title: Dynamic intersection of the longitudinal muscle and external anal sphincter in the layered structure of the anal canal posterior wall.
abstract: OBJECTIVE
  The minute details of the structure of the anal canal are still not well understood. The complex structural configuration of the muscles, ligaments and raphes remains unclarified. This study was undertaken to determine the precise structure of the posterior part of the anal canal and to facilitate an understanding of previous studies.
  
  
  METHODS
  For macroscopic examination, 14 right pelvic halves from 14 Japanese cadavers were used. Observation and dissection were performed from the median plane. In the histological examination, six left pelvic halves were used. The sections of the posterior parts of the anal canal were stained with hematoxylin and eosin, Elastic van Gieson, anti-smooth actin antibody and anti-skeletal myosin antibody.
  
  
  RESULTS
  We identified the following muscles arranged from the internal side to the external side: internal anal sphincter, longitudinal muscle (LM), external anal sphincter (EAS) and levator ani muscle (LAM). Two different types of conformation of the posterior part of the anal canal were found, each bearing a different shape of EAS. In both types, LM penetrated the inferior part of EAS. After penetrating EAS, some fibers of LM ran posterosuperiorly and attached to the "the posterior fibers" which reach the dorsal side of the coccyx.
  
  
  CONCLUSIONS
  We defined and labeled the connective tissues between the anal canal and coccyx on the basis of their relative position to LAM. Based on a comparison of the two types of the posterior part of the anal canal, we propose that there are two phases due to constriction and relaxation of LM.
authors.affiliation: Unit of Clinical Anatomy, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, 1-5-45 Yushima, Tokyo, Bunkyo-ku, 113-8519, Japan.
authors.forename: Satoru
authors.initials: S
authors.lastname: Muro
authors.forename: Kumiko
authors.initials: K
authors.lastname: Yamaguchi
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Nakajima
authors.forename: Kentaro
authors.initials: K
authors.lastname: Watanabe
authors.forename: Masayo
authors.initials: M
authors.lastname: Harada
authors.forename: Akimoto
authors.initials: A
authors.lastname: Nimura
authors.forename: Keiichi
authors.initials: K
authors.lastname: Akita
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001003:Anal Canal / Q000033:anatomy & histology*; D002102:Cadaver; D004210:Dissection; D005260:Female; D006801:Humans; D007564:Japan; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000033:anatomy & histology*; D010388:Pelvis / Q000033:anatomy & histology*