These muscles—the two sphincters—are separated on the outer side by the attachment of the levator ani, some of the fibers of which are internally connected with the external sphincter; on the inner side the muscles are in contact, the line of union corresponding accurately with the junction of the skin and the mucous membrane. In most cases this junction of the sphincters is marked by a line of condensed connective tissue.[[4]] This line is known as "Hilton's white line."
Hilton has pointed out an important anatomical fact in connection with this line—to wit, that it is the point of exit of the nerves, principally branches of the pudic, which descend between the two sphincter muscles, becoming superficial in this situation, and are there distributed to the papillæ and mucous membrane of the anus (Fig. 1).
Fig. 1—Nervous Supply of the Anus (Hilton). a, mucous membrane of the rectum; b, skin near the anus; c, external sphincter muscle; d, internal sphincter muscle; e, line of separation of the two sphincters; f, the overlying white line marking the junction of the two sphincters; g, nerve supplying the skin near the anus, which it reaches by passing first externally to the rectum and then through the interval between the two sphincters; h, flap of mucous membrane and skin reflected back.
These nerves are very numerous, which accounts not only for the extreme sensitiveness of the part, but also, as stated by Andrews,[[5]] for its very abundant reflex communications with other organs. They play a very important part in the etiology of irritable ulcers. The exposure of one of their filaments, either in the floor or at the edge of the ulcer, is an essential condition of its existence.[[6]]
The upper portion of the rectum possesses very little sensibility, as the chief nerve-supply of the organ is at its termination and around the anus; hence it is that such grave diseases as cancer or ulceration may exist in the higher parts of the bowel and not manifest their presence by pain.
Fig. 2—Diagram of the Nervous Relations of Irritable Ulcer of the Anus (Hilton). a, ulcer on sphincter ani; b, filaments of two nerves are exposed on the ulcer, the one a nerve of sensation, the other of motion, both attached to the spinal cord, thus constituting an excito-motor apparatus; c, levator ani muscle; d, transversus perinæi muscle.
Andrews[[7]] directs attention to Hilton's diagram (Fig. 2), as showing that impressions from a fissure are carried to that part of the cord which supplies the pudic nerves and the ilio-lumbar, lumbar, and sciatics, which include the motor supply of the external sphincters as well as of the bladder and the lower extremities.
From these general considerations we can understand why reflex spasm of the sphincter is so constant and important a sign of this malady, and how other and more general reflexes are to be accounted for,—such as symptoms of bladder and urethral diseases, radiating pains, etc.