The anus guards the outlet of the bowel by its double sphincter muscle, which under normal circumstances affords voluntary control, within certain limits, over defecation. The well-known peculiarity of the vascular supply, a sort of erectile tissue being formed by the inferior hemorrhoidal plexus and the passage of some of the efferent veins through the sphincter muscle, by which they are subjected to pressure, is very favorable to the development of certain forms of disease which will be considered among the local disorders. As embryology has thrown considerable light upon the pathology of morbid growths by demonstrating relationships that were previously unsuspected, so a consideration of the development of the lower portion of the intestinal canal may lead to a better understanding of some of its diseases, especially those which are symptomatic or secondary. In early foetal life the third division of the primitive intestine, the pelvic portion, terminates in a cloaca in common with the urachus; subsequently, about the eighth week, a partition (the perineum) is formed which divides the cavity into two portions, the uro-genital sinus and the anal cavity. In the mean time, at an early period a depression occurs on the cutaneous surface at the site of the anus, which deepens progressively until it encounters the primitive intestine, with which it unites at the end of the fourth week, and the continuity of the tube becomes established. It therefore is seen that the rectum in its upper and middle portions is derived from the internal and middle layers of the blastodermic membrane, while its lower third, with the anus, like the buccal cavity, is formed by the external and middle layers.
In its diseases, then, the greater part of the rectum would seem to naturally participate in those of the large intestine, to which it structurally belongs, while its inferior portion and the anus would partake more in the disorders of the general cutaneous system. This peculiarity of development also explains the difference noticed in the vascular supply. The rectal veins are usually divided, like the rectal arteries, into three sets—superior, middle, and inferior. They are arranged so as to form two distinct venous systems, the rectal returning its blood through the inferior mesenteric veins into the portal system, the anal terminating in the internal iliac. The first system is made up of the superior hemorrhoidal, the second of the remaining veins.
The superior hemorrhoidal forms a venous plexus which surrounds the internal sphincter muscle; the inferior hemorrhoidal vein also forms a plexus, but it is subcutaneous and principally below the inferior border of the external sphincter.
There are, however, a number of communicating branches passing along the walls of the rectum from one plexus to the other. The internal hemorrhoidal veins also communicate freely with the branches of the internal iliac around the trigone of the urinary bladder by means of small vessels, which pass through the prostate gland and seminal vesicles. By this method of anastomosis some relief is afforded when there is an obstruction in the portal circulation, which is such a common cause of turgescence of these veins, often resulting in permanent dilatation or hemorrhoids.
At the lower part, or at the junction of the middle and lower third of the rectum, the internal circular fibres of the muscular coat of the intestine become quite numerous, forming what is called the internal sphincter muscle; it is nearly an inch in breadth, and completely surrounds the lowest part of the rectum. It is about an inch above the margin of the anus; its muscular fibres are of the involuntary or unstriped variety; in function it assists the external sphincter in closing the anus and preventing the involuntary escape of the contents of the bowel.
The external sphincter lies directly under the skin and upon the internal sphincter and the levator ani muscle; its fibres encircle the anus: arising from the coccyx, they are inserted into the tendinous centre of the perineum, joining the transversus perinæi, the levator ani, and accelerator urinæ muscles. The sphincter ani is constantly in a state of tonic contraction, but the force of its contraction may be voluntarily increased. In the skin and superficial fascia are found minute branches of the pudic and small sciatic nerves; in the ischio-rectal space the internal pudic nerve; crossing about the centre are the inferior hemorrhoidal nerves, which are distributed to the anus and the lower portion of the rectum; the perineal nerve is especially distributed to the anterior part of the anus.
Thus it is seen that the rectum and anus have vascular and nervous supplies of considerable diversity and importance.
Congenital Malformations.
The simplest form of congenital malformation in this region consists in an anus of insufficient size for the natural demands of the system, but in no other manner abnormal. The most frequent variety of imperforate anus is where complete occlusion is effected by the common integument or by two cutaneo-mucous flaps, which owing to defective development remain united without forming a raphé or perceptible line of union. The rectum is not involved, and when the child strains the contained meconium causes bulging of the part, which disappears under slight pressure, but reappears when again free. In other cases the occluding tissue is very firm, dense, with a disposition to pucker or form rugæ. The sphincter muscle is rarely perfect, and though an artificial anus may be made, years may elapse before the child can control the evacuation. In conjunction with an imperforate anus the colon may terminate in a cul-de-sac, or it may communicate with the urethra, the bladder, or the vagina.