PRACTICAL CONSIDERATIONS: THE LARGE INTESTINE. 1689 



The Rectum and Anus. — In relation to its diseases and injuries, the rectum 

 may most conveniently be divided into two portions : ( i ) the pelvic, from the ter- 

 mination of the sigmoid flexure, at the middle of the third sacral vertebra, to the 

 level of the reflection of the recto-vesical fascia from the upper surface of the levator 

 ani to the wall of the rectum ; and (2) the perineal, — the "anal canal," — which 

 extends from this level, through the floor of the pelvis, to the anus. 



The recto-vesical fascia (page 1678), while perforated by vessels, constitutes an 

 efficient barrier to the progress upward of infections or collections of pus and ren- 

 ders the surgical relations of the anal canal perineal instead of pelvic. The distinc- 

 tion between these portions is developmental as well as practical. 



The pelvic portion is the termination of the hind-gut, which has a blind caudal 

 end ; the anal portion results from an inflection of the ectoblast. Between them 

 lies the anal membrane, which may be persistent to a greater or less extent, causing 

 various degrees of constriction or resulting in imperforate anus. If thin, it is car- 

 ried downward by the meconium, and may easily be felt and incised. If the sep- 

 tum is thicker and includes a layer of fibro-muscular tissue, a considerable distance 

 may separate the lower end of the rectum and the rudimentary anal canal. Either 

 portion may be completely absent. 



The occasional abnormal opening of the rectum upon the surface of the body 

 has been observed in the pubic, gluteal, lumbar, or sacral region. Its more fre- 

 quent communication with the vagina, urethra, or bladder is explained by persist- 

 ence of the early association of the gut-tube with the genital and urinary canals in 

 the common cloacal space (page 1696). 



In early childhood the pelvic portion of the rectum is straighter, more vertical, 

 more of an abdominal organ, and more movable than later in life. The support 

 given by the fascial reflections from the rectum to the other pelvic organs is less, on 

 account of the undeveloped condition of the prostate and uterus. The sacral curve 

 is less marked. The connective tissue between the mucous and muscular coats of 

 the rectum, always lax, is especially so in children. Prolapsus ani is therefore not 

 infrequent in them, especially when straining has been caused by the presence of 

 lumbricoids or by other sources of rectal irritation. It occurs in adults, chiefly in 

 old age, when muscular tonicity has been weakened, and is favored by chronic vesi- 

 cal or pulmonary conditions producing frequent straining or coughing. Between 

 the normal protrusion from the anus during defecation of a very narrow ring of 

 mucous membrane, which returns when the act is completed, and the extrusion of a 

 large portion of the rectum {procidentia recti^, including all its coats, every degree 

 of prolapse may be met with. The anal canal is so firmly held by the levator ani 

 that it is rarely involved in prolapse. 



In many cases of prolapse the recto-vesical or recto- vaginal pouch is dragged 

 down and is followed by coils of small intestine (which the pouch normally con- 

 tains), so that it constitutes a hernial sac. 



Hemorrhoids. — The anatomical conditions related to the development of vari- 

 cosities or dilatations of the veins of the hemorrhoidal plexus may be summarized as 

 follows : ( I ) The absence of valves and of any muscular or fascial support between 

 the veins and the mucous membrane and the looseness of the submucous connective 

 tissue rendering the effect of gravity in the sitting and standing postures particu- 

 larly harmful. It should be noted in this connection that quadrupeds are almost 

 free from this disease. (2) The passage of the tributaries of the superior hemor- 

 rhoidal vein directly through the muscular wall of the rectum, about three inches 

 above the anus, causing intermittent constriction of the veins at that point. (3) The 

 communication of the superior hemorrhoidal vein — carrying most of the blood — 

 with the inferior mesenteric vein, and thus with the portal system, which is sub- 

 ject to periodic physiological congestions (as during digestion) and to frequent 

 pathological obstruction. (4) The plexiform anastomoses just within the anus, be- 

 tween the inferior and middle and the superior hemorrhoidal tributaries (Fig. 767), 

 so that the former, although connected with the systemic circulation, are subject to 

 dilatation as a result of portal congestion. (5) The relation of the hemorrhoidal 

 veins and of the terminal branches of the inferior mesenteric veins to the fecal con- 

 tents of the sigmoid and rectum, exposing them to frequent pressure. 



