Part 124

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. 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.