THE LAMINATIN OF THE MASSETER OF THE
HUMPBACK WHALE*
TETSUO YOSHIKAWA AND TAKASHI SUZUKI**PREFACE

Basing upon the comparative anatomical study of the masseter of various mammals,
which include the marsupial (kangaroo), the insectivore (mole), the chiroptera(macro- and microchiropteras), the primate (monkey, apes and man), the rodent(rat and marmot), the lagomorph (rabbit), the carnibore (dog, cat and bears),
the artiodactyl (pig, camel, deer, goat, sheep and cattle), the perissodactyl (horse),
Yoshikawa et al (1961, 1962) proposed that the masseter of mammal shows alaminar structure, which consists of the following six elements :A. The proper masseter,
1) M. masseter superficialis, lamina prima,2) M. masseter superficialis, lamina secunda,
3) M. masseter intermedius,
4) M. masseter profundus, pars anterior and pars posterior.
B. The improper masseter,
5) M. maxillomandibularis,
6) M. zygomaticomandibularis.
The above laminar pattern is the modification of that of Seiferle (1958) in thedog, in which Seiferle overlooked the second superficial masseter and the maxillomandibular muscle. Although these six elements receive, of course, the various
modification as the animal varies, the fundamental principle seems to be applicablethrough the various animal. Then, how far the laminar pattern described above
can be applied to the whale, one of the most specialised forms in the mammal?
The authors express the hearty thanks to Dr. Y. Imaizumi, National Science
Museum (Tokyo), for his kind offering of the precious material.
MATERIAL AND METHOD

The male fetus of the humpback whale, Megaptera nodosa Bonn., is used, the bodylength of which measures 17 cm long (Fig. 1).
When the laminar structure is investigated, the tendon of the muscular layer
should never be cut through. It is recommendable to cut the muscular substanceof the layer along the terminal line. By cutting the indicated line, the blade canreach the tendon of the underlying layer. By scraping along the underlying tendon,
the layer can be easily separated. There is the ' reversal relation of the tendon andthe muscular substance ' between the succeeding layers.

* Dedicated to Professor T. Ogawa for his sixtieth birthday* * Institute of Anatomy, Faculty of Agriculture, Tokyo University of Agriculture & Technology
49

50T. YOSHIKAWA AND T. SUZUKI

Fig. I.

The fetus of the humpback whale.

DESCRIPTION

The mandible does not yet ossify completely. The posterior part of the zygomaticarch only develops, while the anterior half of the arch presents the tendinous structure, making the ventral border of the orbit.
The first superficial masseter arises from the ventral border of the orbit with
the narrow tendon, running ventrocaudally. It terminates along the mandibular
angle with the rich muscular substance, spreading in the semicircular form (Fig.
2, A and B). Crossing under this narrow tendon, the slender muscle which takes

Fig. 2. The laminar structure of the masseter of the humpback whale.
buccal ... M. .buccalis, interm ... M. masseter intermedius, maxil.-mand ... M.
maxillomandibularis, p ... M. masseter profundus, sup. I. ... M. masseter superficialis, lamina prima, sup. 2.... M. masseter superficialis, lamina secunda, temp.
superf ... M. temporalis superficialis, temp. prof ... M. temporalis profundus, zyg.mand ... M. zygomaticomandibularis.

its origin at the anterior extremity of the zygomatic process of the temporal boneruns anteroventrally. This is the maxillomandibular muscle (side infra). Whenthe first superficial masseter is overturned by cutting its muscular substance along

LAMINATION OF MASSETER OF HUMPBACK WHALE

51

the mandibular angle, the slender second superficial masseter is found, the direction of which coincides with the narrow tendon of the first superficial masseter(Fig. 2, C). Its tendon is found on the mandibular side and the muscular substance develops on the orbital side. When the muscular substance of the orbitalside is cut cautiously, the tendon of the intermediate masseter makes its appearance,
the muscular substance of which develops on the mandibular side (Fig. 2, D).
Comparing with the first superficial masseter, the second superficial and intermediate
masseters are very narrow, overlapping in the same form.
The deep masseter does not make a good development, spreading as a thinsheet between the root of the condyloid process and the glenoid fossa of the zygomaticarch (Fig. 2, E and F).
Making the X-form with the superficial masseter, the thick muscular column
develops, connecting the anterior extremity of the zygomatic process of the temporalbone and the mandible. This column can be divided into the medial thick column
and the lateral thin muscular plate. The latter is the maxillomandibular muscle,
which arises as the thin plate at the anterior extremity of the zygomatic process ofthe temporal bone, spreading anteroposteriorly (Fig. 2, E). The former is thezygomaticomandibular muscle, which originates from the medial side of the anterior
extremity of the zygomatic process and terminates near the root of the small coronoid process (Fig. 2, F).
The temporal muscle can be divided into the superficial and deep muscles(Fig. 2, A and F). The superficial temporal muscle arises from the dorsal side of
the or bi ta and the anterior half of the temporal fossa and terminates on the vestigial
coronoid process. The deep temporal muscle is recognised between the posterior
margin of the superficial temporal muscle and the zygomatic arch.
CONSIDERATIONIn the specialised mammal such as the whale, whalebone or toothed, we expected
the modification of the laminar pattern of the masseter. The result of investigation,
however, proved that the laminar subdivision after Yoshikawa et al can be also
applied to the whale. The proper masseter, however, shows the atrophy as thelayer deepens; while the improper masseter makes a good development, especiallythe zygomaticomandibular muscle.
Such a poor differentiation of the proper masseter coincides well with the loss
or weak development of teeth, which does not require the quick and strong mastication. On the contrary, the enormously long mandible requires the strong closure.
To this purpose, the development of the improp.er masseter plays a good role.
The analogous model is furnished with the forceps, which is easily closed by pressing
the sides with fingers, while it is not easily closed by manipulating the base of theforceps.
Toldt (1905) reported the masseter of the dolphin, Delphinus delphis, inwhich Toldt mentioned only two layers. The superficial layer is piriform andcorresponds to the first superficial masseter after Yoshikawa et al. The deep

52T. YOSHIKAWA AND T. SUZUKIlayer which is covered by the superficial layer corresponds to the intermediate
masseter after Yoshikawa et al. The existence of zygomaticomandibular muscle
is denied, owing to the incomplete zygomatic arch. If Toldt cuts the muscular
substance of the superficial layer along the mandibular terminal line, he would
easily succeed to discover the tendon of the second superficial masseter. Toldt
mentioned the maxillomandibular muscle in the rodent only and included it in
the zygomaticomandibular muscle in the other mammal.
In the dolphin, the zygomatico- and probably maxillomandibular muscles
may be hidden behind the zygomatic process of the temporal bone. The complete absence of these muscles can not be imagined in the toothed whale from theresult of investigation, not only in the whalebone whale, but also in the generalmammal.
SUMMARY

In the fetal humpback whale, one of the mistacoceti, the proper masseter presents
the atrophy as the layer deepens, while the improper masseter makes a good development, especially the zygomaticomandibular muscle.
In the proper masseter, the first superficial masseter makes fairly a good development, while the second superficial and intermediate masseters remain narrowand thin muscles. The deep masseter remains rudimentary.
In the improper masseter, the maxillomandibular muscle is a broad but thinmuscle. The zygomaticomandibular muscle is the thick muscular column, whichplays the most important role to close the long mandible.
LITERATURESGREGORY, W. K. (1957). Evolution emerging. Vol. II, fig. 22. 12 a, b (p. 902-3). (Anatomy of foetal whalebone whale. After Schulte.) MacMillan, New York.
NICKEL, R., A. SCHUMMER & E. SmFERLE (1958). Lehrbuch der Anatomie des Haustiere. Bd. 1, S. 266. Paul
Parey. Berlin u. Hamburg.
SEIFERLE, E. (1958). See NICKEL, R., A. SCHUMMER & E. SEIFERLE (1958).
ToLDT, C. (1905). Der Winkelfortsatz des Unterkiefers beim Menschen und bei den Siiugetieren und dieBeziehungen der Kaumuskeln zu demselben. Tei! 2. Sitz.-berichte d. Mathern. Nat.-wissenschaftl. Klasse.
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YosHIKAWA, T., T. SuzuKI, R. KrncHI & H. MATSUURA (1961). The comparative anatomy of the M.
masseter of the mammal. Acta Anatomica Nipponica, 36 (1) : 53-71.
YosHIKAWA, T., T. SuzuKI, R. KrncHI & H. MATSUURA (1962). The lamination of the M. masseter of thecrab-eating monkey, orang-utan and gorilla. Acta Anat. Nippon., 37 (3): 206-217.
YosHIKAWA, T. & T. SuzUKI (1962). The lamination of the human masseter-the new identification of M.
temporalis superficialis, M. maxillomandibularis and M. zygomaticomandibularis in the human anatomy.
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YosHIKAWA/:T. & T. SuzuKI (1962). The lamination of the masseter of the bat. Acta Anat. Nippon., 37 (5) :352-358.
YOSHIKAWA, T. & T. SuzuKI, R. KrncHI & H. MATSUURA (1962). The lamination of the masseter of theruminantia. Acta Anat. Nippon., 37 (6): 430-442.