31.13 mm. for the fourteen rights and 30.37 mm. for the ten lefts : again the difference is a little over 27.. Looking ahead to table 5 i t is seen that of seventeen ears with a n organ of Corti over 33 mm. long, twelve, or 71‘;/0, a re rights. These data indicate therefore that normally there is a slight dif- ference in length of the organ of Corti on the two sides of the body, and that on tlie right side it is in most cases longer than on the left.

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LENGTH OF THE ORGAN O F COHTI IN N A N 305

Lengths of half-ticrns in cochleae of difweiat lengths. The distribution of the material by millimeters of total length of the organ of Corti, irrespective of age, race, sex or side of body, is represented in figure A. The polygon of distribution is fairly regular, considering that the sixty-eight cases are divided into eleven groups between the two extremes of 25.26 mm. and 35.46 mm. To study the relationship of differences in total length to differences in lengths of the half-turn seg- ments, the material is divided into only three groups: the short, in which the total length of the organ of Corti is less than 30 mm.; the intermediate, with total lengths between 30 and 33 mm.; and the long, over 33 mm. (table 5, part A).

L - mm. TOTAL LENGTH ORGAN OF CORTI

Fig. A The distribution of the sixty-eight cases wi th respect to differences in total length of the organ of Corti.

These arbitrary boundaries give a reasonably good frequency distribution: thirty-seven, or 54%, of the eases fall in the intermediate group and have an organ of Corti within 1.5 mm. of the average length of the whole material; fourteen fall in the short group and seventeen in the long group. For each half-turn the average length is greater in the long group than in the short group, but the percentages of total length are different, especially at the two ends. !The short spirals have, proportionally, longer lower basal regions (35.370 of the total length) and shorter upper apical regions (2.4% of the total length) than do the long spirals, which have an average of 33.4% of their total length in the lower basal and 4.170

T H E AMERICAN JOURNAL OF ANATOMY, VO'L. 62, NO. 2

306 MARY HARDY

in the upper apical region. This revcrsal in proportional lengths of the two ends indicates that a cochlea with a long organ of Corti is slightly more pointed in general shape; that is, has a height greater in proportion to the width of the base than does a cochlea with a short organ of Corti.

The average age for each group, and the distribution with respect to side of body, sex and race are also given in table 5. The average age for the group with short cochleae is less than for the other two groups. I n the group with long cochleae there is a greater proportion of right than of left ears, and a greater proportion of male than of female ears. The dis- tribution with respect to race is very nearly the same for all three groups. These findings strengthen the conclusions drawn in the sections on age, sex, race and side of body.

The graphic reconstructions show that fifty of the sixty-eight spirals, or 74%, have more than two and a half but not more than two and three-quarters turns, and that nine, or 13%, have exactly two and a half turns. In other words, 87% of the cochleae of this material fall within the limits of the usual textbook statements as to the number of turns of the human cochlea. I n eight ears (nos. 4, 23, 24, 34, 36, 42, 48 and 62, table 1) the number of turns is more than two and three-quarters and less than three turns. Only one of these eight (ear no. 4, from a 4-year-old child) has an organ of Corti shorter than the average for the whole series of sixty-eight ears. The other seven fall in the group of long cochleae of table 5, part A, with lengths between 33.0 and 35.5 mm. All nine of the cochleae with exactly two and a half turns (nos. 12, 18, 39, 46, 49,59, 60, 64 and 65, table 1) are from adults ; the average age for this group is 52 years. Only three of the nine have an organ of Corti less than 30 mm. in length, the other six are within 1.5 mm. of the average of the whole material and the longest is 32.85 mm. One cochlea, no. 13 of table 1, has less than two and a half turns, approximately two and one- sixth. In it the basal and middle turns are so long that, in spite of its exceptional shape, the total length of the organ

The number of tzcrrzs.

LEKGTH O F THE OXGAN O F CORTI IX NAN 307

of Coiti is sliglitly above the average for the whole material. Both parts of the middle turn are, in fact, longer in this cochlea than in any other of the series measnrecl. Its unusual shape is well seen by comparison of the mid-modiolar section (fig. 2 ) with the corresponding sectioii of a cochlea with a n average number of turns (fig. 3). That this extreme variation cannot he regarded as pathologic is indicated by the fact that the patient, a yonng man, had good hearing for all tones.

P a r t B of table 5 presents the data for the material as divided into three groups on the basis of the number of turns.

Fig. 2 Photomicrograph of mid-modiolar section of a liunian cochlea in TThicli the organ of Corti has only two aiid me-sixth turns. This and the succeeding two figurcs are all f rom sections cut in the so-called ‘vertical’ plane of the petrous pyranlid, and all a re S ~ O T T I I a t the Same magnificatioii, about X 4‘: i n reproduction.

The cochlea, with oiily two and a sixth turns is omitted from this grouping. In the group of cochleae with no upper apical region (two and a half turns) the total length of the organ of Corti is on the average 3.05 mm. shorter than in the group with moi-e than two and tliree-quarters turns (designated in tabulation as two and seven-eights). The average lengths, both total and regional, for the middle group are very close to those f o r the whole material (par t C, table 5 ) ; also the range of variation in total length in this middle group is almost as great as for the entire material.

308 MABY HARDY

Of the difference in average total length between the two extreme groups the extra segment, the upper apical, accounts f o r but 1.79 mm., o r 59%. The lower apical half-turn is also longer, by an average of 17%, when a long upper apical segment is present. The difference in the apical turn as a whole, therefore, accounts fo r over three-fourths of the total difference in length. The remaining 23% of the difference is distributed between the half-turns of the basal region. In marked contrast to the other parts is the constant arcrage length of the middle turn. For the entire middle turn, lower

Fig. 3 Mid-modiolar section of a cochlea with the arerage iiuniber of turns.

plus upper half-turns, there is an average difference in length of only 0.03 mm. between the extreme groups.

It is apparent from the above figures that the effect of an increase in number of turns upon shape of the cochlca is more complex than the mere addition of an upper apical segment or a uniform enlargement of all turns. The actual effect is best demonstrated by study of the shifts in average per cents of total length for the respective turns and half-turns in the three groups (par t B, table 5). These percentage figures show that in both basal half-turns the increase in length is proportionally less than is the rate of increase in total length, and that the reverse is true in the lower apical

LENGTH O F THE OEGAN O F CORTI I N N A N 309

half-turn. The constant length of the middle turn results, of course, in a progressive decrease in perccntage length for each of its half-turns as the total length increases. The nearest approach to a four : two : one relationship between leiigths of the successive whole turns of the spiral formed by the organ of Corti is in the group with the longest upper apical segments ; the average percentage figures for this group are 56.0, 28.4 and 15.6, respectively, for the basal, middIe and apical turns. The corresponding figores for the group with only two and a half turns are 59.4, 31.2 and 9.5%.

Fig. 4 Mi&modiolar aectioii of a cochlea. with ne.irlp three complete turns. Conqmre with the preceding figurr.

Figures 3 and 4, from mid-modiolar sections of two cochleae, one with an averagc number of turns and the other with nearly three full turns, aid in visualizing the effect upon shape of the cliff erence in number of turns.

On the whole it would seem that the factors, whatever they are, that permit or cause more than the minimum normal growth of the apical end of the cochlea also favor, but to a less degree, growth of the basal end. To advance possible embryological explanations for these observations would be, in the light of present knowledge, hut to speculate.

SUMMARY AXD COSCLUSIONS

1. Measurements of the leiigtli of the organ of Corti in sixty-eight human cochleae a re given. The age range is from 10 weeks to 85 years; two-thirds of the material is from individuals over 40 years of age. The distribution as to side of bod?; arid as to race, White o r American Negro, is fairly even; only fourteen of the sixty-eight cochleae a re from females.

2. The measurements mere made by a n indirect method, graphic reconsti~nction. A graph was made to scale from the serial sections of each cochlea. From this graph were determined total length of the organ of Corti, length of each turn and half-turn, and the exact number of turns.

3. The average total length, measured along the line of pillar cell junctions, is 31.52 mm. The average lengths of the turns a re : basal, 18.23 mm., or 57.9% of total length; middle, 9.28 mm., or 29.4% and apical, 4.01 mm., or 12.8%.

4. The variation in total length is about 10 mm., from a minimum of 25.26 mm. to a maximum of 35.46 mm. The polygon of distribution between these extremes is fairly regular; thirty-seven, or 54%, fall between 30 and 33 mm., fourteen are shorter and seventeen are longer than these limits.

5. Bilateral symmetry of the organs of Corti in an individual is denionstrated by the fact that the two differ but slightlv in comparison to the range of the whole material. Twenty- two pairs were measured.

6. The number of turns is from two and a half to two and three-quarters in fifty-nine, or 8770, of the cochleae; of these, nine have exactly two and a half turns. Eight have more than two and three-quarters but less than three turns. One spiral is an exception, with only two and one-sixth turns.

7. When a long upper apical segment is present the lower apical half-turn and the basal turn are also longer, on the average, but the middle turn is not. The effect of these difierences in proportions on the shape of the cochlea is dis- cussed and illustrated by mid-mocliolar sections.

LENGTH O F TlIE OCG.\R O F COBTI I K M A S 311

8. The right organ of Corti is on the average about 2% longer than the left.

9. In the male the organ of C'orti is 011 the average about 3% longer than in the female.

10. The data indicate a slight increase in total length of the organ of Corti with advancing age, but no change in the numbel. of turns.

11. There is no evidence that the factor of race influeiices either total length of the organ of Corti or tlie proportioiiate leiigths of tlie several regions.

LITERATUBE CITED

C'ROTVE, 8. .T., S. R. GUILD AND L. sf. POLVOGT 1934 01,-enations oil the Bull. J o h n s Hopkiiis Hosp., vol. 34,

GITLD, S. R. A graphic recoiistructioii method for the s tudy of tlic orgaii Anst . Rec., vol. 2 2 , pp. 141-157.

RF.TZII s, G. 1884 Das Gehoroigaii drr Wrbcltliiere. &I. 11. Btockholrii.

pathology of high-toile deafness. p11. 315-380.

1921 of Corti.