_1.R._, First thoracic vertebra; _1.L._, first lumbar vertebra;
_2.K._, second sacral spine; _S._, scapula; _A._, humerus; _St._,
sternum; _B._, ilium; _O._, femur; _L._, lung in complete
expiration; _l._, dotted line indicating contour of lung in
inspiration; _H_, pericardium; _Z_, costal part of diaphragm; _Z′_,
tendinous center of diaphragm; _l.N._, left kidney; _l.v.C._, left
ventral colon; _v.Q._, sternal flexure of colon; _d.Q._,
diaphragmatic flexure of colon; _D._, small intestine; _M._, small
colon; _M′_, rectum; _a_, coccygeus muscle; _b_, retractor ani; _c_,
_c_, sphincter ani externus; _d_, constrictor vulvæ; _e_,
sacro-coccygeus inferior; _f_, anterior gluteal artery; _g_,
internal pudic artery; _h_, abdominal wall in section. (After
Ellenberger, in Leisering’s Atlas, reduced.)
]
The =jejunum= and =ileum= together (Jejuno-ileum) constitute the
mesenteric or floating portion of the small intestine. No distinct point
exists at which to make the demarcation. With the exception of the last
two or three feet, the mesenteric part of the intestine varies so much
in position that only a general statement can be made. It lies in
numerous coils (Ansæ) mingled with those of the small colon, chiefly in
the dorsal part of the left half of the abdomen, from the visceral
surface of the stomach to the pelvis. It may insinuate itself between
the left portions of the colon and the abdominal wall; also between the
ventral portions of the colon, reaching the floor of the abdomen. The
terminal part of the intestine (ileum) ascends almost vertically, a
little to the right of the median plane, to open into the cæcum at the
lesser curvature of its base. The average diameter of the jejuno-ileum
is about two and a half to three inches (ca. 6 to 7 cm.). In the cadaver
one finds most of the tube presenting irregular constricted and dilated
parts. The last three or four feet (ca. 1 meter) are usually tightly
contracted, resembling somewhat the terminal part of the œsophagus. This
part may be termed the ileum.
The mesenteric part is connected with the dorsal abdominal wall by the
=great mesentery=. This is a wide fan-shaped fold, consisting of two
layers of peritoneum, between which the vessels and nerves reach the
bowel; it also contains the mesenteric lymph glands and some fat. The
visceral border of the mesentery contains the intestine, while the
parietal border or =root= (Radix mesenterii) is attached to a small area
around the great mesenteric trunk under the first and second lumbar
vertebræ. The root is thick, as it contains a large number of vessels
and nerves placed close together. The mesentery is short at first, but
soon reaches a length of one and a half to two feet (ca. 50
cm.)—sufficient to allow coils of the intestine to reach the abdominal
floor, the pelvic cavity, or even to descend into the scrotum through
the inguinal canal. Near its termination the intestine (ileum) leaves
the border of the mesentery, so that the latter has a free edge which
passes to the cæcum.
=Structure.=—The wall consists of four coats—serous, muscular,
submucous, and mucous, enumerated from without inward.
The =serous coat= is complete except at the mesenteric edge, where the
vessels and nerves reach the bowel.
The =muscular coat= consists of an outer longitudinal and an inner
circular layer, the latter being the thicker. In the last few feet of
the intestine the muscular coat is very thick, and being usually firmly
contracted in the dead subject, gives the impression that this part of
the bowel is of smaller caliber; such, however, is not the case during
life.
[Illustration: