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2.Topography of the Abdominal Organs and Their Relation to the Abdominal Wall

Clinical and Functional Anatomy p. 168

For the demonstration of the undisturbed organs of the abdominal cavity (dissections on facing page) the following structures were fully or partially removed: certain ribs and all intercostal tissues, diaphragm except for a narrow median strip, abdominal muscles except the rectus, parietal peritoneum, and the tensor fasciae latae.

Knowledge of the relationship of the abdominal organs to the abdominal wall is essential for the various methods by which these organs can be examined (rectal palpation, percussion, auscultation and abdominocentesis). It must be kept in mind, however, that the intestines in particular, because of their loose attachment, can shift markedly from the position most often depicted in anatomical illustrations. Least accessible are the organs covered by the ribs and costal arch, in the so-called intrathoracic part of the abdominal cavity. Most of them lie against the diaphragm and thus are overlain, in addition to the ribs, by portions of the lungs. The abdominal cavity, as a whole, is shaped like an egg whose pointed end occupies the cupola of the diaphragm, whose blunt end is at the pelvic inlet where the abdominal cavity opens into the pelvic cavity, and whose long axis is oriented from cranioventral to caudodorsal. Clinical examination is, of course, easiest through the soft abdominal wall caudal to the costal arch.

It is important for the diagnosis of positional changes and colic arising from these to understand the ATTACHMENT of the entire colon to the roof of the abdomen. The ascending mesocolon (see p. 71.33) splits off from the right side of the root of the mesentery. It does not form a sheet, but attaches the dorsal and ventral colon layers to each other, being visible only as narrow tissue bridges where the two pieces of gut come together. It extends, therefore, from the mesenteric root to the pelvic flexure (see p. 71.27) where it is sheetlike and spans the short distance between the two limbs of the flexure. Accordingly, the entire ascending colon except for its beginning and end (also adhered to the dorsal abdominal wall and base of the cecum) lies free on the abdominal floor and is subject to twisting or displacement. The descending mesocolon, however, is a regular sheet like the (jejunal) mesentery and allows also the descending colon considerable range in the caudal portion of the abdominal cavity.

a)LEFT SIDE

1Left lobe of liver (1), stomach (2), and spleen (3) are intrathoracic and, with the exception of parts of the spleen covered also by lung.

These organs are related ventrally to the left dorsal colon (9) and the beginning of the dorsal diaphragmatic flexure (7); though still under cover of the ribs, some parts of the left dorsal colon escape being blanketed by the lung. (See p. 71 for an understanding of the double flexures of the ascending colon.)

The upper part of the flank is occupied by a mixture of descending colon (5) and jejunum (4). The former has two bands (teniae) of which the free one as well as the sacculations (fecal balls) are prominent. The jejunum is without bands. Below these coils lie the caudal parts of the left colon (9; 10); the left dorsal has a smaller diameter than the left ventral; both become confluent in the pelvic flexure (6) which, shielded by the thigh, lies in front of the pelvic inlet often extending across the median to the right.

b)RIGHT SIDE

Quadrate (17) and right lobes of the liver (16), right kidney (15), and descending (13) and transverse (12) parts of the duodenum are intrathoracic on the right. Ventral to these organs lie the base of the

cecum (14) and the capacious right dorsal colon (18) turning cranially into the dorsal diaphragmatic flexure. The right lung covers the liver (except for its caudoventral angle), the kidney, and part of the duodenum.

The right flank is occupied by the base of the cecum (14) which is continued ventrally by the body of the cecum (11). This is directed craniomedially to allow the right ventral colon (19) to make contact with the costal arch.

c) VENTRAL ABDOMINAL WALL

The dissection on this page depicts the abdominal organs that lie on the floor of the abdomen; the soft ventral abdominal wall was removed so that the organs are flanked by the right and left costal arches. The ventral loop of the ascending colon—right ventral colon (19), ventral diaphragmatic flexure (8), and left ventral colon (10)—dominates the field. Nestling between its parts are the body (11) and apex (20) of the cecum. The latter, among mammals, has the very unusual position, about 15 cm caudal to the xiphoid cartilage; the former is attached to the lateral free band of the right ventral colon by the cecocolic fold. This is an important landmark for blind orientation during abdominal surgery. Coils of jejunum (4) and descending colon (5; with prominent band) may insinuate themselves between the body of the cecum and the left ventral colon to make intermittent contact with the ventral abdominal wall which, close to the pelvis, rises steeply to the level of the pubic brim.

Ventral View of Opened Abdominal Cavity

Cecocolic fold

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(Left lateral view)

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10 Left ventral colon

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19 Right ventral colon

18 Right Dorsal colon

hk Spinalis thoracis et cervicis l Esophagus

m Aorta

n Caudal vena cava

o Internal abdominal oblique

3.Spleen, Liver and Bile Duct, Pancreas, and Stomach with Omenta

Clinical and Functional Anatomy p. 168–170; 178–179

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a)The SPLEEN (11) in the fresh state is bluish; after death and exposure to air the red pulp becomes more obvious and colors the organ reddish-brown. Smooth muscle in capsule and supporting framework allows much variation in size and is important in the function of the spleen as a blood reservoir. The average weight of the spleen is about 1.5 kg. Thoroughbreds and other racers have relatively larger spleens and greater blood storage capacity than draft horses. The spleen is roughly triangular with a broad dorsal base and a ventral apex that is bent cranially. Its position varies slightly with respiration and with the distension of the stomach to which it is tied by the gastrosplenic ligament (9). The parietal surface of the spleen lies against the diaphragm under cover of the caudalmost ribs, dorsally opposite the last three intercostal spaces and ventrally reaching forward to the 9th to 11th interspaces. Its convex caudal border is roughly parallel and 10 cm cranial to the costal arch,but its caudodorsal end can protrude slightly into the flank (see p. 67). The visceral surface is divided by the long (ca. 50 cm) hilus into a small cranial gastric surface and a much larger intestinal surface and through these is related to stomach, pancreas, and intestines. The spleen is held in place by gastrosplenic (9), phrenicosplenic, and renosplenic (12) ligaments that connect it to stomach, diaphragm, and left kidney, respectively.

b)The LIVER accounts for about 1.5% of body weight and thus is relatively small when compared to that of the dog, a carnivore. Its large right lobe atrophies in many horses as a result of pressure from a chronically distended right ventral colon that lies under it

(see p. 67).

The liver lies entirely within the intrathoracic part of the abdomen and extends from the 6th intercostal space cranioventrally on the left to the 15th interspace caudodorsally on the right. Much more of its mass lies to the right of the median plane (see p. 67). The caudate lobe (18) is notched at its ventral free border. A papillary process, is lacking. Its caudate process carries a distinct impression for the right kidney. The major characteristic of the equine liver is the absence of a gall bladder which in the other domestic mammals marks the border between the quadrate lobe and the large right lobe of the organ. The junction between the quadrate and the left medial lobe is well marked by the round ligament (obliterated umbilical vein) that passes caudally to the umbilicus in the free border of the falciform ligament (13). Before reaching its destination the round ligament courses subperitoneally for a short distance. A fissure separates left lateral (2) and left medial (19) lobes. The large right lobe (15) is undivided.

The liver is held in place by right (17) and left (21) triangular ligaments. They attach it to the fleshy periphery of the diaphragm. The coronary ligament (not shown) attaches the parietal surface of the liver to the diaphragm more centrally.

The hepatic ducts are relatively wide which may be related to the absence of a gall bladder. They merge within the organ to form the common hepatic duct (16). The latter is continued by the bile duct that opens into the duodenum at the major duodenal papilla (31 in Fig. on this page) situated in the second bend of the S-shaped cranial part.

c)The PANCREAS also opens with its pancreatic duct on the major duodenal papilla. Its accessory pancreatic duct, however, opens on the minor duodenal papilla (32) opposite the major papilla. The pancreas is roughly triangular; its body (2) is cranial and lies against the cranial part of the duodenum; it is perforated by the portal vein

(d). The left lobe of the pancreas (10) extends across the median plane to the left where it makes contact with the stomach and the dorsal abdominal wall. The right lobe (4) follows the descending duodenum and is related to the right kidney and nearby base of the cecum.

d)The STOMACH of the horse is of the simple, unilocular kind and with a capacity of only 5–15 liters is small for such a large animal. It lies in the cranial part of the abdominal cavity entirely within the rib cage, mostly to the left of the median plane. It is strongly flexed with the result that entrance and exit, cardia (23) and pylorus (27' in Fig. on this page), lie close together. When moderately filled it lies opposite the 9th to the 14th intercostal spaces; but even when fully distended the stomach remains within the rib cage and fails to contact the abdominal floor. The cardia is fixed fairly solidy to the diaphragm at the esophageal hiatus, so that enlargements of the

organ spread away from that point. Its parietal (cranial) surface is related to the diaphragm and the liver, while the visceral (caudal) surface faces the intestines.

The part of the stomach dorsal to the cardia, the fundus, is distinct in the horse and is known as the blind sac (saccus cecus; 22). This is continued ventral to the cardia by the body which is joined to the pyloric part (25) at the deep angular notch (24). When the stomach is opened in the manner depicted on this page, the thickness of the cardiac sphincter (23) is revealed. This feature among others is thought to account for the horse's near inability to vomit. The two sphincters (26'; 27') at the pyloric end are also well developed, the more distal one especially, where the pyloric canal (27) is continued by the duodenum. The lining of the stomach is divided by the margo plicatus (28) into a proximal nonglandular mucosa (29) in the blind sac and body near the lesser curvature, and a glandular mucosa (26; 30) in the rest of the body and the pyloric part. In the fresh organ the former is light gray and harsh to the touch, while the latter is more reddish, softer, and smooth.

It is not possible to delimit grossly the three glandular zones. Histological sampling places the cardiac glands near the margo plicatus where they mix with the proper gastric glands of the distal body and beginning of the pyloric part; the pyloric glands are found in the rest of the pyloric part.

The stomach is attached to its surroundings by certain portions of the greater, and by the lesser omentum. The greater omentum (5) arises from the greater curvature (6), the lesser from the lesser curvature (8) of the organ. The two omenta and the viseral surface of the stomach enclose a subdivision of the peritoneal cavity known as the omental bursa. This sac-like potential space communicates with the general peritoneal cavity by a narrow (5 cm) canal named epi-

Stomach and Cranial Part of Duodenum,

opened on the visceral (caudal) surface

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ploic foramen (see p. 161, 68.2A). The foramen lies between the right lobe of the liver and the descending duodenum and extends medially between caudal vena cava and portal vein to allow access first into the vestibule, and then into the caudal recess of the omental bursa. The caudal recess is the part of the bursa that is enclosed almost entirely by the greater omentum; it is not extensive and is hidden from view between the stomach and the intestines; it has been removed in the upper dissection on the opposite page. The gastrophrenic ligament, a part of the greater omentum, attaches the greater curvature near the cardia to the crura of the diaphragm. The gastrosplenic ligament (9 on facing page) attaches the stomach to the spleen. The lesser omentum (not shown) spans the short distance between lesser curvature and the visceral surface of the liver which itself is well fixed to the diaphragm.

4. Intestines

a) The INTESTINAL TRACT is divided into small and large intestines, although the terminal part of the latter, the descending or small colon, has a narrow lumen but it is easily distinguished by a prominent free band (tenia) and related sacculations (haustra).

1The small intestine has three parts: duodenum, jejunum, and ileum; by and large it follows the general mammalian pattern.

2The duodenum is about 1 m long. Its cranial part (1) begins at the pylorus with a slight enlargement and forms a sigmoid flexure that lies against the porta of the liver. The second bend of the flexure presents in its interior the major and minor duodenal papillae which lie opposite to each other and mediate the influx of bile and pancreatic juice (see p. 68). The descending duodenum (2) continues the sigmoid flexure and passes caudodorsally under cover of the liver to be followed at the right kidney by the caudal flexure (transverse duodenum; 3) that takes the duodenum from right to left around the caudal aspect of the root of the mesentery. As it enters the caudal flexure, high in the abdominal cavity, it is related and attached to the base of the cecum and to the transverse colon. The last segment, the ascending duodenum (4), is short, and medial to the left kidney it becomes the jejunum that has a markedly longer mesenteric attachment. The duodenojejunal junction is made plain by the cranial free border of the duodenocolic fold (5).

3The jejunum (6), is about 25 m (70 feet) long; it is shorter in life.

Owing to its long mesentery, jejunal coils can be found in many parts of the abdominal cavity; most of them, however, reside near the left flank, ventral to the pelvic inlet, and to the left of the cecum.

4The ileum (7), about 50 cm long, continues the jejunum where the ileocecal fold (8) has its most proximal (oral) extent. It is characterized by a thick muscular coat that tends to narrow the lumen, although when relaxed in the fresh state after death the ileum may not appear or feel much different than the jejunum. The ileum ends on the ileal papilla (9) in the medial wall of the cecum at the junction of this organ's base and body.

The large intestine has three parts: cecum, colon, and rectum and is characterized in the horse by an expansive cecum and equally capacious parts of the colon which allow stasis of the ingesta for the purpose of their microbial destruction.

5The cecum is about 1 m (4 feet) long and has a capacity of roughly

35 liters; it consists of base, body, and apex. The base of the cecum

(10)occupies the right flank but extends forward as well to lie under cover of the last few ribs. (Since the latter part of the base is distal (aboral) to the ileocecal junction, it corresponds actually to the first part of the ascending colon as in most domestic mammals.) The base of the cecum is continued by a cylindrical body (12) that curves ventrally and cranially to follow the slope of the abdominal floor to the vicinity of the xiphoid cartilage where it is succeeded by the apex (13) that occupies the concavity of the ventral diaphragmatic flexure formed by the ascending colon. The cecal wall is gathered into sacculations by four bands of which the medial (16) and lateral (14) ones accommodate blood vessels and lymph nodes. The lateral band releases the cecocolic fold (18) that ties the right ventral colon to the cecum. The dorsal band (15) releases the ileocecal fold that ends on the ileum. The ventral band (17) is unremarkable except for joining the medial band shortly before reaching the apex of the organ.

The colon consists of ascending, transverse, and descending parts of which the first-named is the most capacious. For this reason it is also known as the great colon in contrast to small colon that designates the equine descending colon.

6The ascending colon is about 4 m long, holds on average 80 liters, and is folded together to form a double loop consisting of left (26) and right ventral (20), and left (28) and right dorsal (30) parts. It begins at the cecocolic orifice (11) in the lesser curvature of the cecum that lies roughly opposite the costochondral junctions of the last two right ribs. Its first, slightly enlarged portion (collum coli; 19) deflects the digesta it contains from a caudal to a cranioventral direction which is the orientation of the first of the four parts, the

b)The BLOOD SUPPLY to the gastrointestinal tract follows the general mammalian plan despite the extraordinary shape of the large intestine. The celiac artery (35) splits into splenic (42), left gastric (37), and hepatic (36) arteries that pass to their respective organs. The cranial mesenteric artery (45) of many horses is subject to distortion by the activity of nematode larvae. Its principal branch, the ileocolic artery, gives rise to the colic branch (50) and the right colic artery (54) that supply the right and left ventral colons (first half) and the right and left dorsal colons (second half) of the ascending colon, respectively. These two vessels are of course very long; they anastomose at the pelvic flexure. Several (transverse) anastomoses connect the two vessels also in their initial course where they lie close together between the right dorsal and ventral colons. The lateral (52) and medial (51) cecal arteries also anastomose near the apex of the cecum.

c)The NERVE SUPPLY of the intestines follows the general mammalian plan. The parasympathetic innervation is by the endbranches of the vagal trunks that follow the branches of the cranial mesenteric artery. The sympathetic innervation takes the same route but is brought to the region by the splanchnic nerves and passes through the ganglia in the root of the mesentery.

d)The position of the LYMPH NODES associated with the gastrointestinal tract can be determined by reference to the main drawing on the facing page: celiac lymph nodes (A), at the origin of the celiac artery; splenic lymph nodes (B), along the hilus of the spleen; gastric lymph nodes (C), in the lesser curvature of the stomach along the left gastric artery; hepatic lymph nodes (see p. 69), at the porta of the liver surrounding the portal vein; pancreaticoduodenal lymph nodes (D), between pancreas and descending duodenum; jejunal lymph nodes (E), in the dorsal, gathered part of the mesentery where the jejunal arteries originate; colic lymph nodes (G), between the dorsal and ventral layers of the ascending colon; cecal lymph nodes (F), along the medial and lateral bands of the cecum; and caudal mesenteric lymph nodes (H), grouped around the breakup of the caudal mesenteric artery.

Lymph from the intestinal nodes is gathered by the intestinal lymph trunk that empties into the cisterna chyli. This is continued cranially by the thoracic duct which transports the lymph to the veins at the thoracic inlet.

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ACeliac lymph nodes

BSplenic lymph nodes

CGastric lymph nodes

DPancreaticoduodenal lymph nodes

EJejunal lymph nodes

FCecal lymph nodes

GColic lymph nodes

HCaudal mesenteric lymph nodes

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Ischium

Body of ischium (24)

Flat part (tabula) of ischium (25)

Ramus of ischium (26)

Symphysial surface

Lesser ischial notch (27)

Ischial tuber (28)

Ischial arch (29)

Pubis

Body of pubis (30)

Caudal ramus of pubis (31) Symphysial surface

Cranial ramus of pubis (32) Pecten (33)

Groove for accessory ligament (33') Iliopubic eminence (34)

Ventral pubic tubercle (35)

iTensor fasciae latae

jRectus femoris

kVastus medialis

(Caudoventral view)

oblique muscle

11 External pudendal vessels and genitofemoral nerve

12 Supf. inguinal ring

13 Vascular lacuna

14 Muscular lacuna

15 Femoral vessels

16Iliospoas

17Tendinous femoral lamina

18Sartorius

2. Inguinal Area

The complex inguinal area comprises the attachment of the soft ventral abdominal wall to the bony pelvis and, below this, scrotum and penis. It needs to be exposed both from the outside, and from inside the abdominal cavity, as shown in the lower Fig. on the facing page.

a) The INGUINAL CANAL is a flat, potential space between the internal and external abdominal muscles through which certain structures pass from the abdominal cavity into the subcutaneous tissues of the groin. The canal extends from the deep inguinal ring (6) at the free caudal border of the internal abdominal oblique (5) to the supf. inguinal ring (16) which is a slit in the aponeurosis of the external abdominal oblique muscle. The principal structure to use

1this route of exit in the stallion is the vaginal process and its contents. These are the deferent duct (7), the testicular vessels (17) accompanied by the testicular (nerve) plexus, and lymphatics; and, lying against the outside of the vaginal process, the cremaster muscle (4). The aggregate of these structures is known by the clinician as the spermatic cord. The mare lacks, of course, a vaginal process, but in some a tiny peritoneal and fascial evagination can be detected; the round ligament of the uterus and a weak cremaster muscle can be followed through the canal to the base of the udder (see p.

277). In both sexes the genitofemoral nerve (3), branches of the second lumbar nerve (L2; 2), and the external pudendal vessels (14) pass through the inguinal canal.

I. The skin (9) is reflected over the testes to form the scrotum (s) which has few hairs, is pigmented, and glistens from the secretion of sweat and sebaceous glands. Smooth muscle (tunica dartos) as a deep component of the scrotal skin, but also in the scrotal septum (20), wrinkles the skin in cold weather and helps to draw the testes closer to the body.

II. The elastic abdominal tunic (8; see also p. 73) that covers the external abdominal oblique muscle, in the stallion gives rise at the supf. inguinal ring to the external spermatic fascia (8') which encloses the spermatic cord and the testes. (A homologous but much less developed fascia can be dissected also in the mare; see p. 77.) The abdominal tunic also blends with the fascia lata (19) where thigh and abdominal wall lie against each other. In the ventral midline the abdominal tunic is anchored to the linea alba (18), from which, in the mare, the medial lamina of the suspensory apparatus of the udder arises; the lateral laminae originate from near the supf.inguinal ring and cover the sides of the udder (see p. 77). Similar laminae can be demonstrated in the stallion as the suspensory ligament of the penis (8''') that reaches also the prepuce. An additional sheet (lamina femoralis fascialis*; 8''), unrelated to penis and spermatic cord, extends from the fascia covering the lateral crus of the supf. inguinal ring to the thigh and opens the ring when the stallion spreads its hind limbs apart as occurs during service.

III. The external abdominal oblique (10) presents a slit-like defect between its pelvic (10') and abdominal (10'') tendons which was already described as the supf. inguinal ring (16). The ring is easily seen on the deep (internal) surface of the muscle (right side on lower Fig. on facing page). On the supf. (external) surface of the muscle, the ring is covered by the tube-like external spermatic fascia (8') which has been fenestrated on the left side of the same Fig..

The internal abdominal oblique (5) has a free border where it deviates from its attachment on the inguinal ligament. The free border and the inguinal ligament caudal to it form the deep inguinal ring

Clinical and Functional Anatomy p. 172–173

(6) which is a fusiform opening with lateral (6') and medial angles (6''). The vaginal process (13'') passes through the lateral angle of the ring, while the external pudendal artery (14) passes through the medial angle which lies close to the lateral border of the rectus abdominis (15). The cremaster passes through the ring together with the vaginal process. It is a phylogenetic detachment from transversus abdominis and internal abdominal oblique muscles but has acquired an origin of its own from the inguinal ligament.

The caudal border of the transversus abdominis (11) is at the level of the coxal tuber and thus does not reach the area of the inguinal canal or its rings.

IV. The transverse fascia (12) that lines the internal surface of the abdominal muscles evaginates the internal spermatic fascia (12') at the deep inguinal ring that encloses the vaginal process and with it passes through the inguinal canal. The fascia on its way to the testis changes to loose connective tissue that facilitates the sliding movements between the coverings of spermatic cord and testis. (In a closed castration the testis, still covered by vaginal tunic and spermatic fasciae, can be peeled out of the scrotum and detached after ligation.) In stallions with inguinal hernia, the area where the inter-

nal spermatic fascia becomes loose connective tissue forms a typi- 3 cally contracted ring. Cutting the ring is necessary for returning the herniated intestine to the abdominal cavity.

V. At the vaginal ring (13') the parietal peritoneum (13) forms the tubular but blind vaginal process (13'') which also passes through the inguinal canal. Spermatic cord and testis “sink into” the vaginal process and thus gain their immediate coverings known as the visceral and parietal vaginal tunics. The potential space between the two tunics (vaginal cavity) is continuous with the peritoneal cavity at the vaginal ring.

The wall of the inguinal canal consists of different tissues: the fleshy part of the internal abdominal oblique cranially and the pelvic tendon of the external oblique caudally—not the criteria of a canal. The inguinal canal is actually a potential space between these two muscles that are spread apart by the structures that pass through it and by loose connective tissue. The medial and lateral boundaries of the space are ill defined. The length of canal in a medium-sized horse, when measured along the spermatic cord, is about 15 cm, whereby it is necessary to remember that the cord enters the canal close to the lateral angle of the deep inguinal ring, and leaves it near the medial angle of the supf. inguinal ring.

Upon approaching the pubic brim the rectus abdominis (15) tendons curve toward a median decussation of their fibers to penetrate the prepubic tendon. At the caudal angle of the supf. inguinal ring the abdominal tendon of the external abdominal oblique and the overlying abdominal tunic join the rectus fibers to form the accessory ligament of the femur.

b) The INGUINAL LIGAMENT (1) arises from the coxal tuber and ends within the prepubic tendon. After leaving the tuber it blends with the iliac fascia that covers the iliopsoas muscle. About its middle, the ligament loses contact with the fascia and unites with the pelvic tendon of the external abdominal oblique muscle** with which it concludes its course to the prepubic tendon. The lateral part of the inguinal ligament gives origin to a portion of the internal abdominal oblique by which it is hidden from view. In the lower Fig. on the facing page, the muscle origin has been removed to fully expose the ligament.