Introduction

The muscles of facial expression are grouped mainly around the orifices of the face. Thus, it is often argued that their primary function is to act as sphincters and dilators of those facial orifices and that the function of facial expression has developed secondarily [8]. The nasal muscles that are connected to the external nose and nostril vary considerably in their development, which is reflected by the various descriptions in the literature. However, they essentially consist of dilators and compressors of the nasal aperture [6].

It has not been focused on the accurate anatomy and variations of levator labii superioris alaeque nasi (LLSAN) in detail. Generally, it is known that the LLSAN divides into medial and lateral slips that insert into the nasal ala and upper lip, respectively [8]. However, there is no report about the existence of the deep muscle layer arising from the LLSAN. In addition, anatomical variation of the transverse part of the nasalis has not been described.

The aim of this study was to clarify the muscular arrangement and topography of the deep layer of LLSAN and the transverse part of nasalis. Anatomical variations in the topographic relationships are also described, thereby providing data that are critical to the understanding of the function of the LLSAN and the transverse part of the nasalis.

Materials and methods

Anatomical examination was performed on 40 specimens of embalmed Korean adult cadavers (13 males, 9 females; age 56–96 years). After a detailed dissection of the face, the superficial layer of LLSAN and the transverse part of the nasalis were observed. The origins of the levator labii superioris muscle (LLS) and the superficial layer of LLSAN were cut and reflected anteriorly to reveal the deep layer of LLSAN. And then, the nose was removed en bloc to observe the LLSAN and the transverse part of the nasalis.

Results

The LLSAN was divided into two layers, which were superficial and deep in the LLS, respectively (100% of the specimens). The superficial layer of LLSAN descended on the LLS, and the deep layer was located deep in the LLS and lateral to the transverse part of the nasalis. The deep layer of LLSAN observed in all specimens originated from the superficial layer of LLSAN and the frontal process of the maxilla. It was flat and thin, inserting between the levator anguli oris and the orbicularis oris muscles. The direction of the deep layer of LLSAN was more oblique, toward the corner of the mouth than that of the superficial layer, and parallel to the transverse part of the nasalis (Fig. 1).

Fig. 1
figure 1

Photograph showing the deep layer of levator labii superioris alaeque nasi (LLSAN deep). The LLSAN deep originates from the superficial layer of LLSAN (LLSAN superf) and the maxilla, and inserts between the levator anguli oris (LAO) and the orbicularis oris muscles (OOr). The levator labii superioris (LLS) and the LLSAN superf are reflected to reveal the LLSAN deep

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The transverse part of the nasalis originating from the maxilla was thin and flat with a triangular shape. It was located deep in the alar part and ascended to the dorsum of the nose. The transverse part of the nasalis was “C”-shaped, surrounding the posterior nasal aperture and ascended anteriorly toward the dorsum of the nose. This transverse part lay between the lateral nasal cartilage and the greater alar cartilage in all specimens, and received some muscle fibers from the superficial layer of LLSAN in 90% (36/40) of specimens (Fig. 2).

Fig. 2
figure 2

Photograph showing the muscle fibers (arrows) connecting the superficial layer of levator labii superioris alaeque nasi (LLSAN superf) with the transverse part of the nasalis (N-transv). LLS levator labii superioris, LLSAN deep deep layer of levator labii superioris alaeque nasi, Med medial, Sup superior

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We observed two patterns of the positional relationship of the transverse part of the nasalis with the superficial layer of LLSAN. The transverse part of the nasalis originated from the maxilla and ascended, passing posterior to the superficial layer of LLSAN in 65% (26/40) of specimens. However, it originated as two muscle bellies from the maxilla and the upper half of the alar facial crease, respectively, in 35% (14/40) of specimens. The muscle belly arising from the maxilla ascended deep into the superficial layer of LLSAN, and the one arising from the upper half of the alar facial crease ascended on the superficial layer of LLSAN. After these two muscle bellies passed the medial border of the superficial layer of LLSAN, they merged and became a single muscle belly (Fig. 3).

Fig. 3
figure 3

Phographs showing the positional relationship of the transverse part of the nasalis (N-transv) with the superficial layer of levator labii superioris alaeque nasi (LLSAN superf). a The N-transv originated from the maxilla and ascended, passing posterior to the LLSAN superf. b The N-transv originated as two muscle bellies from the maxilla and the upper half of the alar facial crease. LLS levator labii superioris, OOr orbicularis oris muscle

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Discussion

We found that the LLSAN included a deep layer in all our specimens. Since the deep layer descended obliquely toward the levator anguli oris and orbicularis oris muscles, its function may be to assist elevation of the upper lateral lip and the corner of the mouth superomedially.

Other muscles toward the nose and the mouth in the midface, such as the LLS, the zygomatic minor muscle, the zygomatic major muscle, pull the mouth superolaterally. However, the LLSAN is the only muscle to pull the nostril and the upper lateral lip superomedially. Thus, the deep layer of LLSAN, found in the present study, seems to participate in pulling the mouth corner superomedially. Likewise, the superficial layer including the medial and lateral slips can elevate the nostril and the lateral part of upper lip to the superior and medial direction.

In the present study, a part of muscle fibers of the superficial layer of LLSAN descended to blend with the transverse part of the nasalis in most cases. However, the function of the muscle fibers connecting these two muscles remains unknown, because the LLSAN and transverse part of the nasalis have opposing functions as a dilator and a compressor of the nose, respectively. Waller et al. [9] reported that the movements induced by intramuscular electrical stimulation of the LLSAN bore a resemblance to that of nasalis stimulation, as described by Duchenne [5], whereby the skin of the bridge of the nose is wrinkled and contracted as if pinched. Therefore, they concluded that a portion of the nasalis acts in association with the LLSAN. Thus, we infer that the portion of the nasalis that is associated with the LLSAN comprises the muscle fibers connecting the LLSAN with the transverse part of the nasalis. In addition, it is thought that muscle fibers connecting functionally opposing muscles may represent one of the factors that render it difficult to distinguish the perinasal dilator and compressor muscles clearly. Bruintjes et al. [2] also reported that the nasal dilators and constrictors cannot be clearly differentiated because the nasal muscles consistently show a synergistic action. It seems that the muscle fibers connecting functionally opposing muscles are closely related to this consistent synergic action.

The transverse part of the nasalis, known to be a nose compressor, is one of the muscles responsible for compressing the nasal vestibule. It was observed to surround the posterior nasal aperture and lay between the lateral nasal cartilage and the greater alar cartilage, and as such it might compress slightly the posterior nasal aperture and nasal vestibule. It appears that the nasalis of other mammals is not an important muscle, or else it acts only as a dilator of the nose. Waller et al. [9] reported that they did not attempt to locate and stimulate the chimpanzee’s nasalis muscle due to its small size and thus the difficulty of achieving intranasal electrical stimulation. Burrows et al. [3] dissected the facial muscles of chimpanzees, but did not mention the nasalis muscle. The transverse part of the nasalis does not exist in mice and rats [7].

Clark et al. [4] reported that the transverse part of the nasalis helps to completely close the oral cavity by attempting to occlude the posterior vestibular nasal space to allow the production of certain sounds, [P] and [B]. They also stated that it is highly active in producing the sound [OO] and in the act of whistling. Breitsprecher et al. [1] stated that the nasalis muscle and orbicularis oris muscle are not independent muscles. Hence, the function of the transverse part of the nasalis in humans may be also related to phonation and facial expression.

In conclusion, the nasal dilators were located closer to the nostril and nasal vestibule than the constrictor, the transverse part of the nasalis. The arrangement of the perinasal muscles appears to participate in widening the nostril and nasal vestibule rather than in narrowing the nostril. These findings will be crucial data to understand the structure and function of the LLSAN and the transverse part of the nasalis. Surgeons can encounter with these unexpected muscle fibers during operative procedures. In additions, these muscle fibers can easily be interpreted as other structures in MRI and CT. Thus, detailed anatomy of the muscles is also important for clinical aspects.