This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
If the buttocks are drawn aside the mucous membrane of the anus is everted and a considerable portion of the anal canal becomes visible. The lower part of the columns and crypts of Morgagni and the anal valves are seen. If the patient strains or bears down, the mucous membrane of the anal canal is brought into view in almost its entire length. One is thus enabled to see dilated veins or hemorrhoids, ulcers, fissures, foreign growths, both benign and malignant, and the openings of fistulae. By means of a speculum the entire anal canal can be seen. It should be introduced pointing obliquely anteriorly, and if it is desired to view the interior of the rectum above after it has passed the internal sphincter it is to be directed obliquely upward and backward. In digital examination the first resistance encountered is that of the external sphincter; as its edge is passed a sulcus can often be felt, immediately following which the internal sphincter is passed and the finger enters the rectum. The sulcus is about opposite the crypts of Morgagni and is formed by the interval between the contraction of the external sphincter below and the internal sphincter blended with the insertion of the levator ani above. It is just above Hilton's white line.
In an early stage of the development of the embryo the cloaca is the common termination of the genito-urinary system and the intestinal canal. Later the cloaca becomes divided by a septum into the urogenital sinus in front and the rectum behind. A depression in the skin called the anal pit appears opposite the rectum and the membrane between disappears in the fourth month. This membrane is produced by the growing together of the ectoderm and entoderm, the mesoderm being pushed aside. The failure of this membrane to perforate forms imperforate anus. The method of development explains the various malformations of these parts. The anal pit may be absent; the membrane may not perforate; the rectum may end in a blind pouch some distance up from the anus; or it may discharge through a sinus into the bladder or vagina.
Hemorrhoids or piles are varicosities or dilatations of the veins of the anus or anal canal. The middle hemorrhoidal veins are not enlarged because they do not drain the mucous membrane, they are not inside but outside the rectum. When the inferior hemorrhoidal veins are dilated they form external hemorrhoids and are situated at the margin of the anus below the white line and external sphincter, and they cannot be replaced in the rectum. When the superior hemorrhoidal veins are dilated they form internal piles (Fig. 448). The dilatation involves the superior hemorrhoidal veins from the beginning of the mucous membrane at the white line up the entire length of the anal canal and sometimes a short distance up the rectum. There is a natural tendency for external piles to be covered almost wholly by skin and for internal piles to be covered solely by mucous membrane. Inflamed internal piles can be pushed back in the rectum. If an internal pile is continued down over the white line or an external pile is continued up over the white line then they are called intero-external piles. Hemorrhoids consist almost wholly of dilated venous sinuses. The existence of arterial hemorrhoids is now denied although small arterial branches are sometimes encountered in the ordinary venous pile. The strawberry pile is composed of venous capillaries instead of the larger venous canals usually present. They bleed more freely than does the ordinary venous pile. When external hemorrhoids are operated on they are usually thrombosed. They are then incised and the clots turned out; at other times when not inflamed they are excised and the edges stitched with catgut or the wound packed. Internal piles are either ligated or treated with the clamp and cautery. In applying the ligature the base of the pile is loosened below near the white line and detached for some distance above and then ligated. This is facilitated by the loose attachment of the mucous membrane. Bleeding is not marked because the blood-vessels enter the pile from above. In Whitehead's operation, or excision of the pile-bearing area, the mucous membrane is readily separated by blunt dissection from the parts beneath owing to the laxity of the submucous tissue; it is then excised and the cut edge sewn to the skin at the anus.