858 THE INTESTINE. 



muscles, by which it is supported. Lastly, at its termination it is surrounded 

 by the external sphincter aid muscle. In the female, the lower portion of 

 the rectum is firmly connected with the back of the vagina. 



Structure. The rectum differs in some respects from the rest of the large 

 intestine, in the structure of both its muscular and its mucous coats. 



The muscular coat is very thick : the external or longitudinal fibres form 

 a uniform layer round it, and cease near the lower end of the intestine ; the 

 internal or circular fibres, on the contrary, become more numerous in that 

 situation, where they form what is named the internal sphincter muscle. 

 The longitudinal fibres are paler than the circular fibres, but both layers 

 become darker and redder towards the termination of the bowel. 



The mucous membrane of the rectum is thicker, redder, and more vas- 

 cular than that of the colon ; and it moves freely upon the muscular coats ; 

 in that respect resembling the lining membrane of the oesophagus. It 

 presents numerous folds of different sizes, and running in various directions, 

 nearly all of which are effaced by the distension of the bowel. Kear the 

 anus these folds are principally longitudinal, and seem to depend on the 

 contraction of the sphincter muscles outside the loosely connected mucous 

 membrane. The larger of these folds were named by Morgagni the columns 

 of the rectum (columnce rectC). Treitz states that these columns consist of 

 longitudinal muscular fibres, which terminate both superiorly and inferiorly 

 in elastic tissue. Higher up in the intestine, the chief folds are transverse 

 or oblique. Three prominent folds, larger than the rest, being half an inch 

 or more in depth, and having an oblique direction in the interior of the 

 rectum, have been pointed out specially by Houston. One of these pro- 

 jects backwards from the upper and fore part of the rectum, opposite the 

 prostate gland ; another is placed higher up, at the side of the bowel ; and 

 the third still higher. From the position and projection of these folds, they 

 may more or less impede the introduction of instruments. (Houston, Dublin 

 Hospital Reports, vol. v.) 



Vessels and Nerves. The arteries of the rectum spring from three sources, viz. 

 the superior hsemorrhoidal branches from the inferior mesenteric; the middle hse- 

 morrhoidal branches from the internal iliac directly or indirectly ; and, lastly, the 

 external or inferior haemorrhoidal branch from the pudic artery. The arrangement 

 of the vessels is not the same throughout the rectum. Over the greater part the 

 arteries penetrate the muscular coat at short interval?, and, at once dividing into 

 small branches, form a network by their communication. Towards the lower end, for 

 four or five inches, the arrangement differs. Here the vessels, having penetrated the 

 muscular coat at different heights, assume a longitudinal direction, passing in parallel 

 lines towards the end of the bowel. In their progress downwards they communicate 

 with one another at intervals, and they are very freely connected near the orifice, 

 where all the arteries join by transverse branches of considerable size. (Quain, Dis- 

 eases of the Rectum.) 



The veins are very numerous, and form a complex interlacement resembling that 

 of the arteries just described, and named the hsemorrhoidal plexus. After following 

 a longitudinal course upwards similar to that of the arteries which they accompany, 

 they end partly in the internal iliac vein by branches which accompany the middle 

 hgemorrhoidal artery, and partly in the inferior mesenteric vein. Hence, the blood 

 from the rectum is returned in part into the vena cava, and in part into the portal 

 system. (See Fig. 325.) 



The lymphatics enter some glands placed in the hollow of the sacrum, or those of 

 the lumbar series. 



The nerves are very numerous, and are derived from both the cerebro-spinal 

 and the sympathetic systems. The former consist of branches derived from the 

 sacral plexus ; and the latter, of offsets from the inferior mesenteric and hypogastric 

 plexuses. 



