1046 THE DIGESTIVE SYSTEM. | 
small branches of these several arteries unite with one another in the muscular coat. It should 
be remarked that the superior hemorrhoidal artery supplies both the muscular and mucous 
coats above, whilst it is confined to the latter in the lower few inches of the gut, the muscular 
coats here being supplied by the middle and inferior hemorrhoidal vessels. 
Veins of the Rectum and Anus.—These form two chief plexuses of large vessels devoid of 
valves, namely, the internal hemorrhoidal plexus situated in the submucous coat, and the 
external hemorrhoidal plexus in the outer coat. The internal hemorrhoidal 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 hemor- 
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 columns of Morgagni, 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 heemorrhoids. 
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 the two branches of the superior hemorrhoidal vein. 
From the lower part of the internal hemorrhoidal plexus numerous vessels pass through 
the external sphincter to join a venous network on the outer surface of that muscle, from 
which the inferior hemorrhoidal veins arise. This network, as pointed out above, also com- 
municates with the internal hemorrhoidal 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 hemorrhoidal plexus), through which the three 
hemorrhoidal vessels are brought into free communication with one another. Passing off from 
this plexus, the superior hemorrhoidal vein joins the inferior mesenteric, which opens into the 
portal vein; the middle hemorrhoidal joins the internal iliac, a tributary of the vena cava; 
and the inferior hemorrhoidal joins the internal pudic, a tributary of the internal iliac. 
Thus, on the rectum, a free anastomosis is established between the veins of the portal and 
systemic circulations. 
Lymphatics.—Most of the lymphatics of the rectum join a number (four or five) of rectal 
glands found in the connective tissue coat of the bowel along the superior hemorrhoidal 
vein and its two branches, whence they pass to the sacral glands on the front of the sacrum. 
Some of those from the lower part of the anal canal join the cutaneous lymphatics round the 
anus, and pass with them to the oblique set of superficial inguinal glands. A few of the 
lymphatics from the lower portion of the rectum are said (by Quenu) to join the internal iliac 
glands, but these are inconstant according to Gerota. 
Nerves.—The nerves of the rectum come partly from the sympathetic and partly from the 
cerebro-spinal system. The sympathetic fibres are derived on the one hand from the inferior 
mesenteric plexus, and on the other from the right and left divisions of the hypogastric plexus 
(z.e. the pelvic plexuses), the former accompanying the superior hemorrhoidal, the latter the 
middle hemorrhoidal 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 pelvie connective tissue, and 
joiming the pelvic plexuses, reach the side of the rectum. Fibres of the inferior hemorrhoidal 
branch of the internal pudic 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 (‘defecation centre”) is situated in the lumbar region of the cord, and appears to be 
capable of carrying out the whole act of defsecation 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 toa 
partial or complete persistence of the anal membrane (see p. 43), which separates the procto- 
deeum from the hind-gut in the embryo (artresia ani simplex) ; or the hind-gut may be deficient 
in its lower part, when there is a considerable interval between the proctodeeum and the gut 
(defectus recti partialis, vel totalis); or the rectum may open into the vagina, the uterus, the 
bladder, or the ureters, when usually no anus is evident; or finally the cloaca may persist. 
Other forms are also described, but the foregoing are those most commonly found. 
For the development of the rectum and anus, see pp. 32 and 43. 
THE PERITONEUM. 
An introductory sketch of the peritoneum was given on p. 997; subsequently, 
when describing the abdominal viscera, an account of its detailed relations to 
each of these was included. We shall here consider the membrane and its folds as 
parts of one continuous whole; and we shall also describe its arrangement as seen 
