THE RECTUM 131 ! 



or taeniae (taenia; coli) (Figs. 1034 and 1035), each being about half an inch (12 mm.) in width, 

 and named, respectively, the mesocolic, omental, and free longitudinal bands or tsenise. These 

 bands commence at the base of the vermiform appendix, which structure is surrounded by a 

 uniform layer of longitudinal muscular fibres. The bands pass from the base of the appendix 

 to the rectum. At this point they broaden, fuse, and surround the rectum. On the ascending, 

 descending, and sigmoid colon the mesocolic band (taenia mesocolica) is posterior and internal ; 

 the omental band (taenia omentalis) is posterior and external; the free band (taenia libera) is 

 anterior. On the transverse colon the taenia libera is inferior; the taenia mesocolica is poste- 

 ' rior; the taenia omentalis is anterior and superior. These bands are one-sixth shorter than the 

 other coats of the intestine to which they are applied, and serve to produce the sacculi (Fig. 

 1034), which are characteristic of the cecum and colon; accordingly, when they are dissected off, 

 the tube can be lengthened, and its sacculated character becomes lost. There are three rows of 

 the sacculations separated from each other by the longitudinal bands. These pouches are also 

 subdivided by transverse furrows which correspond to concave folds of mucous membrane, 

 called semilunar folds (plicae semilunares coli). In the sigmoid flexure the longitudinal fibres 

 become more scattered, and around the rectum they spread out and form a layer which com- 

 pletely encircles this portion of the gut, but is thicker on the anterior and posterior surfaces than 

 on the lateral surfaces. In addition to the muscular fibres of the bowels, two bands of smooth 

 muscle fibres arise from the second and third coccygeal vertebrae, and pass downward and for- 

 ward to blend with the longitudinal muscle tissue on the posterior wall of the anal canal. They 

 are known as the rectococcygeal muscles (m. rectococcy gens'). 



The circular fibres form a thin layer over the cecum and colon, being especially accumulated 

 in the intervals between the sacculi. In the rectum the circular fibres constitute a thick layer, 

 and in the anal canal they become numerous and constitute the Internal sphincter. 



The submucous coat (tela submucosa) connects the muscular and mucous layers closely 

 together. Solitary follicles are quite numerous here. 



The mucous membrane, in the cecum and colon, is pale, smooth, destitute of villi, and raised 

 into numerous crescentic folds which correspond to the intervals between the sacculi. In the 

 rectum it is thicker, of a darker color, more vascular, and connected loosely to the muscular 

 coat, as in the oesophagus. It consists of simple columnar and goblet cells resting upon a base- 

 ment membrane, beneath which is seen the tunica propria, which contains the capillaries and a 

 considerable amount of diffuse lymphoid tissue. Externally, is seen the muscularis mucosae. 

 Simple tubular glands, lined chiefly by goblet cells, are present. They are much broader than 

 those of the small intestine. 



When the lower part of the rectum is contracted, its mucous membrane is thrown into a num- 

 ber of folds, which are longitudinal in direction and are effaced by the distention of the gut. 

 Besides these, are certain permanent horizontal folds, of a semilunar shape, known as the rectal 

 or Houston's valves (Fig. 1052). They are usually three in number; sometimes a fourth is 

 found, and occasionally only two are present. 1 One is situated near the commencement of the 

 rectum, on the right side; a second extends inward from the left side opposite the middle of the 

 sacrum; a third, the largest and most constant, projects backward from the forepart of the 

 rectum, opposite the base of the bladder. When a fourth is present, it is situated nearly an inch 

 (2.5 cm.) above the anus on the left and posterior wall of the tube. These folds are about half 

 an inch (12 mm.) in width and contain some of the circular fibres of the gut. In the empty 

 state of the intestine they overlap each other so effectually that they render the introduction of 

 a bougie or the finger somewhat difficult, and their use seems to be "to support the weight of 

 fecal matter, and prevent its urging toward the anus, where its presence always excites a sensa- 

 tion demanding its discharge." 



The lumen of the anal canal presents, in its upper half, a number of vertical folds, produced 

 by an infolding of the mucous membrane and some of the muscle tissue. They are known as 

 the columns of Morgagni or anal columns (columnae rectales [Morgagni]) (Figs. 1053 and 1054). 

 There are from five to ten of these folds, each of which is about half an inch long. They 

 are most prominent when the Sphincter contracts. The outer angle of each column below 

 passes into a semilunar valve. The grooves between the columns are shallow above and deeper 

 below, and end in the anal valves. The valves of Morgagni or anal valves (Figs. 1053 and 

 10-54) are folds which stretch from the base of one column to another, and form the anal pockets 

 or crypts of Morgagni (sinus rectales). Just below the anal valves is the junction of the mucous 

 membrane of the anal canal with the skin; this is indicated by the so-called white line of Hilton or 

 anocutaneous line of Hermann (Fig. 1054). 



Vessels and Nerves of the Large Intestine. The arteries (see also p. 665) supplying the 

 large intestine give off large branches, which ramify between the muscular coats supplying them, 

 and, after dividing into small vessels in the submucous tissue, pass to the mucous membrane. 

 The cecum, the appendix, and the ileocecal valve are supplied by the branches from the anasto- 

 motic loops between the right colic and ileocolic branches of the superior mesenteric artery 



1 Dublin Hospital Reports, vol. v, p. 163. 



