The great cul.de-sac of the peritoneum is usually about three-and-a-half inches distant from the anus, so that, making allowance for the curved course of the intestine, we may estimate the length of the perineal portion of the rectum in the adult at somewhat less than four inches. In forming this estimate, the condition of the urinary bladder as regards its distension should not be overlooked, for when that reservoir is empty and contracted, the rec tum receives an extensive serous investment, and at such times the cul-de-sac of the perito neum approaches the anus perceptibly, whilst under the opposite condition (that of 'repletion) the bladder displaces the serous membrane par tially, carrying its cul-de-sac upwards towards the abdomen. Individual varieties, irrespec tive of these changes in the bladder, are, how ever, of constant occurrence, and in many in stances the rectum in the adult is covered by serous membrane anteriorly to within tsvo inches of the anus, the bladder being at the same time fully distended. In the young subject the peritoneum stretches very far downwards along the surface of the bowel, and at birth it very generally covers the front of the rectum to within one inch of the anus; at the age of five years the cul-de-sac of the peritoneum and the anus are still separated by a very trifling interval; but from this period up to puberty the intermediate distance gmdually in creases, pari passu, with the growth of the pelvis and the development of the inferior fundus of the bladder.
At its commencement the perineal portion of the rectum runs obliquely downwards and for wards, this direction it maintains as far as the prostate gland, but it there alters its course and turns slightly backwards to terminate at the anus. Superiorly it presents a slight curva ture concentric with that of the sacrum, so that the anterior surface of the gut is there slightly concave, and its posterior surface slightly convex from above downwards : infe riorly, however, the curvature of the intestine is reversed; it appears as it were to turn round the point of the coccyx to gain the anus, and therefore the convexity of the lower part of the gut is directed forwards whilst its concavity looks backwards. This curved course of the rectum ought to be borne in mind by the sur geon in his attempts to introduce instruments into its interior.
In the child the sacrum and coccyx present but a trifling curvature, and therefore the rec tum reaches the anus by a less circuitous route than that just described, and which is the nor mal condition in the adult; during childhood the inclination backwards of the lower extre mity of the gut scarcely exists, it possesses but a single curve concave forwards, which, like that of the sacrum and coccyx, is but faintly marked, so that the intestine is much straighter in early life than after puberty. In old age the rectum immediately above the anus is sometimes in flected from side to side so as to assume a zig,zag appearance: these lateral inclinations are the result of the enormous enlargement which the bowel occasionally undergoes in the ad vanced periods of life, its length being actually increased at the same time that its cavity is dilated.
In the adult subject the rectum is somewhat cylindrical in shape, but it increases in capacity as it descends, and presents a marked dilatation just above the sphincters, whilst the anus and so much of the gut as is embraced by those muscles exhibit a decided contraction. In the child the dilatation just described is but little marked, whilst in advanced life it very fre quently becomes excessive, and is best appre ciated when the intestine is fully distended with fmces or artificially inflated ; under such cir cumstances the anterior wall of the rectum is hollowed into a deep depression or gutter, in which the prostate gland and base of the bladder are imbedded, and the bowel swells outwards and forwards upon each side of the prostate, losing altogether its cylindrical shape. It will be readily understood that when such a dispo sition prevails in a calculous subject, the rectum must undergo serious danger during lithotomy performed according to the lateral or bilateral methods, and that therefore the precaution of emptying the bowel previous to these opera tions is highly advisable.
The relations of the perineal portion of the rectum deserve from the surgical anatomist his most attentive consideration. Anteriorly the inferior fundus of the bladder, together with the vesiculx serninales and vasa deferentia, come into contact with the rectum immediately beneath the line of reflection of the peritoneum; lower down the prostate gland rests upon the front of the rectum, to which it is very inti mately connected, nothing but some cellular tissue intervening between them, whilst still lower down the membranous portion of the urethra and the bulb are related to the rectum, though not immediately, for neither of those parts of the urinary apparatus is found to touch the parietes of the gut. The bulb of the urethra in the adult is usually situated about half an inch in front of the rectum and about one inch above the anus; the membranous portion of the urethra lies about ten lines anterior to the rectum, and rather more than an inch and a half above the anus, whilst the prostate gland is placed within one line of the anterior wall of the gut, and about two inches above the anus. These anterior relations of the rectum explain how the finger introduced into its cavity may assist the catheter in its passage along the urethra in the living subject; how by the same manceuvre the surgeon obtains valuable infor mation as to the state of the bladder and pros tate gland in various morbid conditions of those organs; how, in sounding, he is able at times to raise up the calculus by his finger so as to bring it into contact with the instrument ; how the bladder may be punctured from the rectum and the urine withdrawn by this route in certain cases of retention ; how, acute inflammations and other diseases of the bladder and urethra or their appendages so frequently occasion mor bid sympathies in the intestine, such as pro lapsus, tenesmus, hemorrhoids, &c.; and above all, how great must be the danger to the bowel, and how urgent the necessity for protecting it during the lateral operation of lithotomy.