Part 39

The principal =relations= of the œsophagus at its origin are: to the cricoid cartilage below; to the guttural pouches and the ventral straight muscles above; and to the carotid arteries laterally. In the middle of the neck the relations are: to the left longus colli muscle above; to the trachea internally; to the left carotid artery, vagus, sympathetic, and recurrent nerves externally. At its entrance into the thorax it has the trachea on its inner side; the first rib, the roots of the brachial plexus of nerves and the inferior cervical ganglion externally. After gaining the upper surface of the trachea, it has the aorta on its left and the vena azygos and right vagus nerve on its right side. In its course through the posterior mediastinum the œsophageal trunks of the vagus nerves lie above and below it, and the œsophageal artery is dorsal to it.

=Structure.=—The wall is composed of four coats: (1) A fibrous sheath; (2) the muscular coat; (3) a submucous layer; (4) the mucous membrane. The =muscular coat= is of the striped variety as far as the base of the heart, where it rapidly changes to the unstriped type. In addition to this change, the muscular coat becomes much thicker and firmer, while the lumen is diminished.[86] The outer fibers are arranged longitudinally, beginning in two bundles attached in the interval between the arytenoid and cricoid cartilages. The inner fibers run in two spiral strata to the terminal part of the tube, where the arrangement is an outer longitudinal and an inner circular layer.[87] The =mucous membrane= is pale, and is covered with squamous stratified epithelium. It is loosely attached to the muscular coat by an abundant submucosa, and lies in longitudinal folds which obliterate the lumen except during deglutition.

=Blood-supply.=—Carotid, broncho-œsophageal, and gastric arteries.

=Nerve-supply.=—Vagus, glosso-pharyngeal, and sympathetic nerves.

THE ABDOMINAL CAVITY

The abdominal cavity (Cavum abdominis) is the largest of the body cavities. It is separated from the thoracic cavity by the diaphragm and is continuous behind with the pelvic cavity.

It is ovoid in form but somewhat compressed laterally. Its long axis extends obliquely from the center of the pelvic inlet to the sternal part of the diaphragm. Its dorso-ventral diameter is greatest at the first lumbar vertebra, while its greatest transverse diameter is a little nearer the pelvis.

The =dorsal wall= or roof is formed by the lumbar vertebræ, the lumbar muscles, and the lumbar part of the diaphragm.

The =lateral walls= are formed by the oblique and transverse abdominal muscles, the abdominal tunic, the anterior parts of the ilia, the cartilages of the asternal ribs, and the parts of the posterior ribs which are below the attachment of the diaphragm.

The =ventral wall= or floor consists of the two recti, the aponeuroses of the oblique and transverse muscles, the abdominal tunic, and the xiphoid cartilage.

The =anterior wall= is formed by the diaphragm, which is very deeply concave, thus greatly increasing the size of the abdomen at the expense of the thorax.

It should be noted that the diaphragm also concurs practically in the formation of a considerable part of the lateral walls, since its costal portion even during ordinary inspiration lies directly on the ribs over a width of four or five inches (ca. 10 to 12 cm.); in expiration this area of contact would be about twice as wide, including about all of the fleshy rim. This fact is of clinical importance, with reference to auscultation and percussion, and penetrating wounds. The cupola of the diaphragm extends as far forward as a plane through the sixth intercostal space to the right of the heart.

There is no wall between the abdominal and pelvic cavities. The line of demarcation here is the =terminal line= (Linea terminalis) or brim of the pelvis; it is formed by the base of the sacrum, the ilio-pectineal lines, and the anterior borders of the pubic bones.

The muscular walls are lined by a layer of fascia, distinguished in different parts as: (1) the diaphragmatic fascia; (2) the transversalis fascia; (3) the iliac fascia; (4) the deep layer of the lumbo-dorsal fascia.

The subperitoneal or extraperitoneal connective tissue (Tela subserosa) unites the fascia and peritoneum. It is composed of areolar tissue, more or less loaded with fat according to the condition of the subject, except over the diaphragm. It sends laminæ into the various peritoneal folds.

The peritoneum, the serous membrane which lines the cavity, will be described later.

The abdominal walls are pierced in the adult by =five apertures=. These are: the three openings in the diaphragm which transmit the aorta, posterior vena cava, and the œsophagus; the inguinal canals, which contain the spermatic cord or the round ligament (in female carnivora). In the fœtus there is the umbilical opening also.

The cavity contains the greater part of the digestive and urinary organs, part of the internal generative organs, numerous nerves, blood-vessels, lymph vessels and glands, ductless glands (spleen and adrenal bodies), and certain fœtal remains.

For topographic purposes the abdomen is divided into nine regions by imaginary planes.[88] Two of these planes are sagittal, and two are transverse. The sagittal planes cut the middles of the inguinal (Poupart’s) ligaments; the transverse planes pass through the last thoracic and fifth lumbar vertebræ, or the lower end of the fifteenth rib and the external angle of the ilium respectively. The transverse planes divide the abdomen into three zones, one behind the other, viz., =epigastric=, =mesogastric=, and =hypogastric=: these are subdivided by the sagittal planes as indicated in the subjoined table.

Left parachondriac Xiphoid Right parachondriac Left lumbar Umbilical Right lumbar Left iliac Prepubic Right iliac.

Other useful regional terms are: sublumbar, diaphragmatic, inguinal. The first two require no explanation. The inguinal regions (right and left) lie in front of the inguinal (Poupart’s) ligament. The flank is that part of the lateral wall which is formed only of soft structures. The depression on its upper part is termed the paralumbar fossa.

THE PERITONEUM[89]

The peritoneum is the thin serous membrane which lines the abdominal and (in part) the pelvic cavity, and covers to a greater or less extent the viscera contained therein. In the male it is a completely closed sac, but in the female there are two small openings in it; these are the abdominal orifices of the Fallopian tubes, which at their other ends communicate with the uterus, and so indirectly with the exterior. The peritoneal cavity is only a potential one, since its opposing walls are normally separated only by the thin film of serous fluid (secreted by the membrane) which acts as a lubricant.

The free surface of the membrane has a glistening appearance and is very smooth. This is due to the fact that this surface is formed by a layer of flat endothelial cells, and is moistened by the peritoneal fluid. Friction is thus reduced to a minimum during the movements of the viscera. The outer surface of the peritoneum is related to the subperitoneal tissue, which attaches it to the abdominal wall or the viscera.

In order to understand the general disposition of the peritoneum, we may imagine the abdominal cavity to be empty and lined by a simple layer of peritoneum, termed the =parietal layer= (Lamina parietalis). We may regard the organs as beginning to develop in the subperitoneal tissue, enlarging, and migrating into the cavity to a varying extent. In doing so they carry the peritoneum before them, producing introversion of the simple sac, and forming folds which connect them with the wall or with each other. The viscera thus receive a more or less complete covering of peritoneum, termed the =visceral layer= (Lamina visceralis). The connecting folds are termed =omenta=, =mesenteries=, =ligaments=, etc. They contain a varying quantity of connective tissue, fat and lymph glands, and furnish a path for the vessels and nerves of the viscera. Some contain unstriped muscular tissue. An omentum is a fold which passes from the stomach to other viscera. There are three of these, namely: (1) the =small= or =gastro-hepatic omentum= (Omentum minus), which passes from the lesser curvature of the stomach to the liver; (2) the =gastro-splenic omentum= (Ligamentum gastrolienale), which extends from the greater curvature of the stomach to the spleen; (3) the =great omentum= (Omentum majus), which passes from the greater curvature of the stomach and from the spleen to the terminal part of the great colon and the origin of the small colon. It does not pass directly from one organ to the other, but forms an extensive loose sac (Figs. 278, 279). A =mesentery= (Mesenterium) is a fold which attaches the intestine to the dorsal wall of the abdomen. There are two mesenteries, namely: (1) the =great mesentery= which connects the greater part of the small intestine with the dorsal abdominal wall; (2) the =colic mesentery=, which attaches the small colon to the dorsal abdominal wall. =Ligaments= are folds which pass between viscera other than parts of the digestive tube, or connect them with the abdominal wall. The term is also applied to folds which attach parts of the digestive tract to the abdominal wall, but do not contain their blood-vessels and nerves. In some cases (_e. g._, the lateral and coronary ligaments of the liver) they are strengthened by fibrous tissue; in other cases (_e. g._, the broad ligaments of the uterus) they contain also unstriped muscular tissue.

THE PELVIC CAVITY

The pelvis is the posterior part of the trunk. It incloses the pelvic cavity (Cavum pelvis), which communicates in front with the abdominal cavity, the line of demarcation being the pelvic brim or terminal line.

The =dorsal wall= or roof is formed by the sacrum and first three coccygeal vertebræ. The =lateral walls= are formed by the parts of the ilia behind the ilio-pectineal lines and the sacro-sciatic ligaments. The =ventral wall= or floor is formed by the pubic and ischial bones. The boundary of the =outlet= is formed by the third coccygeal vertebra dorsally, the ischial arch ventrally, and the posterior edges of the sacro-sciatic ligaments and the semimembranosus muscles laterally. The outlet is closed by the perineal fascia; this consists of superficial and deep layers, which are attached around the margin of the outlet and centrally to the organs at the outlet—the anus and its muscles, the vulva (in the female), and the root of the penis (in the male).

The cavity contains the rectum, parts of the internal generative and urinary organs, some fœtal remnants, muscles, vessels, and nerves. It is lined by the fascia pelvis, and in part by the peritoneum.

The =pelvic peritoneum= is continuous in front with that of the abdomen. It lines the cavity as far back as the third or fourth sacral vertebra in the horse, where it is reflected on to the viscera, and from one organ to another. We may therefore distinguish an anterior, peritoneal, and a posterior, retroperitoneal part of the cavity. Along the mid-dorsal line it forms a continuation of the colic mesentery, the =mesorectum=, which attaches the first or peritoneal part of the rectum to the roof. In animals in fair condition a considerable quantity of subperitoneal and retroperitoneal fat is found on the walls and in the various interstices.

[Illustration:

FIG. 256.—DIAGRAM OF SAGITTAL SECTION OF MALE PELVIS TO SHOW DISPOSITION OF PERITONEUM.

_a_, Pouch between rectum and roof of cavity, continuous laterally with _b_, recto-genital pouch; _c_, vesico-genital pouch; _d_, pouch below bladder and its lateral ligaments. The lateral line of reflection of the peritoneum is dotted. The area of rectum covered by peritoneum varies widely. When the rectum is empty, the reflection dorsally may be at the posterior end of the sacrum; when the rectum is very full, the reflection may occur a short distance behind the promontory. ]

[Illustration:

FIG. 257.—SCHEMATIC CROSS-SECTIONS TO SHOW ARRANGEMENT OF PELVIC PERITONEUM OF HORSE: A, IN MALE; B, IN FEMALE.

_A_: _a_, _b_, Recto-genital pouch, _c_, _c_, vesico-genital pouch; _d_, _d_, pouch below bladder and its lateral ligaments; _1_, mesorectum; _2_, _2_, urogenital fold; _3_, _3_, lateral, _4_, median ligaments of bladder; _v. d._, vas deferens; _u. m._, uterus masculinus. _B_: _a_, _b_, recto-genital pouch; _c_, _c_, vesico-genital pouch; _d_, _d_, pouch below bladder and its lateral ligaments; _1_, mesorectum; _2_, _2_, broad ligaments of uterus; _3_, _3_, lateral, _4_, median ligaments of bladder. ]

In the male the general disposition of the peritoneum here is as follows. If traced along the dorsal wall, it is reflected at the third or fourth sacral vertebra on to the rectum, forming the visceral peritoneum for the first part of that tube. Laterally it is reflected in a similar fashion. If the rectum be raised, it will be seen that the peritoneum passes from its ventral surface and forms a transverse fold which lies on the dorsal surface of the bladder (Fig. 272). This is the =urogenital fold= (Plica urogenitalis). Its concave free edge passes on either side into the inguinal canal. The ventral layer of this fold is reflected on to the dorsal surface of the bladder near its neck. Thus there is formed a pouch between the rectum and bladder—the =recto-vesical pouch= (Excavatio recto-vesicalis), which is partially subdivided by the urogenital fold into recto-genital and vesico-genital cavities. The fold contains the vasa deferentia, part of the vesiculæ seminales, and the uterus masculinus (a fœtal remnant). The space on either side of the rectum is occupied by coils of the small colon and the pelvic flexure of the great colon usually. If the bladder is now raised, it is seen that the peritoneum passes from its ventral surface on to the pelvic floor, forming a median fold, the so-called =middle ligament= (Plica umbilicalis media). It also forms on each side a lateral fold, the =lateral ligament= (Plica umbilicalis lateralis), which contains in its edge the so-called =round ligament= (Ligamentum teres)—the partially occluded umbilical artery, which is a large vessel in the fœtus.

[Illustration:

FIG. 258.—STOMACH OF HORSE, PARIETAL SURFACE, WITH FIRST PART OF DUODENUM.

Fixed _in situ_ when full but not distended. The larger branches of the anterior gastric artery with two satellite veins are shown. ]

In the female the arrangement is modified by the presence of the uterus; the urogenital fold is very large, so as to inclose the uterus and a small part of the vagina. It forms two extensive folds, the =broad ligaments of the uterus= (Ligamenta lata uteri), which attach that organ to the sides of the pelvic cavity and the lumbar part of the abdominal wall (Fig. 271). It thus divides the recto-vesical pouch completely into dorsal and ventral compartments—the =recto-genital pouch= (Excavatio recto-uterina), and the =vesico-genital= pouch (Excavatio vesico-uterina).

Further details will be given in the description of the pelvic viscera.

THE STOMACH

The stomach (Ventriculus) is the large dilatation of the alimentary canal between the œsophagus and the small intestine. It is a sharply curved, U-shaped sac, the right branch being, however, much shorter than the left one. The convexity is directed ventrally. When moderately distended, there is often a slight constriction which indicates the division into right and left sacs. It is relatively small, and is situated in the dorsal part of the abdominal cavity behind the diaphragm and liver, mainly to the left of the median plane.

[Illustration:

FIG. 259.—STOMACH OF HORSE, VISCERAL SURFACE, WITH FIRST PART OF DUODENUM.

Fixed _in situ_ when full but not distended. The posterior gastric artery and its larger branches with two satellite veins are shown. ]

It presents for description two surfaces, two curvatures, and two extremities. The =parietal surface= (Facies parietalis) is convex and is directed forward, upward, and toward the left; it lies against the diaphragm and liver. The =visceral surface= (Facies visceralis), also convex, faces in the opposite direction; it is related to the terminal part of the large colon, the pancreas, the small colon, and the small intestine. The =lesser curvature= (Curvatura minor) is very short, extending from the termination of the œsophagus to the junction with the small intestine. When the stomach is _in situ_, its walls are here in contact, and the cardia and pylorus close together. The =greater curvature= (Curvatura major) is very extensive. From the cardia it is first directed upward and curves over the left extremity; it then descends, passes to the right, crosses the median plane, and curves upward to end at the pylorus. Its left portion is related to the spleen, while its ventral portion rests on the left divisions of the great colon. The =left extremity= or =saccus cæcus= is a rounded cul-de-sac which lies under the upper ends of the fourteenth, fifteenth, and sixteenth ribs and the diaphragm.[90] It is related to the pancreas behind and the base of the spleen externally. The =right= or =pyloric extremity= is much smaller and is continuous with the duodenum, the junction being indicated by a marked constriction. It lies on the liver, a little to the right of the median plane, and a little lower than the cardiac opening. About two or three inches (ca. 5 to 8 cm.) from the pylorus there is a constriction which marks off the =antrum pylori= from the rest of the right sac. The =œsophageal orifice= or =cardia= is situated at the left extremity of the lesser curvature, but about eight to ten inches (ca. 20 to 25 cm.) from the left extremity. The œsophagus joins the stomach very obliquely. The opening is closed by the sphincter cardiæ and numerous folds of mucous membrane. The =pyloric orifice= communicates with the duodenum. Its position is indicated externally by a distinct constriction. Internally it presents a circular ridge produced by a ring of muscular tissue—the =sphincter pylori=.

The stomach is held in position mainly by the pressure of the surrounding viscera and by the œsophagus. The following peritoneal folds connect it with the adjacent parts:

1. The =gastro-phrenic ligament= (Lig. gastrophrenicum) connects the great curvature, from the cardia to the left extremity, with the crura of the diaphragm. This leaves a narrow area uncovered with peritoneum, and here the stomach is attached to the diaphragm by loose connective tissue.

2. The =small= or =gastro-hepatic omentum= (Omentum minus) connects the lesser curvature and the first part of the duodenum with the liver below the œsophageal notch and the portal fissure.

3. The =gastro-splenic omentum= (Lig. gastrolienale) passes from the left part of the great curvature to the hilus of the spleen.

4. The =great= or =gastro-colic omentum= (Omentum majus) connects the ventral part of the great curvature and the first curve of the duodenum with the terminal part of the great colon and the initial part of the small colon.

5. The =gastro-pancreatic fold= (Plica gastro-pancreatica) extends from the left sac above the cardia to the duodenum. It is attached dorsally to the liver and vena cava, ventrally to the pancreas.

[Illustration:

FIG. 260.—EVERTED STOMACH OF HORSE FROM WHICH THE MUCOUS MEMBRANE HAS BEEN REMOVED.

_O_, Œsophagus; _D_, duodenum; _b_, circular layer; _c′_, internal oblique fibers; _c″_, loop around cardia; _c‴_, transition of internal to external oblique fibers; _d_, fibers connecting the two branches of the cardiac loop; _p_, antral sphincter; _p′_, pyloric sphincter. (Ellenberger-Baum, Anat. d. Haustiere.) ]

The stomach of the equidæ is relatively small, its capacity varying from two to four gallons (ca. 8 to 15 liters).

The size, form, and position of the stomach are subject to considerable variation. When the stomach is nearly empty the saccus cæcus contains only gas and is strongly contracted; the middle portion (physiological fundus) contains the ingesta and preserves its rounded character, while the pyloric portion is contracted. When distended the middle portion settles down some four or five inches, pushing back coils of the small colon and small intestine which may lie between the great curvature and the large colon, and also pushing to the left or right the left dorsal part of the great colon; the spleen, small colon, and small intestines are pushed back by the distention of the left sac.

=Structure.=—The wall is composed of four coats—serous, muscular, submucous, and mucous.

The =serous coat= (Tunica serosa) covers the greater part of the organ and is closely adherent to the muscular coat except at the curvatures. It partially bridges over the lesser curvature, and covers here elastic tissue which assists in retaining the bent form of the stomach. The peritoneal folds have been described.

The =muscular coat= consists of three incomplete layers, an external of longitudinal, a middle of circular, and an internal of oblique fibers. The layer of =longitudinal fibers= (Stratum longitudinale) is very thin and exists only along the curvatures and at the antrum. At the lesser curvature it is continuous with the longitudinal fibers of the œsophagus. On the antrum pylori it forms a well developed complete layer. The layer of =circular fibers= (Stratum circulare) exists only on the right sac. At the pyloric orifice it forms a thick ring—the =pyloric sphincter=. Another ring, the antral sphincter, is found at the left end of the antrum pylori. The =oblique fibers= (Fibræ obliquæ) are arranged in two layers; the external stratum covers the left sac and is a continuation (in part) of the longitudinal fibers of the œsophagus; the internal stratum is found also on the left sac, and exchanges fibers with the circular and external oblique layers. It forms a remarkable loop around the cardiac orifice, constituting a powerful =cardiac sphincter= (Sphincter cardiæ).

[Illustration:

FIG. 261.—FRONTAL SECTION OF STOMACH AND FIRST PART OF DUODENUM OF HORSE.

_C_, Cardiac orifice. Photograph of specimen fixed _in situ_. ]

The =submucous coat= is a layer of loose connective tissue which connects the muscular and mucous coats; in it the vessels and nerves ramify before entering the mucosa.

The =mucous coat= is clearly divided into two parts. That which lines the left sac resembles the œsophageal mucous membrane, and is termed =œsophageal= or =cuticular=. It is white in color, destitute of glands, and covered with a thick, squamous, stratified epithelium. At the cardiac orifice it presents numerous folds which occlude the opening.[91] It terminates abruptly at an elevated, denticulated, sinuous line, termed the =cuticular ridge= (Margo plicatus). Below and to the right of this line the mucous membrane has a totally different character, being soft and velvety to the touch, and covered by a mucoid secretion. It is glandular, and three zones may be recognized, although no sharp line of demarcation exists. A narrow zone next to the cuticular ridge has a yellowish-gray color, and contains short tubular =cardiac glands= (Cardiac gland region). Next to this is a large area which has a mottled reddish-brown color, and contains =fundus glands= (fundus gland region). This part of the mucous membrane is thick and very vascular, and corresponds to the fundus of the stomach in man and the dog. The remainder of the mucous membrane has a reddish-gray color and contains branched, tubular, =pyloric glands= (pyloric gland region); it corresponds to the pyloric portion of man and the dog.

The folding of the stomach wall at the lesser curvature produces a prominent ridge which projects into the cavity of the stomach. Circular ridges occur at the antral and pyloric sphincters.

=Blood-vessels and Nerves.=—The stomach receives blood from all the branches of the cœliac artery. The gastric veins drain into the portal vein. The nerves are derived from the vagus and sympathetic nerves.

[Illustration:

FIG. 262.—DIAGRAM OF ZONES OF MUCOUS MEMBRANE OF STOMACH OF HORSE. ]

THE SMALL INTESTINE

The =small intestine= (Intestinum tenue) is the tube which connects the stomach with the large intestine. It begins at the pylorus and terminates at the lesser curvature of the cæcum. Its average length is about seventy feet (ca. 22 meters). When distended its diameter varies from two to four inches (5 to 10 cm.). Its capacity is about twelve gallons (40 to 50 liters).