Carlo Ratto, Angelo Parello, Lorenza Donisi and Francesco Litta (eds.)Colon, Rectum and Anus: Anatomic, Physiologic and Diagnostic Bases for Disease ManagementColoproctology110.1007/978-3-319-09807-4_6

4. Topographic Anatomy in Proctologic Surgery

Ezio Falletto¹

(1)

Division of General and Mini Invasive Surgery, Department of Surgery, San Giovanni Battista Hospital, Città della Scienza e della Salute, Turin, Italy

Ezio Falletto

Email: ezio.falletto@tin.it

Abstract

The pelvic floor’s shape is usually described as a boat’s hull, but the most similar comparison is to a funnel with a wide large upper part (the pelvic floor in the strictest sense of the word) and two lower bottlenecks: the urethral and anal sphincters.

There is no considerable muscle in the body whose form and function are more difficult to understand than those of the levator ani, and about which such nebulous impressions prevail. (Dickinson R: Studies of the levator ani muscle 1889)

1 Introduction

The topographic anatomy of the human perineum is defined by four palpable points of bony pelvis: the symphysis pubis anteriorly, the left and right ischial tuberosities laterally, and the coccyx posteriorly.

The transverse perineal muscles (left and right) set in the middle of the perineum and with a horizontal transverse position divide the perineum in an anterior (with urological and genital apparatus) and posterior (with anal canal) region. Just anteriorly to the transverse perineal muscle on the midline, the central point of the perineum or perineal body is present.

2 The Anus: Macroscopic Aspects

The internal surface of anal canal is characterized by the pectinate or dentate line (from his peculiar shape) at about 2 cm from anal verge. It divides the anal canal in an upper part lined by mucosa and a lower part with the skin. At this level, the embryological junction between postallantoic gut (from endodermic sheet) and the proctodeum (from ectodermic sheet) is placed. At the dentate line, there are little pockets known as crypts of Morgagni in which ducts of anal glands flow out. The internal opening of anal fistula is usually considered to be set in this place.

Above the dentate line, mucosa is thrown in longitudinal folds called Morgagni’s columns . Under the mucosa, the internal hemorrhoidal venous reticulum is often visible. Theoretically in this zone, sensitive touch receptors are missing, so it can be considered an insensitive epithelium to cutting or other trauma (patient feels pain if the rectal wall is dilated, for the presence of distension receptors).

Near 0.5–1 cm below the dentate line, the anocutaneous line could be found. The lining between these two lines is called anal pecten or transitional zone, and it is formed by bright modified keratinized skin but without hair or sebaceous glands and closely adherent to underlying tissues.

Below anocutaneous line, a normal, hyperpigmented skin with hair follicles and glands is present.

3 Anal Sphincters

Anal sphincter system is a double cylindrical muscular structure shaped by an internal cylinder (the internal sphincter) and the external sphincter around it.

The Internal Anal Sphincter: The more internal cylinder of sphincter apparatus is a thickening of the last part of circular muscle coat of the distal rectum. For this reason, it is difficult to measure a precise length, but usually it is defined from 1.5 to 2 cm of length with many sex- and constitution-related differences.

It is composed by smooth muscular involuntary fibers, and it is mainly responsible of anal resting pressure.

The External Anal Sphincter: It is usually considered formed by three bundles of striated voluntary muscle fibers, and it is responsible of anal voluntary contraction. The superficial bundle is subcutaneous, wrapping the internal sphincter, and it is the first muscular structure we run into.

The superior bundle can be divided by puborectalis muscle fibers with difficulty.

Sphincter fibers have a strong link with the central perineal tendon (or body) anteriorly and with the anococcygeal ligament posteriorly.

Its length is 2.5–3.5 cm with many sex- and constitution-related differences. In particular in women it is usually shorter, especially anteriorly.

Intersphincteric Longitudinal Muscle Fibers: They lie between internal and external sphincters. They consist of nonstriated muscle fibers mixed with elastic tissue, and they are the distal continuation of longitudinal muscle layer of the rectal wall. Some of these fibers go through the internal sphincter, and inferiorly they pass over the end of the internal sphincter and are attached to the anal skin forming the anocutaneous line that represents the intersphincteric groove, the macroscopic endoanal border between the internal sphincter and external sphincter.
Musculus Submucosa Ani or Parks’ Ligament: Smooth muscle fibers and elastic connective tissue lying above the internal sphincter and under mucosa and anal skin. It has connections with intersphincteric longitudinal muscle fibers, the superficial part of the external sphincter and anal skin, also called corrugator cutis ani . It was considered an important pathogenic factor (if damaged) in the onset of hemorrhoidal prolapse.

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