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ALIMENTARY CANAL

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Originally appearing in Volume V01, Page 664 of the 1911 Encyclopedia Britannica.
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ALIMENTARY CANAL, in anatomy. The alimentary canal, strictly speaking, is the whole digestive tract from the mouth to the anus. From the one orifice to the other the tube is some 25 to 3o ft. long, and the food, in its passage, passes through the following parts one after the other:—mouth, pharynx, oesophagus, 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 (Gr. oivm, I will carry, and (ID ayeEv, to eat), 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 10 in. long (25 cm.) and half to one inch in diameter. At first it lies in the lower part of the neck, then in the thorax, and lastly, 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 (Gr. o-TOµaXos) is an irregularly pear-shaped bag, situated in the upper and left part of the abdomen. It is somewhat flattened from before backward and so has an anterior and posterior surface and an upper and lower border. 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 pylorus, 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 curvature. 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 mesocolon. These structures form what is known as the stomach chamber. When the stomach is empty it contracts into a tubular organ which is frequently sharply bent, and the transverse colon ascends to occupy the vacant part of the stomach chamber. The last inch of the stomach before reaching the pylorus is Diaphragm Attachment of falciform ligament Right lobe of liver Gall-bladder Transverse colon Small intestine Ascending colon Anterior superior spine Caecum usually tubular and is known as the pyloric canal. Before reaching this there is a bulging known as the pyloric vestibule (see D. J. Cunningham, Tr. R. Soc. of Edinb. vol. xlv. pt. I, No. 2). The pylorus is an oval opening, averaging half an inch 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 during life is probably tightly closed. 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 appearance. Superficial to the mucous coat is a sub-mucous, consisting of loose connective tissue, while superficial to this are three coats of unstriped muscle, the inner oblique, the middle circular and theouter longitudinal. The peritoneal coat is described in the article on the coelom and serous membranes. ' The small intestine is a tube, from 22 to 25 ft. 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 11 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, passing backward and to the right from the pylorus. The second part runs vertically downward in front of the hilum of the right kidney, 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-jejunal 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 abdominal wall. (See
End of Article: ALIMENTARY CANAL
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