The mesoappendix is being defined in this article.It is a double layer of peritoneum, similar to, but on a smaller scale than the mesentery, from the under or left layer of which it is derived. It is either triangular or quadrangular in outline; when the former, its free edge may be considered to form the W of the triangle, while its apex is at the root of the appendix, and the two sides are formed, one by the appendicular attachment, and the other by its origin from the mesentery.
The base, or free edge of the meso-appendix nearly always forms an acute angle with the attachment of the mescntcriolum to the appendix; and in some cases is continued even to the tip of the appendix as an exceedingly narrow fringe, almost invisible to the naked eye. When quadrangular in outline, the fourth side, at the caecum, is usually the shortest of all. The meso appendix usually appears to be too short for the appendix, thus twisting, curving or coiling it as the mesentery does the small intestine throughout its length.
The form of the proximal portion of the meso-appendix varies slightly according to the type of accum: where this is of the first or second type, the proximal part of the meso-appendix is continued as the mesocosm, the left layer of which is continuous above with the under layer of the mesentery of the ileum, and below with the left layer of the meso-appendix; the right layer being continuous below with the corresponding layer of the meso-appendix, and above forming the right layer of the ascending mesocolon.
The upper portion of the posterior surface of the caecum is usually left bare of peritoneum by the divergence of the two layers of the meso-caeum; where this is not the case, abnormal mobility of the caput coli and appendix ensues, as was described above.
In the third type, or in types which are intermediary between the third and the fourth, the meso-caecum Appears at first sight to be absent; search, however, reveals it, though shortened, still formed by the diverging layers of the proximal portion of the meso-appendix The more nearly the ctecum approaches the fourth type, the less distinct becomes the meso caxum, since in this type the meso-appendix arises entirely from the under layer of the mesentery, and the proximal part of its right layer is continuous with the serous coat of the caecum, and with the peritoneal lining of the iliac fossa ; the junction of the last named two portions of peritoneum forming the outer layer of the meso-caecum, while its inner layer is so short as to be practically non-existent.
As the form of caecum approaches the first typo, the freedom of motion of the appendix increases, so that at operations for their excision, appendices of this type are more easily brought through the abdominal wound than are those of other types, since a longer meso-cacum is present. Perforation occurring, as it occasionally may, at the small triangular area nrar the base of the appendix, above described as uncovered by jieritoneum; or in the line of attachment of the meso-appendix, would open into the interval between the two layers of the meso-appendix. As a consequencc of such perforation, pus may pas* into the mesentery and thence to the snbperitoncal areolar tis*ue of the iliac fossa; or, at the proximal portion of the meso-appendix, the pus may enter the post-aecal areolar tissue, and thence gravitate to the iliac fossa; or, rarely, it may burrow upward behind the colon and simulate perinephric abscess.
Between the layers of the meso-appendix are found the arteries, veins, nerves, and lymphatics for the appendix, together with areolar tissue and some fat. In some instances the iliac vessels pass through the meso-appendix, thus accounting for one manner in which collections of pus in the right iliac fossa may find their way beneath Poupart’s ligament into the thigh. In the female the meso-appendix sometimes has running to the ovary a prolongation, which is called by Clado the appendicular-ovarian ligament. It conveys an additional blood supply to the appendix, and will be described in more detail later in connection with that subject. I have myself never met with this structure, and its presence is denied by some excellent authorities.
The mesoappendix acts also as an ap[articular ligament, the mobility of the appendix depending, when adhesions arc absent, largely upon the width and the length of attachment of the meso-appendix. In the rare cases where the mesenterio-lum is entirely absent, the appendix is freely movable in the abdominal cavity.At times in the mesoappendix may be found an opening, in which a coil of small intestine has been known to have become strangulated.