Irrespective of whether the levator ani muscle reached the midsagittal area or not, a fibrous issue extending along the supero-inferior axis between the coccyx and EAS was consistently observed. This seems to correspond to the anococcygeal ligament rather than the raphe, because the term "raphe" should be used for a structure connecting bilateral levator muscle slings. The raphe is thus most likely to run transversely, as seen in the linea alba between the bilateral abdominal rectus sheathes. Our observations thus seem to be consistent with descriptions by Ayoub,18,19
who described that all muscle fibers of the EAS retain skeletal attachments to the coccyx via the anococcygeal ligament. displays one of the classical concepts of the ligament and raphe, as described by Toldt,4
in which the raphe is located along the internal or ventral side of the ligament. In contrast, according to the present histology, we hypothesize that the raphe is a specific subcutaneous tissue on the superficial or dorsal side of the anococcygeal ligament (). This restricted, subcutaneous structure is quite different from the classical concept, but the function as a raphe seems to be the same (see below).
Fig. 6 Anococcygeal raphe: a classical concept and our identification according to the present histology. (A) A classical view of the anococcygeal raphe (raphe) in dorsal views of the female perineum. (B) Our identification according to the present study. Muscles (more ...)
In fetuses, the longitudinal muscle coat of the anal canal gave off smooth muscle fibers into a dorsal band-like tissue, which connected the anal canal with the coccyx, i.e., the primitive anococcygeal ligament. Likewise, also on the ventral side of the anus, similar smooth muscle tissue was seen connecting with a ventral connective tissue mass or the primitive perineal body.20,21
The longitudinal muscle coat thus plays a critical role in connective tissue development around the anal canal. The dorsally extending smooth muscle seems to be maintained as smooth muscle in the superior part of the adult anococcygeal ligament. The EAS also gave off striated muscle into the fetal dorsal band-like tissue - we paid attention to the fact that fetal striated muscle morphology resembled dorsosuperior reflection of the EAS in adults. The parts of the fetal striated muscle derived from the EAS seem to be retained in the adult subcutaneous fibrous tissue, i.e., our identified anococcygeal raphe. We found a specific subcutaneous tissue mass on the dorsal side of the coccyx at 20 weeks. However, neither the levator slings nor EMS was found near the coccyx, consistent with the findings of Niikura, et al.15
Thus, depending on later development of these striated muscles, our identified raphe seemed to develop as a raphe-like structure in the final fetal stage or after birth under the influence of muscle functions.
described the anococcygeal ligament as inserting on the dorsal side of the dorsal end of the levator ani. Between "his ligament" and the external pelvic fascia (superficial fascia), he found a fatty tissue termed Courtney's space. This ligament thus seems to correspond to our identified raphe. Courtney23
was a rectal surgeon who described the raphe clearly in his line-drawings. Notably, the subcutaneously located raphe received the most dorsal muscle fibers of the EAS. This dorsal part of the EAS, in the well-developed cases as shown in , provides a superior reflection toward the coccyx.24
Thus, rather than functioning as a raphe between bilateral levator ani slings, we speculate that this part plays a critical role in coordinating between the contraction and superior shift of the EAS for smooth defecation. Shafik25,26
considered that the width of the raphe changes depending on anal sphincter function. We do not deny his hypothesis because in the elderly individuals bilateral levator slings were not tightly connected.
Rectal surgeons are familiar with Waldeyer's description that, at the level of anorectal junction or just above the levator ani sling, a fascia connects the rectum and presacral parietal fascia (reviewed by García-Armengol, et al.27
). A similar concept is also found for the so-called rectococcygeus muscle.28
However, we suspect that this concept likely represents a bias toward the notion that the coccyx should also connect with the anorectum (rather than the EAS) by a connective tissue structure. The coccyx is connected with the EAS by the anococcygeal ligament, and also by the presently described subcutaneous structure, i.e., our identified raphe. In addition, the term "anococcygeus muscle" may be based on bias from the comparative anatomy of other mammals,29
although striated muscle fibers were contained in both the anococcygeal ligament and our identified raphe.
A major limitation of this study was the small sample size, particularly for elderly specimens because of the possible variations in degeneration of the pelvic floor. The sample size may have been too small to allow for suitable insights into left/right differences or sex differences. Whether the levator ani attaches to the raphe seems to be one of the major reasons for evaluating raphe function. In fetal and elderly specimens, the levator did not often attach to our identified raphe.