Anorectal ring is formed by the following muscles except:
Correct Answer: Superficial part of external sphincter
Description: Ans. B. Superficial part of external sphincter. (Ref BDC 4th/Vol2Jpg. 383; Bailey and Love 25th/pg. 1241).Bailey and Love 25th/pg. 1241........."Anorectal ring marks the junction between the rectum and the anal canal. It is formed by the joining of the puborectalis muscle, the deep external sphincter, conjoined longitudinal muscle and the highest part of the internal sphincter. The anorectal ring can be clearly felt digitally, especially on its posterior and lateral aspects. Division of the anorectal ring results in permanent incontinence of faeces. The position and length of the anal canal, as well as the angle of the anorectal junction, depend mainly on the integrity and strength of the puborectalis muscle sling".Anal canal musculature# The internal sphincter is a thickened continuation of the circular muscle coat of the rectum. This involuntary muscle commences where the rectum passes through the pelvic diaphragm and ends at the anal orifice, where its lower border can be felt. The internal anal sphincter is 2.5 cm long and 2-5 mm thick. Spasm and contracture of this muscle play a major part in fissure and other anal affections.# The longitudinal muscle is a continuation of the longitudinal muscle coat of the rectum intermingled with fibres from the puborectalis. The longitudinal muscle fibres that are attached to the epithelium provide pathways for the spread of perianal infections, and mark out tight compartments that are responsible for the intense pressure and pain that accompany many localised perianal lesions.# Beneath the anal skin lie the scanty fibres of the corrugator cutis ani muscle.# The external sphincter, formerly subdivided into a deep, superficial and subcutaneous portion is now considered to be one muscle. Some of its fibres are attached posteriorly to the coccyx, while anteriorly they are inserted into the midperineal point in the male, whereas in the female they fuse with the sphincter vaginae. In life the external sphincter is pink in colour and homogeneous. Unlike the pale internal sphincter muscle, which is involuntary, the red external sphincter is composed of voluntary (somatic) muscle.# The puborectalis plays a key role in maintaining the angle between the anal canal and rectum and, hence, is essential for the preservation of continence. There is a close association between the puborectalis portion of the levator ani and the external sphincter muscle.# The mucous membrane. The pinkcolumnar epithelium lining the rectum extends through the anorectal ring into the surgical anal canal. Passing downwards where it clothes the series of eight to 12 longitudinal folds known as the columns of Morgagni, the mucous membrane becomes cubical.# Just below the level of the anal valves there is an abrupt, albeit wavy, transition to squamous epithelium, which is parchment colour. This junction constitutes the dentate line. The squamous epithelium lining the lower anal canal is thin and shiny, and is known as the anoderm. At the dentate line the anoderm is attached very firmly to deeper structures. This line is a most vital landmark both morphologically and surgically. It represents the site of fbsion of the proctodaeum and post-allantoic gut, and the position of the anal membrane, remnants of which are frequently seen as anal papillae at the free margin of the anal valves.# The dentate line separates:abovebelow# cubical epithelium;# autonomic nerves (insensitive);# portal venous system;# from squamous epithelium;# from spinal nerves (very sensitive);# from systemic venous system.# Anal valves of Ball are a series of transversely placed semilunar folds linking the columns of Morgagni. They lie along and actually constitute the waviness of the dentate line. They are functionless remnants of the fusion of the post- allantoic gut with the proctodaeum.# Crypts of Morgagni (anal crypts) are small pockets between inferior extremities of the columns of Morgagni.# Anorectal ring marks the junction between the rectum and the anal canal. It is formed by the joining of the puborectalis muscle, the deep external sphincter, conjoined longitudinal muscle and the highest part of the internal sphincter. The position and length of the anal canal, as well as the angle of the anorectal junction, depend to a major extent on the integrity and strength of the puborectalis sling.Educational points:Anal canal anatomy# The internal sphincter is composed of circular, non-striated involuntary muscle supplied by autonomic nerves# The external sphincter is composed of striated voluntary muscle supplied by the pudendal nerve# Extensions from the longitudinal muscle layer support the sphincter complex# The space between sphincters is known as the intersphincteric plane# The superior part of the external sphincter fuses with the puborectalis muscle, which is essential for maintaining the anorectal angle, necessary for continence# The lower part of the anal canal is lined by sensitive squamous epithelium# Blood supply to the anal canal is via superior, middle and inferior rectal vessels# Lymphatic drainage of the lower half of the ana! canal goes to inguinal lymph nodes# Blood supply o f anal canal:Superior, middle and inferior haemorrhoidal arteries. The most important is the superior haemorrhoidal. The superior and middle haemorrhoidal veins drain via the inferior mesenteric vein into the portal system, having become the superior rectal vein en route. The superior haemorrhoidal vein drains the upper half of the anal canal. The inferior haemorrhoidal veins drain the lower half of the anal canal and the subcutaneous perianal plexus of veins: they eventually join the external iliac vein on each side.# Lxmphatic drainage o f anal canal:Upper half of the anal canal drain upwards into the post-rectal lymph nodes and from there drains to the para-aortic nodes via the inferior mesenteric chain. Lymph from the lower half of the anal canal drains on each side first into the superficial and then into the deep inguinal group of lymph glands. However, if the normal flow is blocked, e.g. by tumour, the lymph can be diverted into the alternative route.
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