The Abdomen
3.5 Intestines
Goal: You should know that the intestinal tract lies to the right of the median plane within the supraomental recess. Identify the regions and structures associated with the small intestine (cranial part of the duode- num, sigmoid flexure, descending duode- num, caudal duodenal flexure, ascending duodenum, duodenocolic fold, duodeno- jejunal flexure, jejunum, mesojejunum, ileum, ileocecal fold, and ileal orifice).
Identify regions and structures associated with the large intestine (cecum, cecocolic orifice, ascending colon [has proximal, spi- ral, and distal loops], transverse colon, and descending colon). Identify regions of the spiral loop (centripetal coils, central flex- ure, and centrifugal coils). Know the differ- ence between small and large ruminants in the disposition of large centrifugal coils.
The intestinal tract comprises the small and large intestines. It lies mostly to the right of
median plane because the large ruminoreticular compartment dominates the left side of the abdomen. Use Figure 3.22 to study a global
view of the various parts of the intestinal tract before you start studying the details of each segment.
Abomasum (permanent spiral folds)
Omasoabomasal orifice (vela abomascia)
(opened)
Omasum (opened)
Omasal laminae Pyloric part
(torus pyloricus) Duodenum
(opened)
Proximal Distal
Ox
Torus pyloricus (pyloric pad)
Pyloric gland region
of abomasum (pyloric part) Abomasum (opened) Descending
duodenum (opened)
Pyloric sphincter (cut)
Distal Proximal
Ox
(a)
(b)
Figure 3.21 (a) Bovine omasum, abomasum, and proximal part of the duodenum (opened along the lesser curvature). The muscular projection at the pylorus is the torus pyloricus (see also Figure 3.21b). The mucosa of the abomasum contains cardiac glands (at the omasoabomasal orifice), proper gastric glands (in the fundus and body), and pyloric glands (at the pyloric region). (b) Distal part of the abomasum, and proximal part of the duodenum. The abomasum and duodenum are opened and reflected along the lesser curvature.
With help of Figure 3.22 and the following description, use goat cadavers to identify regions of the small and large intestines in situ and on isolated viscera. In situ, begin by stud- ying the regions of the duodenum. Pull the intestines from the supraomental recess and trace the terminal part of the jejunum to the colon. Identify the ileocecal fold. Next, study regions of the ascending colon. Finally, iden- tify the transverse and descending colon. It will be difficult to identify the transverse colon in situ.
3.5.1 Small Intestine
The small intestine has three parts: (i) duo- denum, (ii) jejunum, and (iii) ileum. The ileum joins the large intestine at the junction between the cecum and the beginning of the proximal loop of the ascending colon.
3.5.1.1 Duodenum
The duodenum has five identifiable parts:
(i) cranial part, having near its end a sigmoid flexure, (ii) descending part, (iii) caudal Box 3.11
Abomasal displacement and abomasal vol- vulus: several factors such as hypomotility and gas production contribute to this condi- tion. Typically, the abomasum can be displaced from its normal position to the left side of the body (left displacement of the abomasum;
LDA) or to the right (right displacement of the
abomasum; RDA). Volvulus occurs when the abomasum rotates on its mesenteric axis.
Surgical correction to prevent recurrence involves attaching the abomasum to the body wall (abomasopexy) through right flank abomasopexy.
Cecum Ileocecal
fold Ileum
Jejunum Proximal loop of the ascending colon
Distal loop of the ascending colon
Descending colon Transverse
colon
Central flexure
Descending duodenum
First centripetal turn of spiral colon
Jejunal (mesenteric) lymph nodes
Last centrifugal turn of spiral colon Duodenocolic fold
Caudal duodenal flexure
Ascending duodenum
Cranial Ox
Figure 3.22 Overview of the bovine intestinal tract: right lateral view. The small intestine includes the duodenum (has several parts), jejunum (longest part), and ileum (defined by the ileocecal fold). The large intestine consists of the cecum, ascending, transverse, and descending colons. The ascending colon is more complex. Trace the three loops of the ascending colon (proximal, spiral, and distal loops). The spiral loop has centripetal turns, central flexure, and centrifugal turns. The last centrifugal fold of the spiral colon in goats lies close to the jejunum and is adhered to the great mesentery (mesojejunum). Its appearance has been likened to that of a “pearl necklace.”
duodenal flexure, (iv) ascending duode- num, and (v) duodenojejunal flexure.
Find the beginning of the duodenum on the right side of the abdomen beneath the visceral surface of the liver. The cranial part (pars cranialis) of the duodenum begins at the pylorus and courses caudodorsally over the dorsal sac of the rumen (Figure 3.23).
Next, it forms a sigmoid flexure (still part of the pars cranialis). The distal bend of the sig- moid flexure is called the cranial flexure.
This is where the descending part (pars descendens) begins.
Reflect the liver dorsally to observe the sig- moid flexure. Trace the descending part of the duodenum toward the pelvic cavity.
Near the pelvic inlet and caudal to the root of the mesentery, the duodenum turns from right to the left of the median plane to form the caudal duodenal flexure. On the right side, it courses cranially to form the ascend- ing duodenum. Note that the ascending duodenum is connected to the descending colon with a peritoneal fold known as the duodenocolic fold (Figures 3.22 and 3.23).
The duodenum terminates by joining the jejunum at the duodenojejunal flexure.
There is no need to identify the duodenojeju- nal flexure.
3.5.1.2 Mesoduodenum
Identify the peritoneum that suspends the duodenum. This is the mesoduodenum (Figure 3.23). The right lobe of the pancreas is enclosed by the double serosal layers of the mesoduodenum.
3.5.1.3 Duodenocolic Fold
The duodenocolic fold attaches the ascend- ing duodenum to the descending colon.
Identify this fold at the beginning of the ascending colon on the right side of the median plane (Figures 3.22 and 3.23).
3.5.1.4 Jejunum
The jejunum is the longest part of the small intestine. Its coils are suspended by the mes- ojejunum around the root of the mesentery.
The mesojejunum is also called the great mesentery (Figure 3.24b).
Sigmoid flexure
Descending duodenum Ascending duodenum
Abomasum Descending colon
Caudal duodenal flexure
Greater omentum (Superficial leaf)
Lesser omentum
∗
Pylorus
Ox
Mesoduodenum
Liver
Figure 3.23 Isolated bovine abdominal viscera showing the course of the duodenum: right lateral view. Trace the cranial part of the duodenum off the pylorus, the sigmoid flexure, cranial duodenal flexure (distal part of the sigmoid flexure), descending duodenum, caudal duodenal flexure, and ascending duodenum. The liver is pulled dorsally to expose the lesser omentum and the origin of the duodenum from the distal part of the pylorus. * Duodenocolic fold.
Cecum
Ileocecal fold
Ileum
Descending duodenum
Greater omentum:
(Superficial leaf)
(Deep leaf)
Jejunum
Goat
Jejunum
Cecum Proximal loop First centripetal coil (turn)
Distal loop Ileocecal fold
Last centrifugal coil
Spiral colon Goat
Central flexure
Mesojejunum
Liver (a)
(b)
Figure 3.24 (a) Goat: right lateral view. The cecum and intestine are pulled out of the supraomental recess. The superficial leaf is incised to open the omental bursa. Star, omental bursa. (b) Small and large intestines of the goat: left view of the cecum and ascending colon. The cecum and ascending colon are pulled cranially. In a normal arrangement, the part of the spiral colon shown here faces the right wall of the rumen. (c) Isolated goat abdominal viscera: lateral right view. The intestines are pulled caudally from the supraomental recess. (d) Abdominal and thoracic viscera in a goat: left lateral view. Note the spillover of the jejunum from the right side to the left side residing behind the blind sacs of the rumen. (e) Abdominal and thoracic viscera in a goat: left lateral view. Note spillover of the jejunum and the spiral colon (left surface) from the right side to the left side because of a relatively empty ruminal compartment. This spill could also be a postmortem event.
Dorsal sac
Descending duodenum
Right kidney
Abomasum Jejunum
Cecum Proximal loop Distal loop
Gallbladder Greater
omentum
Lesser omentum Descending colon
Caudal duodenal flexure
Cranial Goat
Jejunum
Abomasum Spleen
Caudodorsal and caudoventral blind sacs Cranial
Goat
Ventral sac Dorsal sac
Diaphragm
Liver
(c)
(d) Figure 3.24 (Continued)
Note that the presence of elongated jeju- nal (mesenteric) lymph nodes within the double layers of the mesojejunum (Figure 3.22). Understand that most the jejunal coils are contained within the supraomental recess (Figure 3.24a–c) although some coils may migrate out and can be seen on the left side of the median behind the blind sacs of the rumen (Figure 3.24d, e).
Advanced pregnancy and fullness of the rumen influence movement of the jejunum.
3.5.1.5 Ileum
The gut segment next to the jejunum is the ileum. It forms the terminal part of the small intestine and is the shortest part of the small intestine. It is readily identifiable by its mes- enteric attachment to the cecum by the ile- ocecal fold (Figures 3.22 and 3.24a).
3.5.1.6 Ileal Orifice
The ileum enters the large intestine and ter- minates at the ileal orifice. Authors disagree as to where in the large intestine the ileum opens. Some authors are of the opinion that the ileum opens into the proximal part of the cecum, some suggest the ascending colon, and others suggest a “no man’s land” at the
junction between the cecum and the begin- ning of the ascending colon (i.e., at the begin- ning proximal loop). In any case, the disagreement does not have any clinical implications. Opening at the junction between the cecum and the beginning of the ascending colon is a safe bet and the name of the opening (ileal orifice) does not indicate a preference.
3.5.1.7 Ileocecal Fold
Different landmarks define the ileum in domestic species. The ileocecal fold defines the location of the ileum in ruminants (Figures 3.22 and 3.24a).
In horses, the ileum is distinguished by both the ileocecal fold and by its thicker wall that gives the impression of a water hose. In ruminants, the ileum has softer consistency similar to that of the jejunum.
In dogs, the ileum is defined by the presence of antimesenteric vessels that are uniquely located on the opposite side of the mesentery.
3.5.2 Large Intestine
Identify the three regions of the large intes- tine: (i) cecum, (ii) colon (ascending, trans- verse, and descending parts), and (iii) rectum (Figures 3.22 and 3.24a–c).
Rumen
Spleen Left kidney Spiral colon Jejunum
Left Lung
Cranial Goat
(e) Figure 3.24 (Continued)
Of these three parts, the ascending colon is the most complicated in large domestic ani- mals. The simplest form is found in humans and carnivores.
In ruminants, the ascending colon has (i) proximal, (ii) spiral, and (iii) distal loops (Figures 3.22 and 3.24b). Further, three parts form the spiral loop: (i) centripetal coils or turns, (ii) central flexure, and (iii) centrifu- gal coils or turns (Figures 3.22 and 3.24b).
3.5.2.1 Cecum and Cecocolic Orifice
In the live animal, the cecum is accessed in the right dorsal flank. It continues cranially as the proximal loop of the ascending colon without an appreciable sphincter or change in tubular diameter (Figures 3.22 and 3.24a). Consequently, the cecolic orifice is a less defined site. It is gen- erally located at the point where the ileum enters the large intestine at the junction between the cecum and ascending colon (Figure 3.22).
From the ileum, ingesta travels to the colon.
The cecolic orifice is much more well defined in the horse and dog.
3.5.2.2 Ascending Colon
The ascending colon is the more extensive part of the colon. It is divided into the proxi- mal, spiral (which has centripetal, central flexure, and centrifugal turns or coils), and distal loops (Figures 3.22 and 3.24b). The spi- ral loop is sometimes called the spiral colon.
The next segments of the colon, transverse and descending colon, are relatively simple.
3.5.2.2.1 Proximal Loop
The proximal loop begins where the ileum enters the cecum. From there, it continues cranially, turns in a caudal direction, and turns again in a cranial direction before mak- ing a final turn ventrally where it is identified as the spiral loop or spiral colon. The diame- ter of the proximal loop is similar to that of the cecum (Figures 3.22 and 3.24c) but the spiral loop is of a smaller diameter.
3.5.2.2.2 Spiral Loop
To identify the parts of the spiral loop, pull the jejunum, cecum, and ascending colon
cranially out of the supraomental recess. With the help of Figures 3.22 and 3.24(b), (c), trace the proximal loop to the beginning of the spi- ral colon. The spiral colon has three parts where it first turns inwards as centripetal coils or turns, folds upon itself as central flexure, and finally it spirals outward as cen- trifugal coils. In cattle, there are two centrip- etal turns followed by two centrifugal turns.
In goats, there are three or four turns in each direction.
In goats, the last centrifugal coil lies close to the jejunum. It is adhered to the mesojejunum. This coil has the appearance of a “pearl necklace” as it often contains small round fecal balls (Figure 3.24b). In cat- tle, the last centrifugal coil remains close to the rest of the spiral colon (Figure 3.22).
While reviewing the parts of the spiral colon, identify the long jejunal (mesenteric) lymph nodes embedded within the double layers of the mesojejunum.
3.5.2.2.3 Distal Loop
The final segment of the ascending colon is the distal loop (Figures 3.22 and 3.24b, c). It is a single bend of intestine situated slightly dorsal to the second turn of the proximal loop. From the terminal part of the distal loop, the colon crosses from the right to the left of the median plane as the transverse colon. The transverse colon turns caudally and continues as the descending colon on the left side of the median plane (Figures 3.22 and 3.24c).
Note that in small ruminants (goats and sheep), the last centrifugal coil of spiral colon, the distal loop, and the descending colon may have small fecal pellets that are covered with fat (Figure 3.24c).
For easy identification, note the order of the proximal loop, distal loop, and caudal duodenal flexure from ventral to dorsal (study Figure 3.24c). Identify this arrange- ment on the right flank or on isolated whole viscera (Figure 3.24c). The proximal loop is largest and similar in size to the cecum. The caudal duodenal flexure and distal loop are similar in size but the distal loop is covered
by fat deposits while the caudal duodenal flexure is not (Figure 3.24c).