SECTIONS OF THE DOLPHIN'S HEAD
(STENELLA CAERULEOALBA) *HIROSHI HOSOKAWA** AND TOSHIRO KAMIYA**
For the past years one of the authors, Hosokawa, has dissected several specimensof the Stenella (Prodelphinus) caeruleoalba (Meyen), paying speciel attention to theperipheral distribution of cranial nerves. In order to supplement the ordinary
anatomical observations, a couple of heads of this animal were sectioned transversely or lengthwise with the hand-saw. Anatomical features of each section
surface were very interesting and instructive for understanding the three-dimentional
structures of the dolphin's head. Such cross-sectional observation was adopted
partially by Slijper (1936) in his comparative-anatomical study of the cetacean
blood vessels, by Lawrence and Schevill (1956) in their functional anatomical studyof the delphinid nose as well as by Nakajima (1961) in his work on the cetacean
rete mirabile, although they did not give detailed explanation of each structure on
the section surfaces.
Materials used in the present study comprise two heads of adult Stenella caeruleoalba from Kawana, Shizuoka Prefecture, which had been soaked and fixed thoroughly
in 10% solution of formalin. One of the specimens was sectioned transversely into
six slices (Fig. 1), and each section surface was photographed and sketched fromrostral direction. The total length of the head was 45 cm, and each slice was
successively some 16, 5, 6, 4.5, 4.5 and 9 cm in thickness. The second specimen,
46 cm in length, was sectioned at first in the median plane. Then the right half
was cut lengthwise 5 cm apart from the median surface. In the following pages
each section surface will be shown with brief explanations. Since the cross-sections
are seen from rostral, the right side of the head is on the left side of the figures.
CROSS-SECTIONS

Section I (Fig. 2-a, b)
The section passes through the root of the snout or rostrum. The upper andlower jaws are separated from each other by the gap of the oral cavity. The
skeleton of the upper jaw is represented by the maxilla, premaxilla, vomer andmesethmoid cartilage. The last one lies on the median line, enclosed in the canalformed by the premaxillae of both sides and the vomer. The dorsal part of thepremaxilla is composed of particularly compact bony substance. Inside themaxilla and premaxilla we see small canals for conducting the superior alveolar
arteries and nerves, which supply small branches to the alveolar plexus around theroot of the teeth. For reference, the peripheral distribution of the trigeminal nerve
is shown in Fig. 13.
~* To the memory of late Prof. emeritus Tsunetaro Fujita.
* Department of Anatomy, University of Tokyo School of Medicine, Hongo, Tokyo, Japan.
105

106

H. HOSOKA WA AND T. KAMIYA

Needless to say, the Stenella is, like all the other odontocetes, furnished withhomodont teeth, of which the number amounting to some forty or fifty pairs in both
upper and lower jaws. On the left side of the figure the whole contour of one toothis shown. Approximately halfofthe total length is inside the maxilla (radix dentis),while the other half, conical in shape and somewhat curved mediad, is out of the
gmg1va.
Dorsal to the premaxilla and maxilla is a white area occupied by adipose connective tissue. This is the anterior extremity of the so-called " melon". This
peculiar structure, characteristic of most dolphins, lies on the snout and forms
the prominence in front of the nostrils. Although there have been proposed many
hypotheses about the function of this unique structure, we do not know yet anything definite. Probably the spermaceti organ, characteristic of the sperm whale,
represents the highly differentiated " melon" of other odontocetes. The " melon"
and spermaceti organ have been very often thought in relation to the respirationof the cetacea (Raven & Gregory, 1933; Slijper, 1962, etc.).
Under the melon we see the anterior end of the m. rostralis or the pars labialis
of m. maxillonasolabialis (Huber, 1934). These muscle bundles arise from the uppersurface of maxilla and run up- and forwards, so as to insert to the thin fibrous capsuleof the melon. Some of the muscle bundles, which arise from the lateral border ofthe maxilla, are directed up- and backwards, so as to border the anterior end of
the narial muscle group, pars nasalis of m. maxillonasolabialis (see Fig. 10).
The mandibula is cut on both sides in the lower jaw. This bone is furnished witha wide internal space filled with spongy, adipose tissue (mandibular canal or fossa),
and the inferior alveolar vessels and nerves are seen to pass through this fatty substance(confer Fig. 13).
In the middle of the lower jaw is the cut surface of the tongue, representing apeculiar quadrangular shape. Inside the tongue we see the inverted V letter of thegenioglossus muscle, as well as many internal lingual muscle bundles arranged irregularly.
Beneath the tongue are sections of the mylohyoid muscle and of the panniculus carnosus,
which covers the ventral surface of the lower jaw.

Section II (Fig. 3-a, b)
It represents a section through the middle of the " melon ". Anatomical features of this section do not show significant differences from those of the preceding
one, while the melon and the rostral muscle have considerably increased in size.
In the lower jaw the tongue is cut at about the middle of the corpus linguae.
In addition to mm. genioglossus and mylohyoideus, which were already observed in
Section I, the geniohyoideus and hyoglossus are seen in the floor of the oral cavity. The
arrangements of these muscles are shown in Figures 11 and 12. As seen in Fig. 11,
the panniculus carnosus is considerably well-developed on the ventral surface of thelower jaw. Systematic description of the cetacean muscles is to be referred to Stannius (1849), Schulte (1916, 1918), Howell (1930), Slijper (1936), etc.

SECTIONS OF THE DOLPHIN'S HEAD

107

Section III (Fig. 4-a, b)This is a section at the oral angle or the posterior end of the oral cleft. The bonyframework of this section, shaped like a flying bird with extended wings, is composed of premaxilla, maxilla, vomer, os malare or jugale, os palatinum, os pterygoideum aswell as of the mesethmoidal cartilage. The maxilla and os malare form a doublelayered bony plate (confer Fig. 9). On the left side of the figure the malar boneshows on its lower surface a pointed prominence representing the root of the zygomatic process.
Noteworthily there are peculiar cavernous sinuses inside or in close apposition
to the skeleton. The largest one is inside the pterygoid fossa surrounded by ospalatinum and pterygoideum (postpalatine air sinus of Flower, 1876). Hence it iscalled sinus pterygoideus. Other sinuses on the section are secondary expansions of
the pterygoid sinus.
All the sinuses are traversed by fibrous trabeculae and give spongy appearances.
These sinuses belong to the pneumatic sinus system, which characterizes especially theodontocetes. All the sinuses such as pterygoid, epitympanic, peripetrosal, etc. are continuous with the tympanic cavity. The pneumatic sinus system was described by
Boenninghaus (1904), Gruhl (1911), Yamada (1953) etc., who did not however
explain the physiological significance of this peculiar structure. Nerve bundles
belonging to V 2 (n. maxillaris) are seen in the trabeculae of the frontal and maxillarysinus expansions (confer Fig. 13).
Above the bony frame the " melon " is seen in the middle of the section, being
abutted on by the m. narialis on both sides (Fig. 10). The narial muscle of blow holemuscle, pars nasalis of m. maxillonasolabialis of Huber ( 1934), is formed by many layers
and subdivisions, of which the direction of muscle bundles is somewhat different
from one another. Lawrence and Schevill ( 1956) recognized six layers and called
them posteroexternus (pe), intermedius (i), anteroexternus (ae), posterointernus(pi), anterointernus (ai), and profundus (pr).
At the center of the lower half of the section is a transverse slit which is linedwith the thick wall of mucous membrane. This is the posterior part of the oralcavity, of which the floor is represented by the radix linguae (see Section VI). Ventral
to the oral cavity we see m. hyoglossus, genioglossus, geniohyoideus, monogaster, etc. M.
mylohyoideus is cut obliquely on each side of the oral cavity.
The pterygoideus medialis muscle is seen to occupy the space between the ptergoid
process and mandibula, showing close relationship to the frontal sinus expansion.
The n. facialis is cut under the bony shelter formed by the maxilla and os malare.
This nerve trunk can be traced to the rostral until the antorbital notch, where it
turns upwards, returns backward on the dorsal surface of the maxilla, so as to supply
the whole musculature of the nostril (see Fig. 13).
Section IV (Fig. 5-a, b)
The section passes through the bony nasal canal, which courses along the anteriorwall of the cranial case. The nasal canals of both sides are separated by the ovalshaped mesethmoid bone. Above the bony naris the nasal canal continues to the pre-108

H. HOSOKAW A AND T. KAMIYA

maxillray sac and still further to the vestibular sac just under the thick blubber layer.
The complicated structure of the dolphin's nasal passages has been subjected toinvestigations of many scholars such as Rawitz ( 1900), Gruhl ( 1911), Burne ( 1952),
Lawrence and Schevill ( 1958), etc. So the present authors do not intend to go
into its details. A somewhat schematic illustration will outline this labyrinthous
nasal passage (Fig. 14).
On both sides of the nasal ways we see massively developed narial muscle, pars
nasalis of m. maxillonasolabialis. The intimate topographical relations between thismuscle and nasal passage imply their close functional correlations.
The bony frame of the section is composed of premaxilla, maxilla, os frontale
and os pterygoideum. The plate-like lateral projection is formed by the maxilla andos frontale. This peculiar modification and dislocation of the cranial bones areassociated with the famous phenomenon called " telescoping " of the cetacean skull(Kernan, 1918; Kellog, 1928, etc.). Branches of nn. alveolares superiores are seen
between these two bony laminae (see Fig. 13). Lateral to the pterygoid process isseen the sinus pterygoideus of the pneumatic sinus system. Upwards it is elongatedand continuous to the maxillary and frontal expansions.
On the left side of the section we see the cut surface of the 9e ball as well as of
extrinsic 9e muscles. The eye ball is characterized by the extraordinarily thickscleral wall, an adaptation to the hydrodynamic environment of the cetacea. The
floor of the orbit is formed by a tough fibrous membrane, and the facial nerve runs
just under this orbital floor. The :::ygomatic arc is also cut near the tip of the orbitalcone.
It is noteworthy that the eye muscles are covered by a thick cavernous layercomposed of spongy substance. This is nothing but the rete mirabile, or the peculiar
network of arterial blood vessels. As well known, the extraordinary developmentof the rete mirabile is one of the most remarkable characteristics of the cetacean
anatomy (Stannius, 1841; Wilson, 1879; Ommanney, 1932; Slijper, 1936; Kock,
1959; Nakajima, 1961; Slijper, 1962, etc.). The rete is distributed mainly in thetrunk and neck. In the cranial region it is found in the orbit (rete orbitale) and in
the floor of the cranial cavity (rete basis cranii) (see Section V).
In the lower part of the section we see the oral cavity, which is cut at its posterior
end.
Section V (Fig. 6-a, b)
The cranial cavity is cut open and the brain, which is poorly fixed, is exposed.
The brain lies on the cushion of the rete basis cranii. Meningeal coverings of thebrain are quite same as those of man and other mammals.
The bony frame of the section is formed by the maxilla, os jrontale, os parietale,os ethmoidale and vomer. The lateral dislocation of os parietale is a part phenomenonof the " telescoping " of dolphin's skull. The zygomatic process of the squamosalis cut on the left side, while the zygomatic arc is still seen on the right.
The mandibula is cut near its posterior extremity. As seen already in thepreceding section, the medial wall of the mandibula is lost and the mandibular fossa

SECTIONS OF THE DOLPHIN'S HEAD

109

is filled with massive fatty substance, which extends forward into the wide mandibular canal (Sections I-III). Medially the mandibular adipose substance is abuttedon by the pterygoid muscles and pterygoid sinus. Because of these intimate topographical relationships, Yamada (1953) guessed that the movements of the lower jaw
might effect strongly upon the distension of the pterygoid sinus.
The pharynx and larynx are shown on the median line. This peculiar situationthat the pharynx is cut in front of the larynx can be understood by checking theguide line in Fig. 6-a. The pharnyx is surrounded by muscle groups such as m.
monogaster, ~ylohyoideus, styloglossus, hyoideus, sternothyroideus, etc. Os hyoideum is cutat the basihyal as well as at the stylohyal.
The larynx is cut at the tip of the arytenoepiglottideal tube, which protrudes uniquely into the choana. This peculiar tube, characteristically well-developed in
the odontocetes, was described by Hosokawa ( 1950) in the sperm whale (see Fig. 7).
LONGITUDINAL SECTIONS

Section VI (Fig. 7-a, b)This is the median section of the Stenella's head. Because of the slight asymmetry of dolphin's skull, the right nasal passage is cut through, showing the strange
disposition of the nostril (blowhole), vestibular sac, tubular sac, nasal plug, premaxillarysac, etc. The m. narialis is seen in close topographical relationship to the nasal
passage, especially to the nasal plug. In front of the nasal passage we see the" melon ", a peculiar mass of adipose tissue on the dolphin's rostrum.
The skull is represented by the brain case and the long rostrum. The former
is cut through osfrontale fused with mesethmoidale, os supraoccipitale, basisphenoidale andbasioccipitale, while in the elongated snout we see os mesethmoidale, mesethmoid cartilage,
premaxilla, maxilla, os palatinum, os pterygoideum, etc.
The brain shows a remarkable dorsal flexion in the pontine region, representing
a noteworthy characteristic of the cetacean brain. As seen in this section the brain
stem is relatively well developed. The cerebral hemisphere shows a very complicatedpattern of convolutions and sulci. As well known, the cetacean brain stands
highest in the mammalian kingdom, so far as the complexity of the cerebral fissuralconfiguration is concerned. The corpus callosum is relatively small.
The tongue is very long and its greater part is attached to the floor of the oralcavity. Intrinsic and extrinsic lingual muscles are fairly well developed, implying theconsiderable motility of the tongue. The arytenoepiglottideal tube of the larynx isclearly shown at the left and lower end of the figure. As explained in Section V,
this tubular structure formed by modified arytenoid and epiglottic cartilages protrudes into the choana. Probably the dolphin can breathe and swallow foods
simultaneously.
Section VII (Fig. 8-a, b)This section shows a sagittal plane 5 cm to the right side from the median plane.
The medial wall of the section is seen from outside. The section passes through110

H. HOSOKA WA AND T. KAMIYA

the foramen jugulare (foramen lacerum posterius), which is represented by a wide gap
between the basi-exoccipital and alisphenoidal bones. The gap is filled with
vascular plexus, rete mirabile, and nn. IX-XI are cut along the posterior bony wall,
while the n. XII is seen inside the hypoglossal canal within the exoccipital bone.
The os tympanicum is seen under the jugular foramen, surrounded by remarkable spongy cavernae of the peripetrosal pneumatic sinus. The latter continues forward
into the larger pterygoid sinus.
The mandibula is cut at the mandibular fossa, which is filled with adipose masscontaining branches of the n. mandibularis. In the hyoid region we see os basihyale
and stylohyale, surrounded by muscle groups such as m. monogaster, m. mylohyoideus, etc.
IN CONCLUSION

For the purpose of supplementing the ordinary anatomical dissections and observations of the dolphin's body, heads of the Stenella caeruleoalba (Meyen) were sectioned
transversely or lengthwise. Then each section surface was photographed, sketchedand described. Anatomical features of these sections are very interesting andinstructive for understanding the three-dimensional structures of the dolphin's head.
REFERENCES
BoENNINGHAus, A. (1904). Das Ohr des Zahnwales. Z,ool. Jahrb., Abt. Anat. Ontog., Bd. 19. (Cited from
Yamada, '53)
BURNE, R.H. (1952). Handbook of cetacean dissections. British Museum (Natural History). London.
FLOWER, W. H. (1876). An introduction to the osteology of the mammalia. MacMillan Co., London.
GRUHL, K. (1911). Beitriige zur Anatomie und Physiologie der Cetaceennase. Jena.<:,. f. Naturw., 47:
367-414.
HosoKAWA, H. (1950). On the cetacean larynx, with special remarks on the laryngeal sack of the sei whale
and the aryteno-epiglottideal tube of the sperm whale. Sci. Repts. Whales Res. Inst., No. 3: 23-62.
HowELL, A. B. (1930). Myology of the narwhal (Monodon monoceros). Amer.]. Anat., 46: 187-215.
HUBER, E. (1934). Anatomical notes on pinnipedia and cetacea. Carnegie Inst. Washington,publ. No. 447:
105-36. (Cited from Lawrence & Schevill, '56)
KELLOGG, R. (1928). The history of whales-Their adaptation to life in the water. Quart. Rev. Biol., 3:29-76, 174-208.
KERNAN, J. D. (1918). The skull of Ziphius cavirostris. Bull. Amer. Mus. Nat. Hist., 38: 349-94.
KocK, L. L. de ( 1959). The arterial vessels of the neck in the pilot-whale and the porpoise in relation to the
carotid body. Acta Anat., 36: 274-92.
LAWRENCE, B. & ScHEVILL, W. E. (1956). The functional anatomy of the delphinid nose. Bull. Mus. ofComp. Z,ool. at Harvard College, 114: 103-51.
NAKAJIMA, M. (1961). The study of the rete mirabile of the cetacea. (in Japanese) Toho-Igakukai-Z,asshi, 8:1611-24.
0MMANNEY, F. D. (1932). The vascular networks (retia mirabilia) of the fin whale. DiscoveryRepts., 5: 32762.
RAVEN, H. C. & GREGORY, W. K. (1933). The spermaceti organ and nasal passages of the sperm whale(Physeter catodon) and other odontocetes. Amer. 1\1us. Novit., No. 677: 1-18.
RAWITZ, B. (1900). Die Anatomie-des Kehlkopfes und der Nase von Phocaena communis Cuv. Internal.
Monatschr. f. Anat. Physiol., 17: 245-354.
SCHULTE, H. von W. (1916). Anatomy of a foetus ofBalaenoptera borealis. Mem. Amer. Mus. Nat. Hist.,
New Ser., 1 (6): 389-502.

SECTIONS OF THE DOLPHIN'S HEAD

111

SCHULTE, H. von \'\'. & SMITH, M. de F. (1918). The external characters, skeletal muscles, and peripheral
nerves of Kogia breviceps (Blainville). Bull. Amer. Mus. Nat. Hist., 38: 7-72.
SLIJPER, E. J. ( 1936). Die Cetaceen. Vergleichend-anatomisch und systematisch. Martin us Mijhoff, Haag.
SLIJPER, E.J. (1962). Whales. (Translated into English by A.J. Pomerans) Hutchinson Co., London.
STANNIUS, H. ( 1841). Ueber den Verlauf der Arterien bei Delphinus phocaena. Arch. Anat. Physiol., Jahrg.
1841: 379-402.
STANNrus, H. (1849). Beschreibung der Muskeln des Ttimmlers (Delphinus phocaena). /bid.,Jahrg. 1849:
1-44.
WILSON, H. S. (1879). The rete mirabile of the narwhal. J. Anat. Physiol., 14: 377-98.
YAMADA, M. ( 1953). Contribution to the anatomy of the organ of hearing of whales. Sci. Repts. Whales Res.
Inst., No. 8: 1-79.
Addendum
After this paper went to press, we had the opportunity to see the following article which gave brief
outline of cross-sections of the Stenella graffmani.
BorcE, R. C., SWIFT, M. L., & ROBERTS, J.C. (1964). Cross-sectional anatomy of the dolphin. Norwegian
Whaling Gazette, 53 : 177-93.

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SECTIONS OF THE DOLPHIN'S HEAD

Posterior lip\

133

Blowhole

I

I

\

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I
Vesti bulor sac

Valve withligomentous bondTubular sac- - Nosal plug

Bony nostril

Prem axillary

Fig. 14.

Nasal passage.