On the Acoustic System in the Cetacean Brains
Teizo Ogawa and Shozo Arifuku
Department of Anatomy (Director: Prof. T~izo Ogawa),
Medical School, Univl'!rsity of Tokyo

Introduction
It was first pointed out by SPITZKA (1886) and a little later by ZIEHEN..
and KiiKEN'l.TIAL (1893) that the acoustic syst~m in the brains of the Cetacea
is very well developed. HATECHEK and ScHLESI~GER (1902) referred to it
in their study of the brain stem of a Delphinus. Later, HOFMANN (1908)
investigated the superior olive of a few kinds of dolphins and the inferior
colliculus was studied in the same group of animals by VALETON (1908).
Recently I,xsawoRTIIY (1932) examined the whole brain of Tursiops truncatus both macroscopically and microscopically, concluding that its acoustic
system is in a high stage of development. It should be noted here that
only the Odontoceti, mostly the dolphins, have been studied so far in thisrespect and scarcely any reference has ever been made to the acoustic
system of the baleen whales, nor has there been any comparative study
between Odontoceti and Mystacoceti regarding this system.
One of the reasons which prompted us to our present study is, apartfrom the comparative anatomical interest, the desire to estimate the auditory sense of the sperm whale and various Mystacoceti, at least as far as
it is possible through the study of their brain stems. We are sure it is ofpractical importance to know about the sharpness of hearing in those whales, which are the chief objectives of· the present-day whaling enterprise.
Six kinds of whales are studied in our present work : of the dolphins,
Lagenorhynchus obliquidens GILL and Tursio borealis PEALE, of other Odontoceti, Kogia breviceps BLAINVJLLE (pigmy sperm whale) and Physeter
rnacrocephalus LEsso:x (sperm whale, cachalot); of the Mystacoceti, Balaenoptera borealis I~EssoN (sei whale) and Balaenoptera acuto-rostrata LAc:EPEDE (little piked whale). Serial sections of the brain sterns of these six
animals, all stained by the P AL-carmin method, have been examined withthe object of studying the structures in the acoustic system, of determining
the difference between Odontoceti and Mystacoceti with references. to the2

Teizo Ogawa and Shozo Arifuku

acoustic system of other mammals.
The Lagenorhynchus in our present study is an embryo of about 70
cm length, which corresponds, judging by the degree of myelinization inits brain stem, nearly to the newborn human infant. The Tursio is anembryo of about 45 cm length, whose prain stem is in the same stage ofdevelopment as that of 7 months old human embryo. In this Tursio embryo the myelinization of nerve fibers belonging to the acoustic system has
just started, while most of other fi.bers in the brain stem remain as yet
unmyelinated. The very rare brain of Kogia breviceps was collected by
T. 0G.AWA at Shiogama, Miyagi Prefecture on June 7th, 1936. The details
on this specimen were already reported in " Botany and Zoology " Vol. 7,
No. 7 (published by Yokendo, Tokyo. July, 1938). The brain stem ofPhyseter macrocephalus is not in perfect condition and the serial sections
are 'limited only to the levels from the oral part of. pons to the caudal partof mid-brain. Serial sections of the brains of two Mystacoceti are complete,in Balaenoptera borealis in transverse section and in B. acuto-rostrata insagittal section. All the brain stems of four Odontoceti are sectioned
transversely.
The details of our acquisitions will be reported later in another paper,and only summary of our work is presented here due to the limitation of
space.
Results (summarized)
1. " Telescoping " of the dolphin's brain stem.
One of the most characteristic features in the brain stem of dolphinsis the caudal shift of its dorsal structures. This peculiarity is not so apparent in the caudal part of oblongata, but on the level near the caudal endof pons, the axis of the brain stem seems to bend dorsally, presenting a
curious topographical relation, as in one transverse section of the brain
stem one sees in the ventral part structures belonging to the oral proximityof oblongata, in the dorsal part those of mid-brain and in the middle partstructures corresponding to the pontile level.
Strangely enough, this unusual f eatu re seems to have been overlookedby many researchers who have studied the brain stem of dolphins. Either
no notice has been taken of it, or it was simply dismissed as a result of
oblique section. As a matter of fact, however, this is very important inOn the Acoustic System in the Cetacean Brains

3

evaluating the structures in the brain stem of dolphins, and we would liketo emphasize it.
In 1923 MILLER described a phenomenon called " telescoping " of the
Cetacean skulls. According to him, the maxillary bone as well as thefrontal bone is elongated caudally, one over the other, coming in contact
with the occipital. The above mentioned shift in the brain stem of dolphinsseems to have an intimate relation with this unusual configuration of theskull and we wish to use the term " telescoping " also for the brain stemof dolphins. Meanwhile it is as yet impossible for us to decide whether
or not the telescoping of dolphin's skulls is directly responsible for the telescoping of the brain, as no remarkable shift of structures is to be seen in
the brain of other whales such as Kogia, Physeter and Balaenoptera, eventhough l\!ILLER's phenomenon is also present in the skulls of theseanimals.
The brain stem of dolphins is narrow from side to side and broad inventrodorsal direction. This is also one of its characteristics.
LANGWORTHY said that the whole brain of Tursiops is foreshortenedin its anteroposterior extert and widened laterally due to change in theconformation of the skull. This is true, in our opinion, as far as the
cerebral hemispheres are concerned, but in the brain stem, especially in
its caudal part, the transverse dimension is remarkably small. It is ofinterest to note here that the entire oblongata in dolphins, when consideredin its topographical relation to the skull, seems to be caudally shifted, as
both cochlear and vestibular roots run orocaudally to ·reach the brain stem
in its lateral part. The cerebellar flocculus surrounds the ventrolateralsurface of oblongata and pons, and the nucleus ventralis of the cochlear
nerve lies buried deep between flocculus and the brain stem. The cochlear
root approaches the brain stem obliquely and enters the oral part of theventral nucleus.
2. Dolphins and sperm whales.
Though both dolphin and the sperm whale including Kogia, belong to
the Odontoceti, many differences are observed in their acoustic systems, inall probability due to the above mentioned peculiar form of the brain stemat the side of dolphins. There is nothing, however, to indicate the existenceof an essential difference between them.
In the case of Kogia, the ventral nucleus of the cochlear nerve pro-

4

Teizo Ogawa and Shozo Arifuku

trudes from the ventrolateral part of the oral end of oblongata. Its cauqal
half is ventrally exposed to the view owing to the absence of flocculus overits ventral surface, while its oral half lies covered by .the fibers of brachium
pontis. The cochlear ,and vestibular nerves enter the nucleus in its medial
part, as is generally the case in the mammals. The ventral nucleus is as· well developed as that of Lagenorhynchus, and it is hard to decide whichis ·more prominent.
In Odontoceti, the corpus trapezoides, which emerges medially from theventral nucleus, is in a very good state of development ; its transverse
fibers occupy in Lagenorhynchus more than half (603-6') of the dorsoventral
length of the raphe, and in Kogia, a little less than the half (43%). The
same applies to HELn's tract, which runs quite separated from the trapezoidal body and joins this in the median line, where it forms the dorsal
part of the trapezoidal body.
Massive fibers emerge from the dorsal half of the ventral nucleus in
the fetal dolphins, seemingly ending in the superior olive. of the same side.
In Kogia it is hard to distinguish this group of fibers, probably because its
myelinization is in a far advanced stage. In his study of myelinization in
the cochlear system of man, dog, cat, rabbit and rat, one of us (OGAWA,
1936) observed the same kind of homolateral trapezoidal :(ibers as in the
fetal dolphins.
Noteworthy difference is found regarding l\foNARow's striae acusticae.
Those of Lagenorhynchus consist of a great number of fibers, which· form
a compact bundle, while in Kogia they are made of much fewer fibers.
This should not be interpreted as an essential difference, because MoxARow's tract also arises from the ventral nucleus of the cochlear nerve andis nothing but a part (pars dorsalis) of the trapezoidal body. In the sameway HELn's tract should· be considered as its intermediate part. In Odontoceti no trace of the dorsal nucleus of the cochlear nerve, i. e. tuberculum
acusticum, was observed.
One of the most impressive features in the brain stem of Odontoceti
is the remarkable development of the superior olive. We need not enter
into details about this nucleus of dolphins, as it is fully described by HorMAxx, who studied it in Phocaena communis and Delphinus delphis, andalso by LAKGWOnTuY, '''ho investigated that of Tursiops truncatus.
Partition of superior olive in two parts, pars lateralis and pars medialis,On the Acoustic System in the Cetacean Brains

5is hardly recognizable in Lagenorhynhus. It is almost the same with Tursio.
Immature myelinization of the fetal brains is probably responsible for it.
The superior olive of Kogia is relatively larger than that of dolphins. In
fact no lar~er superior olive can be found in all the whales we have investigated. Distinctly divided in two parts, the pars lateralis is disproportionately bigger than the pars medialis. · The nucleus as a whole presents avery light colored aspect, as there are relatively few myelin-stained fibers
within it. The pars medialis is much smaller, its oral end lying more
backward than that of the pars lateralis. It should be noted here that thiscaudal position of pars medialis is an exception to HoF)fAN~'s ·conclusion in
his comparative anatomical study of the superior olive, inasmuch as he saysthat in many animals he studied the pars medialis is invariably found to
be on a more oral niveau than the pars lateralis.
The trapezoidal nucleus is very well developed both in Kogia and in
Lagenorhynchus, with only the following difference: in the latter, probably
owing to the narrowness of the brain stem· this nucleus borders on the
median line, while in the former, there is some space between. the nucleus
and the raphe. Needless to say, this does not mean an .intrinsic· difference.
A part of the trapezoidal nucleus is found 'wedged between the superior
olive- and the lateral lemniscus, protruding into the dorsomedial part of thelatter. This is common to both of the Odontoceti. ·One of the similarities between Kogia and dolphins is the vastness of
the lateral lemniscus. That it is very large in dolphins has already been
The term
known, but in Kogia and Physeter it is strikingly larger.
"colossal " can safely be used in describing it. One is easily led to imagine a malformation of the brain stem. The nucleus of ·the lateral lemniscus and the inferior colliculus with its brachium are also proportionately
large. The medial genicufate body is completely merged in the posterior
part of the optic thalamus, and even though its hugeness can easily be
sctrmised, we were unable to measure its actual size.
Generally speaking, much similarity ·is observed in the acoustic system
of Kogia and dolphins. Remarkable development of the ventral nucleus ofof the cochlear nerve, corpus trapezoides, HELD's tract, trapezoidal nucleus,
superior olive, lateral lemniscus with its nucleus and the nucleus colliculi
caudalis is quite unique· among all the manimals.
J\IOSAKow's striae
acusticae are more prominent in dolphins.· Kogia' and Physeter may well

6

Teizo Ogawa and Shozo Arifuku

be said to surpass dolphins in the development of the superior olive, thelateral lemniscus and the inferior colliculus.
3. Odontoceti and Mystacoceti.
In the topographical relation of a great many structures in the brain
stem, Balaenoptera has much in common with Kogiat presumably because
there is no telescoping in their brain stem. At first sight, it seems as if
Balaenoptera is more akiri to Kogia than to Lagenorhynchus in their acoustic systems. Further investigation, however, reveals the existence of an
intrinsic difference between them.
The ventral nucleus of the cochlear nerve and the corpus trapezoides
are very well developed in Balaenoptera. The latter occupies in Balaenoptera borealis 40% of the raphe and in B. acuto-rostrata 47%, coming
very close to that of Kogia (43%). In Balaenoptera there is not a vestigeof 1-IELD's tract and l\'fONAKow's striae acusticae are poorly developed.
The most significant difference is observed between Balaenoptera andOdontoceti in the development of the superior olive. That of B. borealis isvery small, and it looks very da.°rk as it has many myelinated fibers among
the nerve cells. The pars lateralis of the superior olive in Kogia is disproportionately large compared with the pars medialis, or with the pars lateralisof Balaenoptera. That of B. borealis appears on the oral one third of the
facial nucleus, terminating on the level of the oral proximity of this nucleus (orocaudally 4 mm, transversely 4 mm), while that of Kogia begins on
the middle level of the facial nucleus and reaches the transitional part ofthe superior olive to the lateral lemniscus (orocaudally 5 mm, transversely
6 mm). The pars medialis of the superior olive in Kogia is small anddisappears on a more caudal level, while in Balaenoptera it is fairly longin orocaudal direction, outstripping its pars Iateralis by 4.5 mm. Histologicaldifference of the superior olive between Odontoceti and Balaenoptera seems
also to be of importance. As has been mentioned before, the superior
olive of Kogia presents a very light colored aspect, as there are relatively
few nerve fibers among its cells, while in Balaenoptera it is very dark on
account of abundant fibers within the gray substance.
Meagre development of the superior olive in Balaenoptera in spite ofits large trapezoidal body seems to indicate that a great number of thelatter's fibers reach the lateral lernniscus without interruption, relatively asmall part of it terminating in the superior olive. It is also conceivableOn the Acoustic System in the Cetacean Brains

7

that some of the trapezoidal fibers come directly from the cochlear rootand end in the trapezoidal nucleus. This nucleus is fairly well developedin Balaenoptera but not so well as that of Odontoceti. A small part of
the trapezoidal nucleus, which lies in Odontoceti between the superior olive
and the lateral lemniscus, is not observed in Balaenoptera. The lateral
lemnisct.is is as well developed as in most other mammals but falls shortof Odontoceti in its size. The inferior colliculus of Balaenoptera with its
brachium is above the average of mammals in its development but poorer
in comparison with that of Odontoceti. The medial geniculate body isfairly large, but its inner structure does not seem to be in good differentiation, as we see neither any trace of an incisure on its lateral surface,
nor any difference between the· peripheral and the deeper regions in its
ventral part.
Considered as a whole, there is a remarkable difference between
Mystacoceti and 09.ontoceti in the development of the superior olive andthe lateral lemniscus. We are not at present in a position to say what
functional meanings these structures have in the auditory sense, but judging
from the degree of development of intracerebral stmctures in the acoustic
system, we may assume that Odontoceti must be more acute of hearing.
In this respect Mystacoceti seem to be inferior to Odontoceti.
4. Man and Balaenoptera.
Inferiority of the acoustic sense of Balaenoptera to that of Odontoceti
does not necessarily imply the bluntness of hearing in the former, as structures of the acoustic system developed in Balaenoptera are by no means
Balaenoptera bears at least one resembelow the average of mammals.
blance to man in that the superior olive is very small compared with thewell developed trapezoidal body. In this respect there seems to be something in common between Homo and Balaenoptera in .their auditory sense.
At the same time one should not forget the fact that Balaenoptera's superior olive as well as its trapezoidal body is in a slightly better state ofdevelopment than that of man and the trapezoidal nucleus occupies a large
area, while in man this nucleus is hardly recognizable.
5. The corpus ponto-bulbare of EssICK, the dorsal nucleus ·of the cochlear nerve and the dorsal nucleus of the lateral lemniscus.
The corpus ponto-bulbare, first fully described by EssICK, is hardly
known to exist except in the human brains. Certainly there is no trace

8

Teizo Ogawa and Shozo Arifuku

of it in dolphins, but in Kogia it is very well developed and is also distinctly recognized in Balaenoptera.
When in 1933 WILSON described thebtain stern of Balaenoptera borealis, he mistook this gray matter for theventral nucleus of the cochlear nerve (Journ. Comp. Neurol. Vol. 58, No.
2, p. 455, fig. 18), and the space left blank just lateral to "tpz " in his
illustration also belongs to it. The fibers intimately related to the pontobulbar body are attached to the lateral and the medial sides of the ventral
nucleus both in Kogia and Balaenoptera. In B. borealis, these fibers run
dorsalward parallel to the corpus restiforme, forming the most lateral ofthree bundles directly lateral to this body. Each of these bundles seems
to have a significance of its own.

In the study of the acoustic system of whales, due attention should be
paid to the existence of corpus ponto-bulbare.
The existence of the nucleus dorsalis nervi cochleae, i. e. tuberculum
acusticum, in whales is very doubtful. We could not locate it in Odontoceti and found only a rudimentary one in Balaenoptera. According to. FusE, a partial fusion of the dorsal nucleus of the cochlear nerve to theventral nucleus occurs sometimes in higher animals. In this connection
the question arises whether or not the pars dorsalis of the ventral nucleus
we observed in dolphins is to be considered as the dorsal nucleus itself.
But our reasons for assuming the non-existence of the dorsal nucleus in
Odontoceti are as follows.
In dolphins the ventral nucleus is divided in two parts, pars dorsalis
and pars vetittalis, while in Kogia it is divided in pars lateralis and pars
medialis. The fact that the dorsal and ventral parts of the former correspond respectively to the lateral and medial parts of the latter is out ofquestion, as the cause of this topographical difference lies certainly in thegeneral form of the brain stems. In dolphins the brain stem is broad inventrodorsal direction and narrow transversely, while in Kogia it is quite
the opposite. Had the dolphin's brain stem been shaped just like that of
Kogia, the ventral nucleus of the cochlear nerve might have been divided
in pars" lateralis and pars medialis as in Kogia.
As it is, with its
topographical relation to the cerebellar flocculus and to l\foxAKow's
striae acUsticae being duly considered, we cannot identify in dolphins thedorsal part of the ventral nucleus with the dorsal nucleus of other mammals~·

On the Acoustic System in the Cetacean Brains

9

Next we \vish to call attention to the so-called dorsal nucleus of thelateral lemnicus, which is located both in Kogia and Lagenorhynchus, justventral to the, nucleus of ·the inferior colliculus and seems to be closely
connected with " fibrae perforantes ". This nucleus remains very small
among all other very well developed cochlear structures, and so it is very
doubtful that this tiny gray matter has anything to do with the cochlear
system. Probably it has some other functional significance. In Balaenoptera
no trace of this nucleus is to be seen.
VALE'l'OX, in her comparative-anatomical study of the inferior colliculus,
found a fiber group of small calibre in the lateral !emniscus of Phocaena
communis emerging from its dorsomedial part. Some of it are believed toreach as far as the contra-lateral inferior. colliculus by \Vay of velum
medullare anter.ius and some of it to run dorsally along the medial marginof the inferior colliculus until they disappear near the commis sure of theinferior colliculus.
Described in details as they were, we can not in any
way locate such a fiber group in Lagenorhynchus. Some finely myelinatedfibers are observed by us to arise out of the dorsomedial part of the lateral
lemniscus in Lagenorhynchus. These fibers either pass through the .brachium conjunctivum or over it to reach the central gray matter, wheremost of them disappear with only a small portion of them reaching as far
medial as the surface of the fourth ventricle.
They represent no doubt
Vxr,1;;ToK's fibrae marginales (loc. cit. p. 30 a. 59). In cachalot we saw several thick bundles of the same calibre pass through the brachium conjunctivum and reach the central gray matter dorsal to the fasciculus longitudinalis medialis, where they bend suddenly ventralward. In Kogia theexistence of such fibers is not so clear, while in Balaenoptera borealis athick bundle goes into velum medullare anterius ofter crossing the trochlear
root. We could not decide whether or not this bundle has anything to
do with the acoustic system.
6. Nuclei of the vestibular nerve, especially the DEITERS' nucleus.
In all the cetacean brains studied here, meagre develOpment of thevestibular nuclei as a .whole stands out in striking contrast to the very welldeveloped cochlear nuclei. In Lagenorhynchus, the vestibular root is barely
one tenth of the cochlear one in thickness.
In studying the vestibular system of whales, due consideration should
be given to the existence of a part of the vestibular root, so-called tractus

10

Teizo Ogawa and Shozo Arifuku

solitarius acustici of LEWANDOWSKY, which runs, apart from the maintract, dorsally along the lateral margin of the restiform body.
The cellsof the DEITEHS' nucleus lie scattered along this root, a few of them reaching as far ventral as the ventrolateral margin of the restiform body.
The nucleus triangularis of Kogia and Balaenoptera occupies a smallerarea than that of Lagenorhynchus, as in them the central gray matter ofthe fourth ventricle is more limited in space. This fact, however, should
not be considered as showing some definite difference in vestibular function,
as · the nucleus triangularis is not limited to the vestibular nerve in its
function.
The spinal tract of the vestibular nerve is very meagre m Lagenorhyncl_ms and its fiber bundles are in a looser arrangement. In Kogia andBalaenoptera this tract occupies a far wider area and its fibers are massively collected. Except in Lagenorhynchus, the BECHTEREw's nucleus is hardly
recognized in whales, a finding quite contrary to KAPLAN'S, who foundthis nucleus very well developed both in Phocaena and Balaenoptera.
Incidentally it should be mentioned that according to KAPLAN, this nucleus
in whales is the shortest one of all animals in orocaudal direction, though
it occupies a broad area on a transverse section.
In our opinion, of all the vestibular nuclei only the DEITERS' nucleus
is very well developed in whales. In Lagenorhynchus, Kogia, and Balaeno. ptera, the cells of this nucleus are scattered in a peculiar way and similarly distributed. They first come into view in the lateral part of the
spinal tract of the vestibular nerve which caps the restiform body. Next
some appear along the lateral vestibular root lateral to the restiform body.
Soon they become most abundant in the lateral part of the spinal tract
and also in the gray matter directly dorsal to it. Medium large cells
belonging to the DEITEP..s' nucleus are seen mostly in the dorsomedial
part of the spinal tract and the adjacent gray matter. Only a few large
cells are observed here. The cells of the DEITERS' nucleus are numerous

in number and widely distributed transversely, but is of short duration in
orocaudal direction (in Kogia only 2mm). Only a few cells of the MARTIN'S
nucleus are seen in Kogia and Balaenoptera borealis in the ventral part ofthe spinal tract medial to the restiform body.
The tractus vestibulo-spinalis is very prominent in Lagenorhynchus andTursio, due probably to the immature development of other fiber tracts inOn the Acoustic System in the Cetacean Brains

11

the brain stem. It arises, as a collection of massive fibers, from the dorsolateral part of the DEITERS' nucleus, and arches down ventrally until it reaches the ventromedial part of the reticular formation. This tract is fairly
distinct in Kogia, but in Balaenoptera it is hardly distinguishable.
Resume1. Structures of the acoustic system were comparatively studied in serial
sections of the brain stems of six kinds of whales (Lagenorhynchus, Tursio,
Kogia, Physeter, and two species of Balaenoptera).
2. In dolphins, the dorsal ·structures of mid-brain, pons and oblongata
are remarkably shifted caudally.
This phenomenon, to which the term,"telescoping" of the brain stem may b~ applied, is peculiar to dolphins.
It is also noteworthy that the brain stem of dolphins is narrow transversely
and broad dorsoventrally.
3. No "telescoping" of the brain stem was observed in Kogia, Physeterand Balaenoptera. Its form in these ·animals is broad transversely andnarrow in dorsoventral direction, being hardly different from the brain .
stem of other mammals in general.
4. In Odontoceti (dolphins, Kogia, and Physeter), the ventral nucleus of
the cochlear nerve, corpus trapezoides, superior olive, trapezoidal nucleus,
lateral lemniscus with its nucleus, nucleus et brachium colliculi caudalis
are all in a very good stage of development. They are overwhelmingly~IONAKOW'S
large compared with other structures in the brain stem.
striae acusticae are well developed in dolphins, but less prominent in
Kogia. The HELn's tract is highly developed both in dolphins and othertoothed whales. The superior olive and the lateral lemniscus of Kogia andPhyseter are larger than those of dolphins, probably the largest among all
mammals.

5. In Balaenoptera the ventral nucleus of the cochlear nerve and the
corpus trapezoides are fairly well developed, but the superior olive is smallin size and the trapezoidal nucleus, large as it is, falls short of that of
Odontoceti. This is one of the remarkable differences between Odontoceti
and Mystacoceti. The superior olive of Balaenoptera presents a far darker
aspect, as this nucleus contains many myelinated fibers. Their lemniscus
lateralis with its nucleus is somewhat retarded in its development comparedwith that of Odontoceti. The nucleus of inferior colliculus is fairly large

1'2

Teizo Ogawa and Shozo Arifuku

but inferior to that of Odontoceti in its dimensions. · )foxAKow's striae
acusticae are meagre in development and no HEr,n's tract was observed in
Balaenoptera.
6. That there is noteworthy difference between Odontoceti and Mystacoceti in their .acoustic· sense is easily surmised through the study of theirbrain stems. The acoustic sense of Odontoceti seems to be sharp and tohave reached a peculiar stage of development which is quite unique amongall. the mammals. The acoustic sense of Mystacoceti, though probably notinferior to that of land-living animals in acuteness, seems to be very different from that of Odontoceti. Discussions on the Mystacoceti's acoustic
sense through the study of that of Odontoceti are in no way justified.
7. The corpus ponto-bulbare of EssrcK is very well developed in Kogia
and in a slighter degree in Balaenoptera. In observing the end nuclei ofthe cochlear nerve, the existence of this gray matter should always beborne in mind.
8. Of all the vestibular nuclei only the D:i::rT1ms' nucleus is well developed in the whales we have studied. The cells of this nucleus are widelydistributed in a peculiar way, a part of it lying lateral to the restiformbody. The vestibulo-spinal tract is most prominent in the brains of fetal
dolphins. It also seems to be well developed in Kogia and Balaenoptera,but in these animals the myelinization of fibers in the brain stem is far
advanced and the vestibulo-spinal tract is not so sharply distinguished fromadjacent structures.

On the Acoustic System in the Cetacean Brains

References
1)

Spitzka, E. C.: New York Med. Journ. 1886. (quoted by Hatschek and Schlesinger)2) Ziehen, Th. u. Kiikenthal, w.: Denkschr. Med. Naturw. Ges. Jena. Bd 3, 1893.

3) Hatschek, R. u. Schlesinger, H.: Arb. Neur. Inst. Wien. IX, 1902, S. 1-17.
4)

Hofmann, F.: Arb. Neur. Inst. Wien. XIV, 1908, S. 76-328.

5) Valeton, M. T.:

ibid. S. 29-75.

6) Langworthy, 0. R.:

Journ. Comp. Neurol., Vol. 54, No. 2, 1932, pp. 437-488.

7) Miller, G. S.: Smithsonian Miscellaneous Collections. Vol. 75, 1923.
8)

Ogawa, T.:

Arb. Anat. Inst. Sendai. Heft XVTII, 1936, S. 53-184.

9) Fuse, G. : Arb. Hirnanat. Inst. Zurich. Bd. 7, 1913. S. 147.
10) Kaplan, M. : Arb. Neur. Inst. Wien. XX, 1913, S. 375-559. (esp. S. 446-470)

List of abbreviations in figs. l'-14
be ······Brachium conjunctivumom ······Nucleus olivae accessorius medialis

bck ······Decussatio brachiorum conjunctivorum

osup ···Superior olive (Nucleus olivaris met-

brp ······Brachium pontis

encephali)cc········· Crus cerebri

osupl ···pars Iateralis}

cgm ······Corpus geniculatum mediale

osupm···pars medialis

ci ······ ···Col!iculus caudalis

sgc ······Stratum griseum centrale

cibr ······Brachium colliculi caudalis

sn·· ······ ·Substantia nigra

coch······N. cochleae

tds -- · ·· ·Tractus spinalis n. vestibuli.
.
superior ohve

comci ·· -Commissura colliculi caudalis

tsves · · ·Tractus solitarit:s n. vestibuli

cpb ······Corpus _pontobulbare

tts ·· · · · ·Tractus tectospinalis

cpbf ..... -Fibers belonging to Corpus pontobul-

tvs · · · · · ·Tractus vestibulospinalisbareves · · · · · · Redix n. vestibuli

crs ······Corpus restiforme (restiform bcdy)

IIIr · · ····Radix n. oculomotorii

cs · ·. · · · · · -Colliculus rostralis

IVr ······Radix n. trochlearis

ctr ······Corpus trapezoides

V sp · · · · · ·Tractus spinalis n. trigemini

fs · · · · · · · · ·Fasciculus solit~rius

Vr ······Radix n. trigemini

hi········ ·Fasciculus longitudinalis medialis

Vsens ···Nucleus terminalis n. trigemini

Ht · · · · · · HELD's tract
II · · · · · · · · ·Lemniscus lateralis

VIr ··· .. ·Radix n. abducentis
VIIg ...... Genu (internum) n. facialis

Msa······MONAKOW's striae acusticae

VIIn······Nucleus originis n. facialis

nci ······Nucleus colliculi caudalis

VIlr· ··· .. Radix n. facialis

nil ·· .. ··Nucleus Iemnisci Iateralis

VIII ······N. statoacusticus (N. vestibuli+N.
cochleae)

np·········Nuclei pontis
ntr ······trapezoidal nucleusX

nv ······Nucleus ventralis n. cochleae

Xr · · · ···Radix n. vagi

nvd ······pars dorsalis ) Nucleus ventralis n.
•lVV · · · · · ·pars ventralis fcochleae

XIIr ······Radix n. hypoglossi· · · · · · N. glossopharyngicus et n. vagus

14

Teizo Ogawa and Shozo Arifuku

OrnFig. 1. Lagenorhynchus (x 3)l.

Fig. 2. Lagenorhynchus (x 3.7)

15

On the Acoustic System in the Cetacean Brains

Fig. 3. Lagenorhynchus (x 2.6)

bck

cibr

).:~l.~'-brp·c-1~ ----vlll, Ctr,,,---\ VJ[r_./-Fig. 4. Lagenorhynchus (x 2.2)

16

Teizo Ogawa and Shozo Arifuku

Fig. 5. Kogia (x 6.8)

Fig. 6. Kogia (x 3.5)

On the Acoustic System in the cetacean Brains

nt.r

Fig. 7. Kogia (x 3.2)llhl

Fig. 8. Kogia (x 2.8)

1B

Teizo Ogawa and Shozo Arifuku

ntr
Fig. 9.

Physeter (x 1.6)

Fig. J.O. Balaenoptera borealis (x 2)

On the Acoustic System in the Cetacean Brains

Fig. 11. Balaenoptera borealis (x 1.9)

19

20

Teizo Ogaw'a and Shozo Arifukti

Fig. 13. Balaenoptera borealis. (x 1.8)

Fig. 14. Balaenoptera bor~alis (x 1,.6)