Chapter V: Muscles of the Leg and Foot (3)

The =mucous membrane= of the pharynx is continuous with that of the several cavities which open into it. It is thin and closely adherent to the base of the skull in the vicinity of the posterior nares, where the muscular wall is absent. Behind the Eustachian openings is a median cul-de-sac, the =pharyngeal recess=. The recess is somewhat variable, but is usually about an inch in depth and will admit the end of the finger. In the ass and mule it is much deeper. Here also the muscular wall is absent and the mucous membrane lies against the guttural pouches. From the Eustachian opening a fold of the mucous membrane (Plica salpingo-pharyngea) passes toward but does not reach the laryngeal opening. Below, a horizontal fold, the =posterior pillar of the soft palate= (Arcus pharyngopalatinus), passes along the lateral wall and unites with its fellow over the entrance to the œsophagus. The upper part of the cavity (the naso-pharynx) is lined with a ciliated epithelium, while the lower part (oro-pharynx) has a stratified squamous epithelium. The communication between the two is oval and is bounded by the free edge of the soft palate and its posterior pillars; it is termed the =pharyngeal isthmus=. On either side of the laryngeal opening is a narrow deep depression, the =pyriform sinus= (Recessus piriformis).

The submucous tissue contains numerous mucous glands (Glandulæ pharyngeæ). In the young subject the lymph follicles are numerous and form a collection dorsally and between the Eustachian openings, known as the pharyngeal tonsil.

=Blood-supply.=—External carotid, external maxillary, and thyro-laryngeal arteries.

=Nerve-supply.=—Glosso-pharyngeal, vagus, and sympathetic nerves.

THE ŒSOPHAGUS

The œsophagus is a musculo-membranous tube, about 50 to 60 inches (ca. 125 to 150 cm.) in length, which extends from the pharynx to the stomach. It begins in the median plane above the cricoid cartilage of the larynx. In its course it shows several changes of direction. At the level of the fourth cervical vertebra it inclines to the left side of the trachea, and continues this relation to the level of the third thoracic vertebra. Here it again gains the dorsal surface of the trachea, and passing backward, crosses the left bronchus, being here almost in the median plane. It continues in the mediastinum between the lungs backward, upward, and a little to the left, to reach the hiatus œsophageus of the diaphragm. Passing through this it terminates at once at the cardiac orifice of the stomach, at the level of the fourteenth thoracic vertebra, a little to the left of the median plane, and about four or five inches (ca. 10 to 12 cm.) ventral to the vertebral column.

Viewed with reference to the frontal plane, its course is downward and backward till it enters the thorax and passes upward to gain the dorsal face of the trachea. For a short distance (_i. e._, to the root of the lung) its direction is almost horizontal; behind this it passes somewhat upward to its termination. The =cervical part= of the tube is about four to six inches (10 to 15 cm.) longer than the =thoracic part=, while the so-called abdominal part is about an inch (2 to 3 cm.) long.[85]

FIG. 255.—CROSS-SECTION OF HEAD OF HORSE.

The section passes through the temporo-mandibular articulation, but is
slightly oblique. _1_, Corpus callosum; _2_, lateral ventricle of
brain; _3_, caudate nucleus; _4_, internal capsule; _5_, lenticular
nucleus; _6_, optic chiasma; _7_, middle cerebral artery; _8_,
sphenoidal sinuses; _9_, cavernous sinus; _10_, Eustachian tube,
inner lamina; _11_, _11_, guttural pouches; _12_, soft palate; _13_,
epiglottis; _14_, hyo-epiglottic muscle; _15_, thyro-hyoid muscle.
]

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.

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.
]

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.

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.

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.

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).

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.

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).

It is clearly divisible into a =fixed= and a =mesenteric= or floating portion. The fixed portion is termed the duodenum, while the mesenteric portion (Intestinum tenue mesenteriale) is arbitrarily divided into parts termed the jejunum and ileum.[92]

The =duodenum= is about three to four feet (ca. 1 to 1.25 m.) long. Its shape is somewhat like a horseshoe, the convexity being directed toward the right. The =first part= is directed to the right and forms an ᔕ-shaped curve. The convexity of the first part of the curve is dorsal, of the second ventral. It lies on the middle and right lobes of the liver, and presents two dilatations (Ampullæ) with a constriction between them. The duodenal angle or head of the pancreas lies in the concavity of the second curve, and here, five to six inches (ca. 12 to 15 cm.) from the pylorus, the pancreatic duct and the bile-duct pierce the bowel wall. The =second part= passes upward and backward on the right lobe of the liver and, on reaching the right kidney and the base of the cæcum, it curves toward the median plane, opposite the last rib. The =third part= passes almost transversely from right to left behind the base of the cæcum, crosses the median plane under the third and fourth lumbar vertebræ, and turns forward to become continuous with the jejunum under the left kidney. The sacculations of the first part have a diameter of three to four inches (ca. 7.5 to 10 cm.).

It is attached by a short peritoneal fold termed the =mesoduodenum=. This fixes the first part of the duodenum closely to the liver and the right dorsal part of the colon; the remainder is somewhat less closely attached by it to the cæcum and right kidney, the sublumbar muscles, and (more closely) to the terminal part of the great colon and the first part of the small colon.[93]

FIG. 263.—TOPOGRAPHY OF VISCERA OF HORSE, LEFT VIEW.

_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.

FIG. 264.—DIAGRAM OF SECTION OF DIVERTICULUM DUODENI OF HORSE.

Solid line indicates mucous membrane.
]

The =submucous coat= is a layer of areolar tissue in which the vessels and nerves ramify. It contains also the duodenal glands and the bases of the solitary glands and Peyer’s patches.

The =mucous membrane= is soft and velvety. It has a grayish or yellowish-red color and is very vascular. About five or six inches from the pylorus it forms a pouch, the =diverticulum duodeni=, in which the pancreatic and hepatic ducts open. On a small papilla opposite this is the termination of the accessory pancreatic duct. At the ileo-cæcal opening the mucous membrane projects slightly into the cavity of the cæcum, forming the =ileo-cæcal valve=. The free surface is thickly beset with villi, small projections of the mucous membrane which can be seen well by placing a piece of the membrane in water. They are relatively short and thick and have a distinct neck in the horse. Each contains a central lymph vessel (lacteal), and around this a plexus of capillaries, lymphoid tissue, and unstriped muscle-fibers. They are important agents in absorption from the contents of the intestine. The epithelium is columnar, with many goblet cells. Underneath the basement membrane is a layer of unstriped muscle-fibers, the =muscularis mucosæ=.

The glands of the small intestine are of three kinds:

1. The =intestinal glands= (Glandulæ intestinales Lieberkuehni) are found throughout. They are simple tubular glands which open between the villi.

2. The =duodenal glands= (Glandulæ duodenales Brunneri) are found in the first twenty feet (ca. 6 meters) of the bowel. They are racemose glands, and are situated in the submucosa, so that their ducts perforate the muscularis mucosæ and the mucous membrane.

3. The =lymph follicles= (Noduli lymphatici) are found either scattered or in groups. In the former case they are termed =solitary glands= (Noduli lymphatici solitarii), in the latter =Peyer’s patches= (Noduli lymphatici aggregati). The solitary glands are about the size of a millet-seed or a small sago grain. Peyer’s patches are situated chiefly along the surface opposite to the mesenteric attachment and begin about three or four feet from the pylorus. They number one to two hundred, and are usually one to two inches (2 to 5 cm.) long and a quarter of an inch to one-half inch (ca. 2 to 14 mm.) wide. Larger ones occur in the terminal part, where one patch may have a length of seven to fifteen inches (ca. 17 to 38 cm.) and a width of half an inch to one inch (ca. 5 to 25 mm.) in young horses (Ellenberger). They undergo atrophy in old subjects.

FIG. 265.—PEYER’S PATCHES OF SMALL INTESTINE OF HORSE.
]

=Vessels and Nerves.=—The =arteries= of the small intestine come from the cœliac and anterior mesenteric arteries. The =veins= go to the portal vein. The =lymph vessels= are numerous and go to the mesenteric lymph glands. The =nerves= are derived from the vagus and sympathetic through the solar plexus.

THE LARGE INTESTINE

The large intestine (Intestinum crassum) extends from the termination of the ileum to the anus. It is about twenty-five feet (ca. 7.5 to 8 in.) in length. It differs from the small intestine in its greater size, in being sacculated, for the most part, possessing longitudinal bands, and having a more fixed position. It is divided into cæcum, great colon, small colon, and rectum.

THE CÆCUM

The =cæcum= (Intestinum cæcum) is a great cul-de-sac intercalated between the small intestine and the colon. It has a remarkable size, shape, and position in the horse. Its length is three to four feet (ca. 1 to 1.25 m.), and its capacity about seven to eight gallons (ca. 25 to 30 liters). It is conical in form, and is curved somewhat like a reversed comma. It is situated almost entirely to the right of the median plane, extending from the right iliac and sublumbar regions to the abdominal floor behind the xiphoid cartilage. Both extremities are blind, and the two orifices are placed close together on the concave curvature. It presents for description a base, a body, and an apex.

The =base= (Saccus cæcus) extends from about the thirteenth intercostal space backward almost to the pelvic inlet. Its greater curvature is dorsal, its lesser ventral; connected with the latter are the termination of the ileum and the origin of the colon. The =body= (Corpus cæci) extends downward and forward from the base and rests largely on the ventral wall of the abdomen. Its lesser curvature is about parallel with the costal arch and about five to six inches (10 to 15 cm.) below it. The =apex= (Apex cæci) lies usually on the abdominal floor about a hand’s length behind the xiphoid cartilage.

The base is attached dorsally by connective tissue and peritoneum on the ventral surface of the pancreas and right kidney, the psoas muscles, and the iliac fascia; internally, it is attached to the terminal part of the great colon, and ventrally to the origin of the great colon. The body is attached dorsally to the first part of the colon by the cæco-colic fold. The apex is free, and consequently may vary in position.

FIG. 266.—DIAGRAM OF CÆCUM AND LARGE COLON OF HORSE.
]

The cæcum has four =longitudinal bands= (Tæniæ), situated on the dorsal, ventral, right, and left surfaces; these cause four rows of =sacculations= (Haustra). The ventral band is entirely exposed or free (Tænia libera); the dorsal band is free on the apex. The cæcal arteries are placed on the other two. The right or parietal surface of the cæcum is related chiefly to the right abdominal wall, the diaphragm, duodenum, and liver. The left or visceral surface lies against the left divisions of the colon, the root of the great mesentery, and the small intestine.

The =ileo-cæcal orifice= (Ostium ileocæcale) is situated in the lesser curvature of the base, about four or five inches (ca. 10 to 12 cm.) to the right of the median plane and about opposite the lower end of the last rib. The end of the ileum is partially telescoped into the cæcum, so that the orifice is surrounded by a fold of mucous membrane, forming the =ileo-cæcal valve= (Valvula ileocæcalis). The peritoneum and longitudinal muscle-fibers do not take part in its formation.

The =cæco-colic orifice= (Ostium cæcocolicum) is placed above and external to the preceding one; the interval between them is only about two inches (ca. 5 cm.), and they are separated by a distinct ridge which projects into the interior of the cæcum. The orifice is slit-like and is small in relation to the size of the cæcum and colon. It has a =valvular fold= (Valvula cæcocolica) at its lower margin and a =muscular ring= (Sphincter cæci). Large crescentic or semilunar folds (Plicæ cæci) project into the cavity of the bowel, and between these are large pouches (Cellulæ cæci).

FIG. 267.—TOPOGRAPHY OF VISCERA OF HORSE, RIGHT VIEW.

_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._, right lung; _H._,
pericardium; _Z._, diaphragm (pars costalis); _r.v.C._, right
ventral colon; _v.Q._, sternal flexure of colon; _d.Q._,
diaphragmatic flexure of colon; _C._, body of cæcum; _C′._, base of
cæcum; _C″._, apex of cæcum; _D._, small intestine; _F._, pelvic
flexure of colon; _M._, rectum; _a._, abdominal wall in section;
_b_, duodenum; _c_, coccygeus muscle; _d_, retractor ani; _e_, _e′_,
sphincter ani externus; _f_, anterior gluteal artery; _g_, internal
pudic artery; _h_, xiphoid cartilage. (After Ellenberger, in
Leisering’s Atlas.)
]

=Vessels and Nerves.=—The cæcal arteries come from the great mesenteric artery. The veins go to the portal vein. The nerves are derived from the great mesenteric plexus of the sympathetic.

THE GREAT COLON

FIG. 268.—ILEO-CÆCAL ORIFICE OF HORSE.

_a_, Mucous membrane of cæcum; _b_, ileum; _c_. ileo-cæcal orifice;
_d_, fold of mucous membrane. (After P. Schumann.)
]

The =great colon= (Colon crassum) begins at the cæco-colic orifice, and terminates by joining the small colon behind the saccus cæcus of the stomach. It is ten to twelve feet (ca. 3 to 3.7 m.) long, and its average diameter (exclusive of its narrowest part) is about eight to ten inches (ca. 20 to 25 cm.). Its capacity is more than double that of the cæcum. When removed from the abdomen, it consists of two parallel portions, which are connected by peritoneum and partially by areolar tissue also. _In situ_, it is folded so that it consists of four parts, which are designated according to their position or numerically. The three bent connecting parts are termed the flexures. The first, =right ventral part= (Colon ventrale dextrum), begins at the cæco-colic orifice about opposite the middle of the last rib, passes downward and forward along the right costal arch, and reaches the floor of the abdomen. Over the xiphoid cartilage it bends sharply to the left and backward, forming the =sternal flexure= (Flexura diaphragmatica ventralis). The second, =left ventral part= (Colon ventrale sinistrum), passes backward on the left part of the abdominal floor, and, on reaching the pelvic inlet, bends sharply dorsally and forward, forming the =pelvic flexure= (Flexura pelvina). This is continued by the third, =left dorsal part= (Colon dorsale sinistrum), which passes forward above the left ventral portion. On reaching the stomach, diaphragm, and left lobe of the liver, it turns to the right, forming the =diaphragmatic= or =gastro-hepatic flexure= (Flexura diaphragmatica dorsalis). The fourth or =right dorsal part= (Colon dorsale dextrum) passes backward above the first portion, and on reaching the inner or left surface of the base of the cæcum it turns upward and to the left behind the left sac of the stomach; here it becomes constricted, and joins the small colon below the left kidney.

FIG. 269.—ABDOMINAL VISCERA OF HORSE, VENTRAL VIEW.

The ventral wall and part of the lateral walls of the abdomen are
removed. _C_, Cæcum; _r.v.C._, right ventral part of colon; _v.Q._,
sternal flexure of colon; _l.v.C._, left ventral part of colon;
_d.Q._, diaphragmatic flexure of colon; _D_, small intestine; _M_,
small colon; _a_, ventral free band of cæcum; _b_, inner band of
cæcum; _c_, lateral band of ventral part of colon; _d_, ventral band
of ventral part of colon; _e_, point of elbow; _f_, anterior end of
sternal region; _g_, xiphoid cartilage; _h_, teats. (After
Ellenberger-Baum, Top. Anat. d. Pferdes.)
]

The =caliber= of the great colon varies greatly at different points. At its origin it is only about two to three inches (ca. 5 to 7.5 cm.) in diameter. This soon increases to about eight to ten inches (ca. 20 to 25 cm.) for the ventral portions. Beyond the pelvic flexure the diameter is reduced to about three or four inches (ca. 8 to 9 cm.). Near the diaphragmatic flexure the caliber rapidly increases, and reaches its maximum in the last division, where it may be about twenty inches (50 cm.) in its widest part. This is succeeded by a somewhat funnel-shaped terminal contraction.

FIG. 270.—TOPOGRAPHY OF VISCERA OF HORSE, RIGHT SIDE, DEEPER VIEW.

_1.R._, First thoracic vertebra; _1.L._, first lumbar vertebra; _2.
K._, second sacral spine; _S_, scapula; _A_, humerus; _B_, ilium;
_O._, femur; _Sch._, pubis; _Si._, ischium; _St._, sternum; _L._,
right lobe of liver; _r.N._, right kidney; _C._, body, _C′._, base,
_C″._, apex of cæcum; _r.v.C._, right ventral colon; _v.Q._, sternal
flexure of colon; _d.Q._, diaphragmatic flexure of colon; _r.d.C._,
right dorsal colon; _F._, pelvic flexure of colon; _D._, small
intestine; _H._, urinary bladder; _M._, rectum; _a_, left ventricle;
_a′_, right ventricle; _b_, _b′_, right coronary artery; _c_, left
atrium; _c′_, right atrium; _d_, small (left) coronary vein; _e_,
vena azygos; _f_, anterior vena cava; _g_, posterior vena cava; _h_,
sinus venosus; _i_, right phrenic nerve; _k_, right vagus, with its
dorsal (_k′_) and ventral (_k″_) divisions; _l_, aorta; _m_,
œsophagus; _n_, trachea; _o_, diaphragm (median section); _p_, right
lateral ligament of liver; _q_, duodenum; _r_, dotted line
indicating position in median section of diaphragm in inspiratory
phase; _s_, recto-coccygeus; _t_, suspensory ligament of rectum;
_u_, sphincter ani; _v_, vesicula seminalis; _w_, prostate; _x_,
bulbo-urethral (Cowper’s) gland; _y_, urethra; _z_, abdominal wall;
_z′_, xiphoid cartilage. (After Ellenberger, in Leisering’s Atlas,
reduced.)
]

The first part of the great colon is attached to the lesser curvature of the cæcum by two layers of peritoneum which form the =cæco-colic fold=. The ventral parts of the colon are similarly connected to the dorsal parts of the same side by the mesocolon. The right portions are united also by areolar tissue and muscular fibers, the surface of contact being about four or five inches (ca. 10 to 12 cm.) wide; the left portions are attached to each other by a peritoneal fold wide enough to allow them to be drawn apart five or six inches (ca. 12 to 15 cm.) near the pelvic flexure. The terminal part of the colon is attached by peritoneum and areolar tissue to the ventral surface of the pancreas dorsally and to the base of the cæcum externally. It is connected indirectly with the diaphragm and liver by means of a fold derived from the right lateral ligament of the liver.

The =relations= are complex, but the more important facts are as follows: The ventral portions (first and second) have extensive contact with the abdominal wall ventrally and laterally. On the right side the colon is almost entirely excluded from contact with the flank by the cæcum; on the left side it has no contact with the upper part of the flank, being excluded here by coils of the small colon and small intestine. Dorsally the chief relations are to the stomach, duodenum, liver, pancreas, small colon, small intestine, aorta, vena cava, and portal vein. Since there are no transverse attachments of the right and left portions, and the latter have no attachment to the wall, they are subject to considerable displacement.[94] The pelvic flexure is variable in position, but usually it is directed to the right across the pelvic inlet.

FIG. 271.—TOPOGRAPHY OF VISCERA OF HORSE, LEFT DEEP VIEW.

_1R._, First thoracic vertebra; _1L._, first lumbar vertebra; _2K._,
second sacral spine; _S._, scapula; _A._, humerus; _B._, ilium;
_F._, femur; _Sch._, pubis; _Si._, ischium; _L._, liver (left lobe);
_Ma._, stomach, the posterior contour of which is indicated by
dotted line _x_; _Mi._, spleen; _l.N._, left kidney, concealed part
indicated by dotted line; _M._, small colon; _D._, small intestine,
parts of which have been removed; _l. d. C._, left dorsal colon;
_l.v.C._, left ventral colon; _v.Q._, sternal flexure; _d.Q._,
diaphragmatic flexure; _O._, left ovary; _U._, cornu uteri; _L.1._,
broad ligament; _M′._, rectum; _V._, vagina; _H._, bladder; _a_,
left ventricle; _a′_, right ventricle; _b_, left coronary artery
with descending (_b′_) and circumflex (_b″_) branches; _c_, left
auricle; _d_, pulmonary artery (cut); _e_, aorta; _f_, ligamentum
arteriosum; _g_, brachiocephalic trunk (anterior aorta); _h_,
trachea; _i_, œsophagus; _k_, left phrenic nerve; _l_, diaphragm in
median section; _m_, Fallopian tube; _n_, bursa ovarica; _o_,
urethra; _p_, cut edge of broad ligament; _q_, line of reflection of
pelvic peritoneum; _r_, recto-coccygeus; _s_, so-called suspensory
ligament of rectum; _t_, sphincter ani internus; _u_,
sacro-coccygeus inferior; _v_, abdominal wall in section; _w_,
xiphoid cartilage. (After Ellenberger, in Leisering’s Atlas.)
]

The ventral portions of the colon have four longitudinal =muscular bands= (Tæniæ) which produce four rows of sacculations (Haustra). On the first part the external and internal bands are free; the dorsal band is covered by the adhesion to the overlying dorsal division, while the ventral band is largely covered by the adhesion to the cæcum. On the left ventral part the external and ventral bands are free; of the two dorsal bands, the inner one is concealed by the mesocolon, the outer one is chiefly free. The pelvic flexure has a band on its lesser curvature, which is continued on to the third part, concealed by the peritoneal attachment; these parts are practically non-sacculated. Near the diaphragmatic flexure two other bands appear, so that the flexure has three bands; two of these are dorsal and free, the third is ventral and concealed. The last part has three bands, of which the inner and outer ones are free, the ventral one covered.

THE SMALL COLON

The =small colon= (Colon tenue) begins at the termination of the great colon, behind the saccus cæcus of the stomach and below the left kidney, and is continued by the rectum at the pelvic inlet. Its length is about ten to twelve feet (ca. 3.5 m.), and its diameter about three to four inches (ca. 7.5 to 10 cm.).

FIG. 272.—PELVIC INLET AND POSTERIOR PART OF ABDOMINAL WALL OF HORSE,
VIEWED FROM THE FRONT.

The left inguinal canal is partially opened. The peritoneum is
retained except over a part of the left cremaster muscle. The
sublumbar region is greatly foreshortened in this view. _a_, _a′_,
Kidneys; _a″_, adrenal; _b._, ureter; _c_, renal artery; _d_, aorta;
_e_, cœliac artery (cut); _f_, anterior mesenteric artery (cut);
_g_, posterior mesenteric artery; _h_, circumflex iliac artery; _i_,
external iliac artery; _k_, internal iliac artery; _l_, spermatic
artery in peritoneal fold (plica vasculosa), _m_; _n_, vas deferens,
inclosed in urogenital fold, _o_; _p_, urinary bladder; _q_, round
ligament of bladder; _r_, middle ligament of bladder; _s_, rectum;
_t_, margin of vaginal ring; _u_, posterior abdominal artery; _v_,
external pudic artery; _w_, rectus abdominis muscle; _x_,
transversus and obliquus internus abdominis; _y_, _y′_, cremaster
externus; _z_, posterior vena cava (cut). (After Ellenberger-Baum,
Top. Anat. d. Pferdes.)
]

Its coils lie in the space between the stomach and the pelvic inlet, dorsal to the left portions of the great colon. They are mingled with those of the small intestine, from which they are easily distinguished by the tæniæ and sacculation.

It is attached to the sublumbar region by the =colic mesentery=, and to the termination of the duodenum by a short peritoneal fold (Lig. duodeno-colicum). The great omentum is also attached to the origin of the bowel. The colic mesentery is narrow at its origin, but soon reaches a width of about three feet (ca. 80 to 90 cm.). Its parietal border is attached along a line extending from the ventral surface of the left kidney to the sacral promontory; it is continuous in front with the root of the great mesentery, and behind with the mesorectum.

There are two =tæniæ= and two rows of sacculations. Of the tæniæ, one is free, the other concealed by the mesentery. When the bowel is hardened _in situ_ its lumen between the pouches is reduced to a narrow slit.

THE RECTUM

The =rectum= (Intestinum rectum) is the terminal part of the bowel; it extends from the pelvic inlet to the anus.[95] Its length is about one foot (ca. 30 cm.). Its direction may be straight or oblique. The first or =peritoneal part= of the rectum is like the small colon, and is attached by a continuation of the colic mesentery termed the =mesorectum=. The second or =retroperitoneal part= forms a flask-shaped dilatation termed the =ampulla recti=; it is attached to the surrounding structures by connective tissue and muscular bands.

The first part of the rectum is related to the small colon and the pelvic flexure of the great colon. It is frequently deflected to the left by the latter. The second part of the rectum is related dorsally and laterally to the pelvic wall. Ventrally the relations differ in the two sexes. In the male they are the bladder, the terminal parts of the vasa deferentia, the vesiculæ seminales, the prostate, the bulbo-urethral (Cowper’s) glands, and the urethra. In the female they are the uterus, vagina, and vulva.[96]

=Structure of the Large Intestine.=—The =serous coat= covers the different parts in varying degrees. It does not cover (_a_) the opposed surfaces of the cæcum and colon which are between the layers of the cæco-colic fold and mesocolon; (_b_) the areas of attachment of the cæcum and colon to the pancreas, right kidney, and sublumbar region; (_c_) the second part of the rectum.

The =muscular coat= consists of longitudinal and circular fibers. The bulk of the former is in the bands already described. Some of the circular fibers pass from one part of the colon to another, where they are attached to each other, forming the fibræ transversæ coli. The muscular coat of the second part of the rectum presents special features. The longitudinal layer of fibers is very thick and consists of large bundles, loosely united. A large band, the =recto-coccygeus=, is detached from it on either side, and passes upward and backward to be inserted into the fourth and fifth coccygeal vertebræ.

The =submucous tissue= is abundant in the wall of the rectum, so that the mucous membrane is loosely attached to the muscular coat, and forms numerous folds when the bowel is empty.

The =mucous membrane= of the large intestine is thicker and darker in color than that of the small intestine. It forms large crescentic or semilunar folds corresponding to the external constrictions. It has no villi, Brunner’s glands, or Peyer’s patches. The =intestinal glands= (of Lieberkühn) are large and numerous. =Solitary glands= are also numerous, especially at the apex of the cæcum and in the left dorsal part of the colon.