ALIMENTARY CANAL, in anatomy. The alimentary canal is the digestive tract from the mouth to the anus. It is some 25 to 3oft. long, and the food, in its passage, passes through the following parts one after the other : mouth, pharynx, oesoph agus, stomach, small intestines, caecum, large intestines, rectum and anus. Into this tube at various points the salivary glands, liver and pancreas pour their secretions by special ducts. As the mouth (q.v.) and pharynx (q.v.) are separately described, the detailed description will here begin with the oesophagus or gullet.
The oesophagus, a muscular tube lined with mucous membrane, stretches from the lower limit of the pharynx, at the level of the cricoid cartilage, to the cardiac orifice of the stomach. It is about loin. long (25 cm.) and lin. to 'in. in diameter. At first it lies in the lower part of the neck, then in the thorax, and last ly, for about an inch, in the abdomen. As far as the level of the fourth or fifth thoracic vertebra it lies behind the trachea, but when that tube ends, it is in close contact with the pericardium, and, at the level of the tenth thoracic vertebra, passes through the oesophageal opening of the diaphragm (q.v.), accompanied by the two vagi nerves, the left being in front of it and the right behind. In the abdomen it lies just behind the left lobe of the liver. Both in the upper and lower parts of its course it lies a little to the left of the mid line. Its mucous membrane is thrown into a number of longitudinal pleats to allow stretching.
The stomach is an irregularly pear-shaped bag, situated in the upper and left part of the abdomen. When moderately distended the thick end of the pear or fundus bulges upward and to the left, while the narrow end is constricted to form the py/orus, by means of which the stomach communicates with the small intestine. The cardiac orifice, where the oesophagus enters, is placed about a third of the way along the upper border from the left end of the fundus, and, between it and the pylorus; the upper border is concave and is known as the lesser curvature. From the cardiac to the pyloric orifice, round the lower border, is the greater cur vature. The stomach has in front of it the liver (see fig. I), the diaphragm and the anterior abdominal wall, while behind it are the pancreas, left kidney, left adrenal, spleen, colon and meso colon. When the stomach is empty it contracts into a tubular organ, and the transverse colon ascends to occupy the vacant space.
The pylorus is an oval opening, averaging tin. in its long axis but capable of considerable distension; it is formed by a special development of the circular muscle layer of the stomach, and dur ing life is tightly closed, except during the periodic escape of gastric contents into the duodenum. The mucous membrane of the stomach is thrown into pleats or rugae when the organ is not fully distended, while between these it has a mammillated ap pearance.
Superficial to the mucous coat is a submucous, consisting of loose connective tissue, while superficial to this are three coats of unstriped muscle, the inner oblique, the middle circular and the outer longitudinal. The peritoneal coat is described in the article on the coelom and serous membranes.
The small intestine is a tube, from 22 to 25ft. long, beginning at the pylorus and ending at the ileo-caecal valve; it is divided into duodenum, jejunum and ileum.
The duodenum is from 9 to r 'in. long and forms a horseshoe or C-shaped curve, encircling the head of the pancreas. It differs from the rest of the gut in being retroperitoneal. Its first part is horizontal and lies behind the fundus of the gall-bladder, pass ing backward and to the right from the pylorus. The second part runs vertically downward in front of the hilum of the right kid ney, and into this part the pancreatic and bile ducts open. The third part runs horizontally to the left in front of the aorta and vena cava, while the fourth part ascends to the left side of the second lumbar vertebra, after which it bends sharply downward and forward to form the duodeno- j e j unal flexure.
The jejunum forms the upper two-fifths of the rest of the small intestine; it, like the ileum, is thrown into numerous con volutions and is attached by the mesentery to the posterior ab dominal wall. (See COELOM and SEROUS MEMBRANES.) The ileum is the remaining three-fifths of the small intestine, though there is no absolute point at which the one ends and the other begins. Speaking broadly, the jejunum occupies the upper and left part of the abdomen below the subcostal plane (see ANATOMY: Superficial and Artistic), the ileum the lower and right part. At its termination the ileum opens into the large in testine at the ileo-caecal valve.
The caecum is a blind sac occupying the right iliac fossa and extending down some 2 or Sin. below the ileo-caecal junction. From its posterior and left surface the vermiform appendix pro trudes, and usually is directed upward and to the left. This worm like tube is blind at its end and is usually 3 or 4in. long. Its in ternal opening into the caecum is about iin. below that of the ileum. On transverse section it is seen to be composed of (r) an external muscular coat, (2) a submucous coat, (3) a mass of lymphoid tissue, which appears after birth, and (4) mucous mem brane. In many cases its lumen is wholly or partly obliterated, though this is probably due to disease. Guarding the opening of the ileum into the caecum is the ileo-caecal valve, which consists of two cusps projecting into the caecum; of these the upper forms a horizontal shelf, while the lower slopes up to it obliquely. At birth the caecum is a cone, the apex of which is the appendix; it is bent upon itself to form a U, and sometimes this arrange ment persists throughout life.
The ascending colon runs up from the caecum at the level of the ileo-caecal valve to the hepatic flexure beneath and behind the right lobe of the liver; it is about Bin. long and posteriorly is in contact with the abdominal wall and right kidney. It is covered by peritoneum except on its posterior surface (see fig. I).
The transverse colon is variable in position, depending largely on the distension of the stomach, but usually corresponding to the subcostal plane (see ANATOMY: Superficial and Artistic) . On the left side of the abdomen it ascends to the splenic flexure, which may make an impression on the spleen (see DUCTLESS GLANDS), and is bound to the diaphragm opposite the i i th rib by a fold of peritoneum. The peritoneal relations of this part are discussed in the article on the coelom and serous membranes.
The descending colon passes down in front of the left kidney and left side of the posterior abdominal wall to the crest of the ilium ; it is about 6in. long and is usually empty and contracted while the rest of the colon is distended with gas ; its peritoneal relations are the same as those of the ascending colon, but it is more likely to be completely surrounded.
The iliac colon stretches from the crest of the ilium to the in ner border of the psoas muscle, lying in the left iliac fossa, just above and parallel to Poupart's ligament. Like the descending, it is usually uncovered by peritoneum on its posterior surface. It is about 6in. in length.
The pelvic colon lies in the true pelvis and forms a loop, the two limbs of which are superior and inferior while the convexity reaches across to the right side of the pelvis. In the foetus this loop occupies the right iliac fossa, but, as the caecum descends and enlarges and the pelvis widens, it is usually driven out of this region. The distal end of the loop turns sharply downward to reach the third piece of the sacrum, where it becomes the rectum. Formerly the iliac and pelvic colons were spoken of as the sigmoid flexure.
The rectum, according to modern ideas, begins in front of the third piece of the sacrum. It ends in a dilatation or rectal ampulla, which is in contact with the back of the prostate in the male and of the vagina in the female and is in front of the tip of the coccyx. The rectum is not straight, as its name would imply, but has a concavity forward corresponding to that of the sacrum and coccyx.
At the end of the pelvic colon the mesocolon ceases, and the rectum is then only covered by peritoneum at its sides and in front ; lower down the lateral covering is gradually reflected off and then only the front is covered. About the junction of the middle and lower thirds of the tube the anterior peritoneal cover ing is also reflected off on to the bladder or vagina, forming the rectovesical pouch in the male and the pouch of Douglas in the female. This reflexion is usually about Sin. above the anal aperture.
The anal canal is the termination of the alimentary tract, and runs downward and backward from the lower surface of the rectal ampulla between the levatores ani muscles. It is about an inch long and its lateral walls are in contact.
The submucous coat is very strong and consists of loose areolar tissue in which the vessels break up.
The mucous coat is thick and vascular (see fig. 2) ; it consists of an epithelial layer nearest the lumen which forms the intestinal glands (see also the articles EPITHELIAL and ENDOTHELIAL. External to this is the basement membrane, outside which is a layer of retiform tissue, and this is separated from the submu cous coat by a very thin layer of unstriped muscle called the mus cularis mucosae. In the duodenum and jejunum the mucous mem brane is thrown into a series of transverse pleats called valvulae conniventes (see fig. 3) ; these begin about an inch from the pylorus and gradually fade away as the ileum is reached. About 4in. from the pylorus the common bile and pancreatic ducts form a papilla, above which one of the valvulae conniventes makes a hood and below which a vertical fold, the frenulum, runs down ward. The surface of the mucous membrane of the whole of the small intestine has a velvety appearance, due to the presence of closely-set, minute, thread-like elevations called villi (see fig. 2). Throughout the whole length of the intestinal tract are minute masses of lymphoid tissue called solitary glands (see fig. 2) ; these are especially numerous in the caecum and appendix, while in the ileum they are collected into large oval patches, known as