The caecum is the blind pouch of the large intestine which extends beyond the opening of the ileum. It is about 7.5 cm. (3 in.) broad and 6.25 cm. (2 1/2 in.) long. Its three longitudinal bands converge to the appendix and are continued over it. It lies in the right iliac fossa on the iliacus and psoas muscles, more on the latter, and reaches nearly or quite to its inner edge. It is in contact with the abdominal wall above the outer half of the inguinal (Poupart's) ligament. In fetal life the caecum is cone-shaped and passes gradually and regularly into the appendix. It increases in size more rapidly on its outer side, so that the appendix, which was before opposite the long axis of the gut, becomes placed to the inside just below the ileocaecal valve.

Four varieties of caecum are given by Treves: (1) the conical or fetal type, (2) a globular or quadrilateral type, in which the development of both sides is even, (3) the adult type, in which the outer side is much larger than the inner, (4) an irregular type, in which there is an excess of development of the outer side and an atrophy of the inner side resulting in placing the root of the appendix close to the lower and posterior portion of the ileocaecal junction.

Cunningham makes three varieties: (1) a fetal conical type, (2) an infantile type, in which the outer side is somewhat larger than the inner, and (3) an adult type, 93 or 94 per cent., in which the outer side is much the larger, and the root of the appendix is on the inner wall just below the ileocaecal valve (in the adult about 2 cm. - 3/4 in.).

The ileocaecal valve marks the emptying of the ileum into the large intestine. On the surface of the body it corresponds to a point 2.5 cm. (1 in.) below the middle of a line joining the anterior superior spine and the umbilicus and the same distance above the middle of a transverse line drawn from the anterior superior spine to the median line. This point is about on the linea semilunaris and directly above the point where the external iliac artery passes under Poupart's ligament. Normally the ileocaecal valve will allow of the passage of gas from the colon into the ileum, as in Senn's hydrogen test for perforation, but not of liquids or solids.

Vermiform Appendix (Processus Vermiformis)

The appendix varies much, both in length and diameter. In health its average length may be given as from 8.75 cm. (3 1/2 in.) to 10 cm. (4 in.) and its diameter as 6 mm. (1/4 in.). It is pale in color and soft in consistence, with its blood-vessels barely visible. In disease it becomes hard and red and the injected vessels are distinctly seen. It becomes much increased in diameter, equalling in size a finger or thumb, and lengthens to 15 cm. (6 in.) or even more. It possesses a serous peritoneal coat, a longitudinal muscular, a circular muscular, a submucous and a mucous coat. The lumen of the appendix has been found to be partially occluded in at least one-fourth of all adults. This occlusion occurring toward its distal extremity is not regarded as pathological, but constrictions occurring elsewhere in the length of the tube are probably the result of previous disease.

The opening of the appendix in the caecum is about 2.5 cm. (1 in.) below and a little behind the ileocaecal opening. The fold of mucous membrane guarding it has been named the valve of Gerlach but it is not generally regarded as a distinct valve. The root of the appendix is only about 2 cm. (3/4 in.) below the lower edge of the ileum and is often even closer on account of the lower surface of the ilium being in contact with the caecum at that point. It corresponds to a point on the surface of the body where the right semilunar line crosses a line joining the two anterior superior spines (Fig. 423).

The meso-appendix (mesenteriolum) comes off the lower surface of the mesentery. It is shorter than the appendix, hence the twisting and curling of the latter. It usually, but not always, extends to the tip and contains toward its left or free border the appendicular artery.

The ileocolic artery, from the superior mesenteric, as it approaches the ileocaecal junction divides into five branches: (1) the colic, distributed to the colon; (2) the ileal, to the upper surface of the ileum; (3) the anterior ileocaecal branch, to the front of the caecum, passing through the ileocolic fold; (4) the posterior ileocaecal artery, to the posterior part of the caecum; (5) the appendicular artery. The appendicular artery descends behind the ileum to enter the meso-appendix and, after sending one recurrent branch to the root of the appendix and another to the ileocaecal fold, passes along the left or free edge of the meso-appendix, and, if this is short, it may be continued on the surface of the appendix to its extremity (Fig. 424).

In removing the appendix this artery requires ligation and if the ligature is not placed close to the root the recurrent branches will not be included and may cause dangerous bleeding.

Fig. 423.   The relations of the appendix. The ileocaecal junction is seen to be about one inch below the middle of a line joining the anterior superior spine and umbilicus or where this line crosses the linea semilunaris. The base of the appendix is under the point of crossing of the linea semilunaris and the middle of a horizontal line running from the anterior superior spine to the mid line of the body; it is one inch below the ileocaecal junction.

Fig. 423. - The relations of the appendix. The ileocaecal junction is seen to be about one inch below the middle of a line joining the anterior superior spine and umbilicus or where this line crosses the linea semilunaris. The base of the appendix is under the point of crossing of the linea semilunaris and the middle of a horizontal line running from the anterior superior spine to the mid-line of the body; it is one inch below the ileocaecal junction.

The veins of the appendix and the caecum end in the ileocolic vein, which joins the superior mesenteric vein and helps to form the portal vein. Hence infection is carried by the blood stream from the appendix and caecum directly to the liver.