By hematoxylin and eosin staining, the tarsus was reported to be a connective tissue plate running along the palpebral conjunctiva. It remains unknown whether the tarsal plate of the eyelid is a one-plate structure, as its name suggests. The upper-eyelid specimens from the 32 Japanese cadavers were stained using the Azan method to examine the tarsal structure, composed of dense fibrous connective tissue. The tarsus was stained the darkest blue as a plate extending along the palpebral conjunctiva. In addition, the intensity of the dark-blue staining in the eyelid margin increased toward the cutaneous side; thus, the tarsal tissue resembled the head of a golf club (Fig. 1). Therefore, if the tarsal tissue that was stained dark blue by the Azan method could be regarded as the tarsus, it could then be defined as a “composite connective tissue,” including the musculature, tarsal glands, eyelashes, and sweat glands. It appeared that the eyelashes hardly shed even during dynamic blinking, because the hair follicles were buried in this hard connective tissue.1
In general, the musculature that exists in the eyelid margin under the pretarsal orbicularis oculi muscle is identified as Riolan's muscle. Riolan's muscle was located around the meibomian gland ducts in the tarsal connective tissue,2,3 exhibited a striated structure similar to that of the orbicularis oculi muscle, and extended from the medial to the lateral canthus. Therefore, the tarsal musculature could be generically referred to as the “intratarsal orbicularis oculi muscle” or the “fourth part of the orbicularis oculi muscle,” besides the preorbital, preseptal, and pretarsal orbicularis oculi muscles. Moreover, a part of the intratarsal orbicularis oculi muscle was attached to the muscle fibers surrounding the lacrimal canaliculus (Fig. 2); this component is known as Horner's muscle.4
The new concept related to the intratarsal orbicularis oculi muscle, which extends into the tarsus from the medial to the lateral canthus and includes Horner's and Riolan's muscles, could facilitate an understanding of the mechanisms underlying the meibomian secretion and lacrimal drainage systems that are stimulated by blinking.
Masaaki Iwanami, M.D., Ph.D.
Shin-Yokohama Keisei Clinic
Kazumi Tsurukiri, M.D., Ph.D.
Tsurukiri Keisei-Hifuka Clinic
1. Tsurukiri K, Iwanami M, Tsutsumi M, et al. Histological study of eyelash follicles-growth environment and morphological change during hair cycle. J Jpn Society Aesth Plast Surg.
2. Lipham WJ, Tawfik HA, Dutton JJ. A histologic analysis and three-dimensional reconstruction of the muscle of Riolan. Ophthal Plast Reconstr Surg.
3. Iwanami M, Tsurukiri K. Histological comparison between young and aged specimens of the Oriental lower eyelid using sagittal serial sections. Plast Reconstr Surg.
4. Halben R. Beiträge zur Anatomie der Tränenwege. Albrecht von Graefes Arch Klin Exp Ophthalmol.
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