Intussusception or invagination means the prolapse of one part of the intestine into the lumen of an immediately adjoining part. An intussusception shows in a vertical section six layers of intestine, three on either side of the central canal, which are more or less parallel to one another. The arrangement of the layers is such that mucous membrane is in contact with mucous membrane, and peritoneum with peritoneum. On transverse section the invaginated mass shows three concentric rings of bowel.

The external of the three layers is called the intussusci-piens, the sheath, or the receiving layer. The innermost cylinder is called the entering layer and the middle one the returning layer. The latter two together form the intus-susceptum. The neck of the intussusceptum is at its upper part where the returning layer joins the sheath.

In case the intussusception lasts for some time the serous surfaces of the gut touching each other may become glued together and ultimately- adherent. This will prevent the disengagement of the invaginated portion, while its further passage into the other bowel will not be interfered with. The mesentery always participates in the invagination and becomes more or less compressed and wedged in by the sheath. The whole mass of a simple intussusception may in its turn become invaginated and give five instead of three coats, or even seven if the process is repeated, so that the upper edge of the intussuscipiens is rolled over like a cuff. These double and triple intussusceptions are comparatively rare.

With regard to the mechanism of intussusception Noth-nagel's experiments on animals have proven of greatest value. According to this writer intussusception may be due either to a localized spastic contraction of a portion of the bowel or to a total paralysis. The normal gut immediately below the contracted part slips upward to a slight extent over this strongly contracted and greatly narrowed portion, and invagination is thus produced. Again if a segment of the bowel is paralyzed, the gut lying immediately below it, on contraction will slip into the paralyzed portion and thus an invagination may arise.

Intussusception may take place at any point within the entire small and large intestines. Over fifty per cent of the cases consist of the invagination of the ileum into the colon. With regard to the remote cause of intussusception Treves 1 has examined a number of reported cases and found it in one hundred examples of intussusception distributed as follows:

1.

No evident exciting cause...............

62

per cent.

2.

Diarrhoea, dysentery, enteritis, marked irregularity of the bowels........................

8

"

3.

Polypi.........

5

"

4.

Ingesta.............

5

"

5.

Injuries and exposure to cold................

5

"

6.

Certain acute and chronic ailments which may or may not have had a concern in the etiology, such as typhoid fever, whooping-cough, measles, scarlet fever, smallpox, cholera, and hernia; with these may be included pregnancy and labor.........

15

"

Total.............................

100

"

This clinical form of intussusception must not be confounded with agonal intussusception, which, as the term indicates, occurs shortly before death and is purely of anatomical importance. The agonal form of intussusception is sometimes found multiple and is met with frequently at autopsies of children who have died from affections of the brain.