ANAL CANAL. 1233 



rhoidal artery is distributed chiefly on the posterior, and the middle hamiorrhoidal chiefly on the 

 anterior aspect of the lower part of the bowel. 



4. One or more small branches of the middle sacral artery reach the posterior surface of the 

 rectum, where they are distributed chiefly, if not solely, to the muscular coat. 



Anastomosis of the Hsemorrhoidal Arteries. The superior and middle haemorrhoidal arteries 

 anastomose freely in the hsemorrhoidal plexus of the submucosa, and also by a few large branches 

 on the exterior of the bowel : some perforating branches of the middle sacral and inferior 

 haamorrhoidal arteries also join the plexus in the submucous layer at the lower part of the rectum. 

 In addition, small branches of these several arteries unite with one another in the muscular coat. 

 It should be remarked that the superior haemorrhoidal artery supplies both the muscular and 

 mucous coats in the superior part of the rectum, but the muscular coats in the inferior part are 

 supplied by the middle and inferior haemorrhoidal vessels only. 



Veins of the Rectum and Anus. These form two chief plexuses of large vessels devoid of 

 valves, namely, the internal hsemorrhoidal plexus situated in the submucous coat, and the 

 external haemorrhoidal plexus in the outer coat. The internal haemorrhoidal plexus takes origin 

 near the margin of the anus in a number of small (anal) veins, which are radially disposed 

 beneath the skin of the anus, and communicate below with the rootlets of the inferior hsemor- 

 rhoidal vein over the external sphincter. These anal veins, traced upwards, join together, and 

 are joined by others from the surrounding parts to form larger and often tortuous vessels, which 

 ascend in the columnae rectales, where they frequently present ampullary enlargements, varying 

 in size up to that of a small pea, which are said to be the starting-points of haemorrhoids. 

 Passing upwards, the veins are known as the " terminal veins " ; they communicate freely with 

 one another, forming the plexus, and unite into still larger vessels, which pierce the muscular 

 coat about the middle of the rectum, and join to form the superior haemorrhoidal vein. 



From the inferior part of the internal haemorrhoidal plexus numerous vessels pass through 

 the external sphincter to join a venous network on the outer surface of that muscle, from 

 which the inferior hsemorrhoidal veins arise. This network, as pointed out above, also com- 

 municates with the internal haemorrhoidal plexus, through the anal veins which descend from 

 the latter beneath the skin of the anal canal, to the exterior of the sphincter. 



The various veins which pass out through the walls of the rectum unite freely on its 

 exterior to form a rich venous plexus (external hsemorrhoidal plexus), through which the three 

 hsemorrhoidal vessels are brought into free communication with one another. Passing off from 

 this plexus, the superior hsemorrhoidal joins the left colic vein and forms with it the inferior 

 meseiiteric vein, which opens into the splenic ; the middle haemorrhoidal joins the hypogastric, 

 from which the blood passes through the common iliac to the vena cava inferior; and the 

 inferior haemorrhoidal joins the internal pudendal, a tributary of the hypogastric vein. Thus, 

 on the rectum, a free anastomosis is established between the veins of the portal and systemic 

 circulations. 



Lymph Vessels. Most of the lymph vessels of the rectum pass to the lymphoglandulae 

 sacrales, of which some lie close to the muscular coat on the side of the rectum along the 

 superior hasmorrhoidal vessels, while others, four or five in number, and of a larger size, lie in 

 front of the promontory of the sacrum, between the layers of the pelvic mesocolon. The glands 

 of opposite sides are connected with one another by the middle sacral plexus and with the 

 hypogastric and mesocolic lymph glands. The efferent vessels from these pass to the lumbar 

 glands. Some of those from the lower part of the anal canal join the cutaneous lymph vessels 

 round the anus, and pass with them to the inguinal and subingtiinal glands. A few of the 

 lymph vessels from the lower portion of the rectum are said (by Quenu) to join the hypogastric 

 glands. 



Nerves. The nerves of the rectum come partly from the sympathetic and partly from the 

 cerebro-spinal system. The sympathetic fibres are derived from the inferior mesenteric plexus, 

 through the superior haemorrhoidal nerve and the corresponding plexus, and from the upper 

 and lower divisions of the hypogastric plexus, the former accompanying the superior haemor- 

 rhoidal, the latter the middle haemorrhoidal vessels, to the rectum. The cerebro-spinal fibres 

 arise from the second, third, and fourth sacral nerves soon after these leave the sacral foramina 

 (and constitute the " pelvic splanchnics " of Gaskell). They run forward in the pelvic con- 

 nective tissue, and joining the pelvic plexuses, reach the side of the rectum. Fibres of the 

 inferior haemorrhoidal branches of the pudendal nerve (third and fourth sacral) are also distributed 

 to the lower part of the anal canal as well as to the external sphincter. 



It has been shown by experiments on animals, that the cerebro-spinal nerves (from the 

 second, third, and fourth sacral) convey motor impulses to the longitudinal fibres, but inhibitory 

 impulses to the circular muscular fibres. In like manner the branches from the sympathetic 

 convey motor fibres (derived from some of the lumbar rami communicantes) to the circular 

 muscle, and inhibitory fibres to the longitudinal muscle of the rectum. 



The reflex centre which governs the action of the sphincters and the muscular fibres of the 

 rectum (" defaecation centre ") is situated in the lumbar region of the spinal medulla, and appears 

 to be capable of carrying out the whole act of defaecation even when separated from the brain. 



Variations. The best known anomalies found in connexion with the rectum are those 

 classed under the term imperforate anus or atresia ani. The atresia may be simply due to a 

 partial or complete persistence of the anal membrane (see p. 42), which separates the procto- 

 aaeuni from the hind-gut in the embryo (atresia ani simplex) ; or the hind-gut may be deficient 

 in its lower part, when there is a considerable interval between the proctodaeum and the gut 

 (defectus recti partialis, vel totalis) ; or the rectum may open into the vagina, the uterus, the 



79 



