The opening out of the rectum (a) on the surface of the body is called the anus. At this place there is a double narrowing, caused by circular bands of muscular fibre round the canal. These bands form what is called the sphincter of the anus, and prevent the accumulated mat ters in the rectum passing out until the sphincter is relaxed by an effort of will.
Observe that the bend at the sigmoid flexure (m) helps to relieve the rectum and sphincter of the anus of the pressure of the matter con tained in the descending part of the large bowel. Without that relief there might be difficulty in maintaining the closure of the sphincter.
The cavity of the abdomen has been spoken of. It is formed of muscular walls which, directly or indirectly, are supported by the back-bone behind and by the ribs above and the haunch-bones below. In the middle of the back wall of the cavity is the back-bone, but it is covered over, and its irregularities and hard nesses masked by muscle and other soft tissues. Now it must not be supposed from the diagram (Fig. 99) that the intestines lie in a loose heap in this cavity. Lining the cavity, just as, to use a very rough figure, a paper lines the walls of a room—lining the walls of the cavity is a delicate fibrous membrane called the peritoneum. When the peritoneum, in its course of lining the walls, comes in contact with the large bowel it passes over it, just as the paper of a room would pass over, say, a gas-pipe which ran along the surface of a wall, and in passing over the large bowel it binds it down to the wall of the cavity.' The small bowel is, however, not lying against the wall of the cavity as is the large one ; it is out towards the centre of the space. In order to reach it, therefore, the peritoneum from its course over the back wall passes out to reach the small bowel, passes round it so as com pletely to envelop it, and then passes back to the wall again to continue its course. Thus there is a double fold formed by the membrane as it passes out and as it passes back, and this double fold suspends the small bowel from the back wall of the cavity. It is called the mesentery. Besides the stomach and bowels, the abdomen contains other organs, the liver, spleen (melt), pancreas (sweet-bread), kidneys, and low down in the pelvis the bladder and generative organs. The positions of these
ether organs will be referred to immediately, Ind will be more fully stated when each comes xl be discussed. The general position, which most of the organs named occupy, can be roughly made out from the outside. Take the suc ceeding figure (100), and study it in relation to Fig. 99. It represents the front wall of the belly as mapped out into regions by the lines shown on the figure. The regions are marked AT by drawing two cross lines, one connecting the lower edge of the ribs on each side, and the other by connecting the highest point of the haunch-bone on each side. The two upright lines are drawn straight upwards from the middle point of the groin on each side. There are thus marked off nine regions, each one being called by a special name, the advantage of them consisting in the list, which anatomists are able to draw up for the guidance of physicians and surgeons, of the different organs which are found to lie beneath each region. • The first region is called epigastric (Greek, epi, upon, and gaga., the stomach), because it is over the stomach ; the second, immediately below it, is umbilical, because it Meioses the umbilicus or navel, and the third hypogastric (hupo, under, and gaster), because it is below the stomach. These are the middle divisions. On the left side, the fifth division is left hypochondriac (Greek, hupo, under, and chon dros), because it is the region under the ribs. Below it is the seventh division—the lumbar region, and next is the ninth—the iliac region, because the region of the ilium or flank-bone (Latin, ilia, the flank), the name for the chief portion of the haunch - bone (p. 63). Similar names apply to similar regions on the right side, right being substituted for left. By referring to Page 63 it will be seen that these three lower divisions, namely, the 3rd, 8th, and 9th, are ranged round the upper edge of the pelvic bones, and that beneath them is the cavity of the pelvis--the lower portion of the belly—which has no region marked externally corresponding to it. As already noted, in this lower cavity lie the genitourinary organs.
These points being understood, by consulting the following list it will be seen how one could with some ease determine the position of any particular organ from the outside. Or, again,