© Karen Wild & Claus-Peter Richter

Previous literature suggests an essential role of fascia lines in posture and movements. According to Myers, the mandibular bone is one end of the deep front line and the inner ear is the top of the lateral line. Based on the phylogeny and ontogeny of the middle ear, we suggest that the deep front myofascial line should be extended beyond the mandibular bone ending at the inner ear too because the middle ear phylogenetically and ontogenetically have the same origin. Phylogenetically, the middle ear separated from the mandible creating the definite mammalian middle ear. Ontogenetically, the mandible and the middle ear ossicles originate from the mandibular neural crest stream that undergoes endochondral ossification. Note that the stapes has a second origin, the mesoderm. The two middle ear muscles, the stapedius and tensor tympani muscle, differentiate from arch mesenchyme.

The interesting functional aspect of myofascial lines refers to the possibility of diagnosis and treatment of musculoskeletal disorders. Interlinked trains of muscles and fascia, so-called myofascial lines, connect the body in an endless web and have implications for posture, movements, training, and therapy (Ahmed et al., 2019; Kamani et al., 2021; Myers, 2021; Sheikhi et al., 2021; Wilke et al., 2016).

The three-dimensional fascial system and its potential functional contribution to the body received little attention over many years. However, with increasing awareness of the fascial tissue’s role in musculoskeletal disorders and a better anatomical and physiological characterization of the tissue, a vivid debate on how to anatomically define fascia started about a decade ago (Adstrum, 2015; Adstrum et al., 2017; Bordoni et al., 2021a; Bordoni et al., 2022; Bordoni et al., 2021b; Bordoni et al., 2018; Bordoni et al., 2019; El-Boghdadly et al., 2022; Ercoli et al., 2005; Hedley, 2016; Kumka, 2014; Kumka & Bonar, 2012; Langevin, 2014; Myers, 2014; Schleip et al., 2019a; Schleip et al., 2019b; Schleip et al., 2012; Schleip & Klingler, 2014; Sharkey, 2019; Stecco, 2014; Stecco et al., 2018; Tozzi, 2014; van der Wal, 2015; Wendell-Smith, 1997).

Based on dissection studies and previous literature, Myers defines 11 myofascial meridians connecting distant parts of the body using muscles and fascial tissues. Simply, a myofascial line connects two longitudinally adjacent and aligned structures. However, fascia lines, such as the deep front line, render this basic principle more complex. The deep front line includes large body cavities, which are three-dimensional structures rendering the connection of two longitudinally adjacent and aligned structures impossible. Moreover, some myofascial lines connect and form slings (Hoepke & Landsberger, 1936; Myers, 2021).

Although not obvious, we argue that the deep front and the lateral myofascial lines are connected and form a sling at the inner ear. We argue that the cranial ending of the deep front myofascial line should be extended beyond the mandibular bone, including the middle ear ossicles, ligaments, and middle ear muscles.

The lateral and deep front lines

As described, for example, by Myers (Myers, 2021), the lateral myofascial line connects, along the lateral side of the body, the first to fifth metatarsal bases at the foot with the inner ear. The inclusion of the inner ear into the lateral line originates from a comparison with the lateral line organ of fish (Myers, 2021).

The deep myofascial front line is sandwiched between the superficial dorsal and the superficial front line. It connects the plantar tarsal bones and the plantar surface of the toes with the mandibular bone (Myers, 2021). The deep myofascial frontal line is unique because of its complex structure. This myofascial line does not simply connects two adjacent and aligned structures but forms complex three-dimensional constructs at the pelvis, abdomen, thorax, mediastinum, and neck. The deep myofascial front line connects via the hyoid bone to the mandibular bone (Myers, 2021). Like the lateral myofascial line, we suggest extending the deep frontal line to the inner ear, where it merges/connects with the lateral line.

Arguments to include the middle ear and mandibular bone into the fascia line system.

Phylogenetically, about 299 to 251 million years ago, two significant evolutionary steps led to the incorporation of the primary jaw joint of one of the largest terrestrial vertebrates, the premammalian synapsids, into the definite mammalian middle ear (Anthwal et al., 2017; Ji et al., 2009; Lautenschlager et al., 2018; Luo & Manley, 2020; Urban et al., 2017). In the first step, a mammalian middle ear formed, in which the ectotympanic (tympanic ring) and malleus were still connected to the mandible by an ossified Meckel’s cartilage (Anthwal et al., 2017). A second evolutionary step freed the tympanic ring and the malleus from the mandible through osteoclastic activities and created the definitive mammalian middle ear (Anthwal et al., 2017; Luo & Manley, 2020).

The tympanic ring and the malleus supporting the tympanum are less developed in Mesozoic mammals than in living mammals. This is consistent with their simple cochleae, suggesting that the ancestral, less sensitive middle ear was correlated with the limitation of their inner ear (Luo & Manley, 2020; Urban et al., 2017). Neomorphic inner ear structures of mammaliaforms include the petrosal fused from the periotics, the promontorium for the longer cochlea, and a vasculature similar to those of extant mammals. Fossil trechnotherians are the earliest to show a therian-like perilymphatic conduit configuration.

Ontogenetically, two cranial neural crest cell streams are important for the middle ear formation of amniotes; the mandibular stream of neural crest cells derived from the posterior midbrain and hindbrain rhombomeres and the hyoid stream (Ellies et al., 2002; Koentges & Lumsden, 1996; Sienknecht, 2013). In mammals, Meckel’s cartilage, malleus, and incus are derived from the mandibular neural crest stream, while stapes and the otic capsule arise from the hyoid stream. The phylogenetical and ontogenetical connection between the mandibular bone and the middle ear ossicles provide a strong argument to extend the deep front line to the stapes, which inserts in the round window of the cochlea forming a connection between the lateral and deep front lines.

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