The Vermiform Appendix in the adult is situated on the inner and posterior aspect of the caecum, and is normally about 3½ inches long, and about ½ inch in diameter. Its position is very variable, even when the caecum retains its normal situation, and it also varies greatly in length. It may lie curled up behind the caecum and ileum, or its extremity may extend over the brim into the pelvis, or even be found on the left side of the abdomen, or up under the liver. The position of its orifice on the posterior and inner caecal wall is about 1 inch below the ileo-caecal valve, and generally presents a valved aperture. It is enveloped in peritoneum, its mesentery, which is derived from that supplying the lower end of the ileum, extending generally to the tip, although the terminal position may be represented by a slight ridge. Shortness of this mesentery may cause ņends of the appendix.

The peritoneum in this neighbourhood presents several small fossae, which are possibly of importance in relation to herniae and appendicitis. One of these, the ileo-ceecal fossa, whose orifice is directed away from the caecum, is situated below the ileum, in the angle between it and the caecum. It is bounded behind by the meso appendix, and in front by a fold of peritoneum (the ileo-caecal fold), which runs from the ileum down to join the front of the mesoappendix. The ileo-colic fossa, similar but smaller, is situated on the upper surface of the ileum, being bounded behind by the mesentery of the ileum, and in front by a small fold of peritoneum (the ileo-colic fold). Retro-colic fossee are, occasionally, present, and are seen by turning the caecum upwards. They lie behind the beginning of the colon on either its outer or inner margin, or both. When they are present, the appendix is said frequently to lodge in them, and so be more prone to appendicitis, presumably from becoming caught.

In structure the appendix closely resembles the large intestine. The lymphatic follicles are most prominent in younger subjects, and occur particularly at the distal portion of the appendix.

The ileo-colic division of the superior mesenteric artery branches to supply the lower end of the ileum, caecum, appendix, and portion of the ascending colon. The branch which supplies the anterior surface of the caecum runs down in the ileo-colic fold, and gives off the appendicular artery, which runs behind the ileum to reach the mesoappendix. It in turn gives off a recurrent branch, which runs in the ileo-caecal fold to reach the ileum, and then runs forward supplying the appendix to its tip by means of numerous branches. The veins are similar, and empty into the superior mesenteric.

The nerves of the caecum and appendix are derived from the superior mesenteric plexus, and the lymphatics pass to the mesocolic glands, lying behind the ascending colon. The common nerve-supply of ileum and appendix should be noted. Thus in operative cases it has been seen that, when the lower portion of the ileum is stimulated, the secretion of the appendix comes in little jets, and, as the ileo-caecal valve directs the intestinal contents, as they enter the caecum, over the mouth of the appendix, they are thoroughly mixed with this secretion. While this secretion may have a purely digestive function, it has been suggested that the appendix may also exist as a culture chamber for the Bacillus coli, and that it impregnates the contents with the bacilli at this point (Macewen). Certainly a number of obscure digestive troubles have been traced to affections of the appendix, and it is well known that obstinate constipation frequently follows the removal of the appendix. It has further been noted in actual cases that mental impressions have a marked effect upon the secretion of appendix and caecum, bad news producing an entire cessation of secretion for some hours. Thus the mental condition as a cause of indigestion cannot be disregarded. In cases of mucous colitis it is frequently of advantage to be able to wash out the colon, and this is readily done by bringing out the appendix through the abdominal wall, cutting off its tip after it has contracted adhesions, and then irrigating by means of a narrow tube passed down through the appendix (Macewen). In such cases it is possible to observe the appendix, and it is worthy of note that, save in cases of severe purgation, faecal matter rarely finds its way into the appendix. Even in such purgation only a slight brown stain generally results. Foreign bodies, such as pins and orange-pips, do occasionally find their way into the appendix, but generally such bodies are composed of gradually accumulated faecal matter, and are termed coproliths.

The appendix is a frequent seat of inflammatory and suppurative mischief, which sometimes subsides after the pus has become discharged into the bowel, or may proceed toulceration, or gangrene, with localized or generalized peritonitis. As already-explained, the omentum and various bowel loops frequently take part in walling off an abscess arising in connection with the appendix, and thus preventing general peritonitis. Such appendicular abscesses may be found in almost any portion of the abdomen, and are frequently met with in the pelvis, and less commonly up under the liver, or diaphragm, or on the left side. A subphrenic abscess may arise from lymphatic extension from the appendix. When the appendix is inflamed, it may become attached to the ovary, and frequently it is difficult to discriminate between the two organs as the seat of pain. From the involvent of the psoas muscle and anterior crural nerve the thigh is frequently flexed, and pain is referred to the inside of the knee, thus simulating hip disease. Again, when the abscess is up under the liver, the condition may be diagnosed as one arising in connection with liver or kidney, etc.

Why the appendix should thus be affected by inflammatory mischief is not easy to explain. Its blind extremity, narrow lumen, power of movement, unequal length and position of its mesentery, and abundant supply of Bacillus coli in its interior, may all be predisposing causes. In operations for appendicitis an incision is generally made with its centre over McBurneys point, situated 2 inches from the anterior superior spine on a line from the spine to the umbilicus. The incision runs obliquely downwards and inwards, and the abdominal muscles, where possible, are split, rather than cut, in the direction of their fibres. Where a circumscribed abscess exists, an incision farther out is generally better, it being frequently possible to evacuate the abscess without opening the general peritoneal cavity. The scar of a wound which has healed by connective tissue is liable to stretch and cause a ventral hernia. Such herniae occur probably most often through appendix scars, where, owing to suppuration, the wound has been kept open and allowed to granulate.

The normal position of the ileo-caecal valve is indicated on the surface by a point slightly above that midway between the anterior superior spine and umbilicus. It is situated somewhat posteriorly, at the junction of caecum and colon, and consists of a narrow opening about ½ inch long, the long axis lying horizontally, and the aperture, which is bounded by upper and lower crescentic margins, looking forwards and to the right. At the junction of the upper and lower margins at each end of the valve, a ridge, or frenula, extends round the caecum. While the peritoneal and longitudinal muscular coats of the bowel pass sharply from the ileum to the colon, the mucous and part of the circular muscular coats are in-vaginated at the valve into the caecum to form the above-mentioned crescentic margins, a markedly pouting mouth being produced, which directs the bowel contents over the orifice of the appendix as they enter the caecum. This mouth is also supposed to act in preventing regurgitation of food into the ileum, since, as the caecum distends and the frenula becomes tight, the valve is mechanically closed for the time being. It is at this valve that intussusception most frequently occurs, the narrower ileum being, as a rule, projected into the wider colon, carrying the valve inwards as its apex (ileo-caecal). To such an extent may the intussusception go that the valve may ultimately appear at the anus. Occasionally the valve retains its position, and the ileum alone forms the intussusception (ileo-colic) ; this, however, is not common.