Anatomical and physiological studies on the growth of the inner ear of the albino rat 3 (1923)

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Wada T. Anatomical and physiological studies on the growth of the inner ear of the albino rat. (1923) Memoirs of the Wistar Institute of Anatomy and Biology, No. 10, Philadelphia. Rat Inner Ear (1923): I. Cochlea growth | II. Inception of hearing and cochlea growth | III. Growth of largest nerve cells in ganglion vestibulare | Final Summary | Literature Cited

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Anatomical and Physiological Studies on the Growth of the Inner Ear of the Albino Rat

III. On the Growth of the Largest Nerve Cells in the Ganglion Vestibulare

Material and technique

The material used for the present study was in a great part the same that was employed for the studies reported in chapter 1, with the addition of some new specimens as shown in table 114 and table 94. In the slides obtained in the radial vertical section we see the vestibular ganglion cells situated in a single group at the radix of the cochlea (Fig. 3 G. V.). As four ears were used in each age group, four cell groups were examined at each age. Besides these fourteen age groups, six rats used for cross-sections, in chapter 1, were also included.

The measurements were made in the same way and under the same conditions as those described earlier for the cells of the spiral ganglion. Since the ganglion vestibulare consists of two parts, the ganglion vestibulare superius and inferius, the ten largest cells were taken from each part and the results averaged.

Observations

By way of introduction I wish to say a word about equilibration in the young rat. The young just born crawl over on each other and seem to attempt to find the mothers nipples. They turn the head to and fro and roll over on the flanks, belly, or back. While resting they take their normal position or lie on the side. When turned on their backs they endeavor to regain the normal position. The fore legs are of more use than the hind in making readjustments. The tails hang down between the hind legs.

TABLE 114

Data on rats used for the study of the cells of the ganglion vestibulare (radial section).

See also table 94


AOB


BOOT WEIGHT


BODY LENGTH


BEX


BIDE


AUDITOHT RESPONSE


days


grams


mm.





1


5


44


9


R.




4


44


9


R.




5


48


d 1


R. L.



3


9


60


<?


R. L.




8


56


9


R. L.



6


10


64


a


R.




10


64


9


R. L.




11


62


(7


R.



9


11


67


<?


R. L.


+



9


58


9


R.




10


57


tf


R.



12


13


70


d 1


R. L.


+



12


68


9


R.


+



15


72


c?


R.


+


15


13


74


d"


R. L.


+



14


75


9


R. L.


+


20


30


.96


d 1


R. L.


+



28


94


c?


R. L.


+


25


34


101


9


R. L.


+



34


100


d 1


R. L.


+


50


58


121


9


R. L.


+



43


104


(f


R. L.


+


100


146


176


c?


L.


+



103


154


9


L.


+



101


152


9


R. L.


+


150


154


184


9


R. L.


+



189


191


c?


R.


+



199


192


d 1


R.


+


260


137


162


9


R.


+



140


171


9


R. L.


+



134


178


9


R.


' +


367


205


202


rf


L.


+



170


182


9


L.


+



179


196


9


R. L.


+


546


282


222


d 1


R. L.


+



227


204


d 1


R. L.


+


At three days they move and crawl very actively. They tend to assume the normal position. When rolled over on the back or side they succeed in regaining the normal position in


158


ANATOMICAL AND PHYSIOLOGICAL STUDIES ON


a few seconds. When six days old the rats have fairly well coordinated movements. They use their fore and hind legs effectively and in the same way. When the mother 's body touches them they respond quickly by searching for the nipples.

At nine days they move much more quickly and the movements are well coordinated. .Though the eyes are still closed, they


TABLE 115

Diameters of the cells and their nuclei in the ganglion vestibulare in radial vertical section

(Chart 43)


AGE


BODY WEIGHT


DIAMETERS IN M


CELL BODY


NUCLEUS


Long


Short


Computed


Long


Short


Computed


days


grams








1


5


21.2


19.5


20.3


12.4


11.1


11.7


3


9


23.7


22.2


22.9


12.5


11.6


12.0


6


11


24.0


22.1


23.0


12.3


11.9


11.9


9


10


24.8


23.0


23.9


12.5


11.6


12.0


12


13


24.9


23.0


23.9


12.5


11.7


12.1


15


13


24.8


23.0


23.9


12.5


11.6


12.0


20


27


25.0


23.3


24.1


12.3


11.6


11.9


25


34


25.2


23.6


24.4


12.5


11.8


12.1


50


50


25.6


23.6


24.5


12.5


11.6


12.0


100


112


25.5


23.9


24.7


12.8


11.9


12.3


150


174


25.4


23.5


24.4


12.8


11.6


12.2


260


138


25.8


23.4


24.6


12.4


11.7


12.0


367


184


26.2


24.9


25.5


12.9


11.8


12.3


546


255


26.5


24.2


25.3


12.8


11.8


12.2


Ratios 1

-367 days




1 1.3




1:1.1


1546 "




1.2




1.0


15367 "




1.1




1.0


crawl toward the object sought. When turned over on the back they regain the normal position immediately. While resting they lie on their bellies with all the legs spread well apart.

Twelve-day-old rats, though the eyes are still closed, go to and fro actively with good coordination, but are somewhat slower than the adults. The body loses its fetal red color through the development of the first hairs. After this period the rats do not differ greatly from the adult in their general behavior.


GROWTH OF THE INNER EAR OF ALBINO RAT


159


The growth changes in the ganglion cells of the ganglion vestibulare. In table 115 (chart 43) are given the values for the diameters of the cell bodies and their nuclei in the largest cells of the ganglion vestibulare. At the bottom of the last column for the cell body and for the nucleus, respectively, are recorded the ratios at 1 to 367, 1 to 546, and 15 to 367 days. The last ratio was taken


25

a

20 15 10


  • GEDAYSH




25


50


5O 1OO 2OO 30O 4OO 5OO


Chart 43 The diameter of the largest cell bodies and of the nuclei from the ganglion vestibulare. table 115.

Cell bodies. -.-.-.-.-. Nuclei.

to facilitate a comparison with the data in table 118 which begin at 15 days.

Looking at the ratios of the cell bodies and of their nuclei from 1 to 546 days, it appears that the ganglion cells increase 1.2 in diameter, while their nuclei have only a very slight increase, and therefore the ratio is 1 : 1.0. This increase in the cell bodies is continuous from birth to old age, but after fifteen days is very slow. In the nucleus we see a slight increase at the earlier ages, after which the values are nearly constant. This means that after birth the size of the cell bodies and their nuclei does not increase so much as do those of the spiral ganglion cells, or, expressed in another way, the cells in the vestibular ganglion have developed earlier than those of the spiral ganglia and at birth have already attained nearly their full size.

On the comparison of the diameter of the cell bodies and their nuclei in the nerve cells of ihe ganglion vestibulare according to sex. For this purpose twelve age groups of albino rats were used. In seven cases we have two cochlea in each group in the same sex, in which the average value is recorded. In table 116 are entered the values for these diameters and at the foot of the table the data are analysed. They reveal no evidence of a significant difference in the diameters according to sex.

On the comparison of the diameters in the cell bodies and nuclei of the nerve cells in the ganglion ves ibulare according to side. For the present study fourteen age groups were employed. As indicated in table 117, the data in five instances are based on the average of two cochleas of the same side. Table 117 enables us to make the comparison of the diameters of the cell bodies and their nuclei on both sides, and the analysis of the data given at the bottom of the table shows that there is no difference in these characters according to side.

On the morphological changes in the cells of the vestibular ganglion. Figure 14 illustrates semi-diagrammatically the ganglion cells in the vestibular ganglion of the albino at birth, 20 and 367 days of age. These figures, as in the ganglion spirale, have been magnified 1000 times and the absolute values of the diameters are given in table 115.

As seen in figure 14, both the cell body and the nucleus are at birth already well developed and more precocious in their development than the cells in any of the other cerebrospinal ganglia thus far examined. The cytoplasm is relatively abundant and the Nissl bodies are present, though both of these characters become more marked later.

The nucleus is also large, the chromatin somewhat differentiated and the so-called 'Kernfaden' often occur. Generally speaking,




I Day


20 Days


14


366 Days


Fig. 14 Showing setrii-diagrammatically the size and the morphological changes in the ganglion cells in the ganglion vestibulare of the albino rat at the age of 1, 20 and 366 days. All cell figures have been magnified 1000 diameters.


GROWTH OF THE INNER EAR OF ALBINO RAT


161


therefore, the cells have the characteristics of the mature elements though they stain less deeply than in the adult. At twenty days of age the cell body is enlarged and fully mature. The Nissl

TABLE 116

Comparison of the diameters of the cells and their nuclei in the ganglion vestibidare

according to sex


AGE


BODY WEIGHT


NUMBER OF RATS


8EX


COMPUTED DIAMETERS


Cell body


Nucleus


days


grams






1


6


2


f


20.8


11.9





9


19.9


11.5


3


9


2


tf


21.7


11.8



8


2


9


23.8


12.2


6


11


2


tf


22.7


11.9



10


2


9


23.1


12.1


9


11


1


d*


23.8


12.5



9


1


9


23.8


12.1


12


15


1


cf


24.4


12.2



12


1


9


23.1


11.9


15


13


2


cf


24.3


12.2



13


2


9


23.4


11.9


20


30


1


cf


24.7


11.9



19


1


9


24.6


12.6


25


34


2


d"


24.4


11.9



34


2


9


24.4


12.4


50


43


2


cT


26.1


12.4



58


2


9


22.6


11.4


100


146


1


<?


26.3


12.8



103


1


9


23.4


12.6


150


194


2


rf 1


24.4


12.5



154


2


9


24.4


12.0


365


205


1


<f


24.2


11.7



170


1


9


24.6


12.1


Average for male


24.0


12.1


Average for female


23.4


12.1


Males larger


6


7


Females larger


3


5


Males and females equal


3




bodies are more differentiated and the nucleus is mature, though it shows only a slight increase in size.

At 367 days the histological structures appear much as at twenty days, but the diameters of both the cell body and the nucleus have very slightly increased. This is in contrast to the change which occurs in the cells of the spiral ganglion.


162


ANATOMICAL AND PHYSIOLOGICAL STUDIES ON


In order to study the form of the cells of the ganglion vestibulare the measurements also were made on the cross-sections. Table


TABLE 117


Comparison of the diameters of the cells and their nuclei in the ganglion vestibulare according to side


AGE


BODY WEIGHT


NUMBER OF RATS


SIDE


COMPUTED DIAMETERS


Cell body


Nucleus


days


grams






1


4


1


R.


20.1


12.0



5


1


L.


22.0


12.5


3


9


2


R.


23.0


11.8





L.


22.6


12.3


6


10


1


R.


23.2


12.1





L.


23.5


12.0


9


11


1


R.


25.1


12.3





L.


23.8


12.5


12


15


1


R.


24.4


12.2



13


1


L.


25.1


12.5


15


13


2


R.


24.2


12.2





L.


23.6


11.9


20


30


1


R.


24.7


11.9





L.


23.5


11.4


25


34


2


R.


23.9


12.1





L.


24.9


12.2


50


50


2


R.


23.1


11.6





L.


25.6


12.3


100


101


1


R.


25.0


12.0





L.


24.8


11.7


150


199


1


R.


25.1


12.8



154


1


L.


25.4


12.5


263


140


1


R.


26.5


12.3





L.


25.1


12.4


368


179


1


R.


27.2


12.6





L.


26.2


13.0


546


255


2


R.


26.0


12.4





L.


24.6


12.0


Average right side


24.4


12.2


Average left side


24.3


12.2


Right larger


8


6


Left larger


6


8


118 (chart 44) shows the results. Looking at the ratios of 15 to 371 days, we see the same rate of increase in the cell bodies and the nuclei as that in the radial section; i.e., in the cell bodies 1 : 1.1 and in the nuclei 1 : 1.0. Comparing the diameters at each


GROWTH OF THE INNER EAR OF ALBINO RAT


163


TABLE 118

Diameters of the cell bodies and their nuclei in the ganglion vestibulare, on crosssection (chart 44)




DIAMETERS M


AOK


BOOT WEIGHT


CELL BODY


NUCLEUS




Long


Short


Computed


Long


Short


Computed


days


grams








15


20


25.1


22.8


23.9


12.4


11.6


12.0


20


27


25.2


23.4


24.3


12.5


11.7


12.1


25


39


25.2


24.0


24.6


12.3


12.0


12.1


100


95


26.6


24.7


25.6


12.8


11.8


12.3


150


169


26.7


24.7


25.7


13.0


11.7


12.3


371


220


26.8


25.3


26.0


12.8


11.8


12.3


Ratio 15-371 days 1:1.1




1 :1.0


25


20


15


10


25


50


50 10O 20O 300 40O 5OO


Chart 44 The diameters of the cell bodies and of their nuclei from the ganglion vestibulare, after fifteen days (cross-section), table 118. Cell bodies. -.-.-.-.-. Nuclei.


164


ANATOMICAL AND PHYSIOLOGICAL STUDIES ON


age in both the radial and cross-sections, they are almost the same, with a slight tendency for the cells in the cross-section to give higher values, which suggests that the long axes of these cells tend to lie in the plane of the section.

On the nucleus-plasma relations of the ganglion cells in the ganlion vestibulare. In table 119 are entered the computed diameters of the cell bodies and their nuclei in the radial section, and in the last column the ratios of the volume of the nucleus to that of the cytoplasm obtained by the method previously given. As

TABLE 119

Nucleus-plasma ratios of the cells in the. ganglion vestibulare radial vertical section




COMPUTED DIAMETERS M


AGE


BODY WEIGHT


Cell body


Nucleus


Nucleus-plasma ratios


days 1


grams 5


20.3


11.7


1 :4.2


3


9


22.9


12.0


5.9


6


11


23.0


11.9


6.2


9


10


23.9


12.0


6.9


12


13


23.9


12.1


6.7


15


13


23.9


12.0


6.9


20


27


24.1


11.9


7.3


25


34


24.4


12.1


7.2


50


50


24.5


12.0


7.5


100


112


24.7


12.3


7.1


150


174


24.4


12.2


7.0


260


138


24.6


12.0


7.6


367


184


25.5


12.3


7.9


546


255


25.3


12.2


7.9


seen, the ratio is at birth relatively large, 1 : 4.2, and this increases with age, in the earlier stages considerably, but in the later, less rapidly. In the oldest age group it is largest, 1: 7.9.

On the cross-section the nucleus-plasma ratio is also progressive and the increase is very regular (table 120). Comparing the ratios in the radial with those in the cross-sections, they are found to be nearly the same at fifteen twenty and twenty-five days, but at the later ages those in the cross-sections are somewhat larger than in the radial. It is difficult to determine whether the ratios on the cross-section are really larger or whether the


GROWTH OF THE INNER EAR OF ALBINO RAT


105


result depends on the fact that the number of the cells here measured is only one-fourth of that measured in the radial section, and hence fewer cells of smaller size were included. At any rate, these ganglion cells in both the radial and crosssections of the cochlea appear to grow at about the same rate. The statistical constants for these cells and their nuclei are given in tables 121 and 122.

Discussion

The nerve cells in the ganglion vestibulare are, as seen from the above description, already well developed at birth both in size and histological structure. After that time they grow con TABLE 120

Nucleus-plasma ratios of cells of the ganglion vestibulare, in cross-section




DIAMETERS COMPUTED M



BOOT



AGE


WEIGHT






Cell body


Nucleus


Nucleus-plasma






ratios


days


grams





15


20


23.9


12.0


1 :6.9


20


27


24.3


12.1


7.1


25


39


24. ti


12.1


7.4


100


95


25.6


12.3


8.0


150


169


25.7


12.3


8.1


371


220


26.0


12.3


8.4


tinuously but slowly so long as followed. The increase from 1 to 546 days in the ratios of the diameters is in the cell body 1: 1.3, in the nucleus 1: 1.1, and is therefore very small. In the cerebrospinal ganglion cells and in the cells of the cerebral cortex, studied in the albino rat, there is no case which shows such a small rate of increase between birth and maturity. The following table 123 shows the ratios of increase which have been found.

It is to be noted that for the cells of the seventh spinal ganglion and the spinal cord, the ratios were taken from 17 to 360 days. If we had the ratios from 1 to 360 days, they would be without question much larger.

There are a few measurements on the size of the ganglion cells in the vestibular ganglion of various animals in the liter


166


ANATOMICAL AND PHYSIOLOGICAL STUDIES ON


ature. Schwalbe ('87) and Alexander ('99) report measurements on these cells in several animals, but for the reasons already given when considering the diameters of the cells in the ganglion spirale, the values obtained by the authors are not repeated here.

TABLE 121

Giving the mean, standard deviation and coefficient of variability, with their respective probable errors, for the diameters of the cells in the ganglion vestibulare, in radial-vertical section


AGE


CELL NUCLEUS


MEAN


STANDARD DEVIATION


COEFFICIENT OF VARIABILITY


days 1


Cell


20.1 0.16


1.46 0.11


7.3 0.55



Nucleus


11.7 0.11


0.99 0.07


8.5 0.64


3


Cell


22.6 0.14


1.33 0.10


5.9 0.44



Nucleus


11.9 0.07


0.63 0.05


5.3 0.40


6


Cell


22.8 0.13


1.23 0.09


5.4 0.41



Nucleus


11.9 0.05


0.43 0.03


3.6 0.27


9


Cell


23.6 0.16


1.48 0.11


6.3 0.48



Nucleus


12.0 0.0<9


0.82 0.06


6.8 0.52


12


Cell


23.6 0.14


1.28 0.10


5.4 0.41



Nucleus


12.0 0.06


. 59 . 04


4.9 0.37


15


Cell


23.6 0.13


1.21 0.09


5.1 0.39



Nucleus


12.1 0.06


0.60 0.05


5.0 0.38


20


Cell


23.9 0.16


1.54 0.11


6.5 0.49



Nucleus


11.9 0.10


0.90 0.07


7.6 0.55


25


Cell


24.2 0.16


1.48 0.11


6.1 0.46



Nucleus


12.1 0.08


0.74 0.06


6.1 0.46


50


Cell


24.1 0.30


2.80 0.21


11.6 0.88



Nucleus


11.8 0.09


0.86 0.06


7.3 0.55


100


Cell


24.3 0.20


1.86 0.14


7 . 7 . 58



Nucleus


12.2 0.09


0.86 0.06


7.0 0.53


150


Cell


24.1 0.18


1.70 0.13


7.1 0.53



Nucleus


12.2 0.09


0.83 0.06


6.8 0.52


260


Cell


24.3 0.26


2.45 0.18


10.1 0.76



Nucleus


11.9 0.07


0.67 0.05


5.6 0.42


367


Cell


25.2 0.22


2.07 0.16


8.2 0.62



Nucleus


12.3 0.09


0.88 0.07


7.2 0.54


546


Cell


25.0 0.19


1.80 0.14


7.2 0.54



Nucleus


12.1 0.09


0.81 0.06


6.7 0.50


On the differences between the growth of the cells in the ganglion spirale and ganglion vestibulare. The foregoing discussion has made plain that the vestibular ganglion cells grow not only in size, but also in histological structure very much before birth, while after birth they grow slowly though continuously. On the other hand, the spiral ganglion cells are relatively immature at


GROWTH OF THE INNER EAR OF ALBINO RAT


167


birth, but in the earlier stages after birth grow very rapidly, reach at twenty days their maximum size, and then diminish slowly. This great difference in the course of growth is probably related to the maturity of the functions of the animal.

TABLE 122

Giving the mean, standard deviation and coefficient of variability with their respective probable errors for the diameters of the cells in the ganglion vestibulare on cross-section


AGE

days


CELL NUCLEUS


MEAN


STANDARD DEVIATION


COEFFICIENT OF VARIABILITY


15


Cell


23.8 0.21


1.00 0.15


4.2 0.58



Nucleus


12.0 0.12


0.55 0.08


4.6 0.69


20


Cell


23.9 0.20


0.92 0.14


3.9 0.58



Nucleus


12.1 0.06


0.30 0.05


2.5 0.37


25


Cell


24.4 0.20


0.94 0.14


3.9 0.58



Nucleus


12.1 0.03


0.16 0.02


1.3 0.20


100


Cell


25.4 0.32


1.51 0.23


5.9 0.90



Nucleus


12.3 0.15


0.72 0.11


5.9 0.88


150


Cell


25.6 0.20


0.94 0.14


3.7 0.55



Nucleus


12.4 0.09


0.42 0.06


3.4 0.51


371


Cell


25.9 0.41


1.91 0.29


7.4 1.11



Nucleus


12.3 0.06


0.26 0.04


2.1 0.32


TABLE 123 Ratios of diameters between the ages given.



CEREBRAL CORTEX



DONALDSON AND



(SUGITA, '18)



NAOABAKA. '18


CELL GROUP


LAMINA


LAMINA


OA88ERIAN


SPIRAL


VESTIBULAR


7TH


EFFERENT



PYHA

GANO

GANGLION


GANGLION


GANGLION


SPINAL


SPINAL



MIDIS


LIONARIS


NITTONO


WADA


WADA


GANGLION


CORD





C20)





CELLS


Age









days


1-730


1-730


1-330


1-546


1-546


17-360


17-360


Cell









body


1 :1.6


1 : 1.6


1 : 1.69


1 : 1.6


1 :1.2


1 :1.3


1 :1.2


Nucleus


1.5


1.5


1.20


1.2


1.0


1.2


1.2


As a consequence, in the nucleus-plasma ratio there is also a large difference between the cells in the two ganglia. Table 124 shows this.

The ratio at birth in the ganglion vestibulare is large as compared with that in the ganglion spirale, but the increase in this ratio


168 ANATOMICAL AND PHYSIOLOGICAL STUDIES ON

at 546 days is relatively slight as compared with what takes place in the cells of the ganglion spirale. It appears, therefore, that the cells in the vestibular ganglion are at birth in a more mature condition.

As to the correlation between the development of the ganglion cells and the equilibrium function, we have noted that the albino rats, even just after birth, show some sense of equilibrium, though the movements are lacking in coordination. With advancing age the balance of the body is held much better and all the movements gradually become coordinated. The histological structure and the size of the cells at birth suggest that they are functional at that time, and the later increase in the volume and maturity of the cells is accompanied by a corresponding

TABLE 124



GANGLION VESTIBULARE


GANGLION SPIRALE


Nucleus-plasma ratio at one day Nucleus-plasma ratio at 546 days


1 :4.8

7.9


1 : 1.3

4.2


increase in the functional development. When the tactile sense is well developed and the eyes open equilibrium is almost perfected. It is a well-known fact that these two senses have very intimate relations to the maintenance of equilibrium. In this case, as we might expect, the early development of a function is accompanied by an early maturing of the neural mechanism on which it depends.

Conclusions

(for the ganglion vestibulare)

1. The measurements were taken on the largest nerve cells of the ganglion vestibulare in the radial section of the cochlea, and the developmental changes during portnatal growth studied in fourteen age groups, comprising four ears in each group. Further, in six age groups the cell size was determined in crosssections. The results have been given n tables 115 and 118 and charts 43 and 44.

2. The computed diameter at birth is 20.3 [x for the cell body and 11.7 ^ for the nucleus, and at 546 days, 25.3 and 12.2 n, respectively. Therefore the cells at birth are comparatively large and increase in size very slowly, but the increase is continuous.

3. The increase in the ratio of the cell body is as 1 : 1.3, of the nucleus as 1 : 1.1. We have between the same age limits no such small rate of increase in any other cerebrospinal ganglion studied in the albino rat. This small ratio indicates that the cells in the vestibular ganglion are well developed at birth.

4. We find no appreciable difference in the diameters of the cell bodies or the nuclei according either to sex or side.

5. Morphologically, the cells at birth are well differentiated. The form of the cells is ovoid.

6. The nucleus-plasma ratios are large at birth and increase regularly with age.

7. Comparing the development of the function of equilibrium with the growth of the cells, we see that these are correlated.

Final summary

This study is concerned with the age changes in the organ of Corti and the associated structures. The changes in the largest nerve cells which constitute the spiral ganglion and the vestibular ganglion, respectively, have also been followed from birth to maturity. On pages 116 to 124 are given the summary and discussion of the observations on the growth of the tympanic wall of the ductus cochlearis.

The conclusions reached from the study of the largest nerve cells in the ganglion spirale appear on pages 143 to 145. On pages 155 and 156 are presented the results of the study on the correlation between the response to sound and to the conditions of the cochlea.

Finally, the observations on the growth of the largest cells in the ganglion vestibu'are are summarized on pages 168 and 169.

It is not necessary to again state in detail the conclusions reached in the various parts of this study.

At the same time, if we endeavor to obtain a very general picture of the events and changes thus described, this may be sketched as follows:


170 ANATOMICAL AND PHYSIOLOGICAL STUDIES ON

Within the membranous cochlea there occurs a wave of growth passing from the axis to the periphery as shown in figures 4 to 13. The crest or highest point of the tissue mass appears at birth near the axis, in the greater epithelial ridge, and then progressively shifts toward the periphery, so that at maturity it is in the region of the Hensen cells. With advancing age the hair cells come to lie more and more under the tectorial membrane and the pillar cells seem to shift toward the axis.

At from 9 to 12 days the tunnel of Corti appears and the rat can hear.

All of these changes occur first in the basal turn and progress toward the apex. The mature relations are established at about twenty days. There are thus two waves of change in the membranous cochlea, from the axis to the periphery and the other from the base to the apex. The rat can usually hear at twelve days of age or about three days before the eyes open.

The largest cells in the ganglion spirale are very immature at birth, reach their maximum at twenty days, and after that diminish in size, slightly but steadily. The rat hears, therefore, before these cells have reached their full size.

The largest cells in the vestibular ganglion are precocious and remarkably developed, even at birth. They cease their rapid growth at about fifteen days of age, but increase very slightly though steadily throughout life.