The “middle rectal artery” has been frequently mentioned in many Chinese literature in recent years. However, a review of the Chinese anatomical literature on rectal blood supply does not include the name “middle rectal artery”, which corresponds to the blood vessels of the “inferior rectal artery”. Moreover, the shape of this artery, which has different names in China and abroad, is still very controversial. The traditional view is that it enters the rectum from the lateral rectal ligament and needs to be separated and ligated during surgery to avoid bleeding during surgery, but some studies have shown that it originates from the internal pubic artery, travels on the surface of the levator ani muscle, and enters the rectum at the lower end of the rectum without entering the lateral rectal ligament, so the name “inferior rectal artery” is more consistent with the actual course of the vessel. Moreover, whether the inferior rectal artery enters the rectal mesentery has rarely been mentioned. In order to further clarify the supply of the inferior rectal artery and the relationship between the artery’s course and the lateral rectal ligament and rectal mesentery, we performed an autopsy study on 10 cadaveric specimens and report as follows. 1. Materials and methods (1) Study materials The cadaveric specimens were obtained from 10 voluntarily donated adult cadavers collected by the Red Cross Society of the Second Military Medical University and provided by the Department of Anatomy and Research of the Second Military Medical University from September 2008 to September 2009. Among them, 7 were male and 3 were female, with an average age of (66.8±11.1) years, including (70.9±4.2) years for males and (57.3±17.6) years for females. (2) Apparatus and equipment: SF2000 three-button electronic digital display calipers (Guilin Guanglu Digital Measurement and Control Co., Ltd.) were used for measurements, and digital cameras (Nikon COOLPIX885, Nikon E8800) were used to obtain photographic data. (2) Study method Experimental preparation The right femoral artery was cannulated, and the red latex solution was instilled from the femoral artery at a pressure of 3.0 to 3.5 kg/cm2 , and the superficial temporal artery was observed to be well filled. The pressure was maintained for 10 min to make the artery fill better. The perfused cadaver was immersed in 4% formaldehyde solution for fixation. Specimen sampling The pelvic specimens were taken from the fourth lumbar plane up to the upper 1/3 of the thigh transecting the cadaver. The anterior abdominal wall was excised from the superior border of the pubic symphysis along the inguinal ligament to the iliac crest, and the small intestine and colon above the sigmoid colon were removed to clear the contents of the pelvis and intestinal cavity. Revealing the rectal mesentery and inferior rectal artery Free the rectum: pull the sigmoid colon to the right and cut the peritoneum along the left root of the sigmoid mesentery in a reverse fold, extending it toward the pelvic region to the rectal bladder sink (rectal uterine sink in women). The peritoneum was separated to the left to reveal the left ureter. The sigmoid colon was then turned to the left, and the right root of the sigmoid colon was incised and separated upward to the root of the inferior mesenteric artery and downward to the rectal bladder recess (or rectal uterine recess), where it met the opposite side. The sigmoid colon and its mesentery were lifted, and the peritoneum on both sides of the rectum was incised medially in the ureter to the recto-rectal recess of the uterus or the recto-rectal recess of the bladder anteriorly. The anterior sacral hiatus posterior to the rectum was fully exposed by drawing toward the bladder, and the lax tissue within the anterior sacral hiatus was sharply separated with a scalpel to reach the sacrorectal ligament. The anterior lobe of the pelvic floor at the lowest point of the peritoneal cut is clamped up with a vascular clamp, while the anterior bladder or uterovagina is pulled forward and upward with a deep pulling hook to reveal the tougher Denonvilliers fascia, and the freeing continues anterior to this fascia to reach the seminal vesicles and prostate or the posterior vagina. If exposure is difficult, the anterior pubic symphysis can be excised to reveal the posterior pubic space, and the anterior rectal organs can be dissected medially and sagittally. Confirmation of the inferior rectal artery and rectal mesentery: at the point where the ureter enters the pelvis, the cord of the umbilical artery is confirmed and the lateral bladder gap between the lateral wall of the pelvis and the bladder is entered by incising the peritoneum with its superior edge. The umbilical artery was traced to the beginning of the internal iliac artery, and the fascia was cut along the fusion of the inferior ventral fascia of the bladder with the pelvic wall to reveal the main trunk of the visceral branch of the internal iliac artery and traced toward the pelvic floor to the point where the internal pubic artery exits the pelvic floor. The branches of the internal pudendal artery in the pelvis were identified, and the arterial vessels entering the rectum were identified by dissection. Those that reach the rectum are marked and identified as inferior rectal arteries. Once the inferior rectal artery crosses the medial layer of the inferior ventral fascia of the bladder, it is considered to enter the rectal mesentery. (3) Observations General conditions of the inferior rectal artery Vessel of origin of the inferior rectal artery; outer diameter of the vessel at the beginning of the inferior rectal artery; outer diameter of the vessel of the inferior rectal artery reaching the end of the rectum; length of the inferior rectal artery; branching of the inferior rectal artery The distribution of the inferior rectal artery in the pelvic cavity The unilateral and bilateral distribution of the inferior rectal artery; the relationship between the inferior rectal artery and the rectal mesentery; the site where the inferior rectal artery enters the rectum. Statistical methods The data obtained were statistically described by SPSS 16.0 software. 2. Results (1) General condition of the inferior rectal artery In 10 specimens, 9 were found to have inferior rectal artery; a total of 12 inferior rectal arteries were found in 20 lateral pelvises. Among them, 10 inferior rectal arteries originated from the internal pubic artery, one from the gluteal artery, and one from the inferior bladder artery. The outer diameter of the origin of the inferior rectal artery ranged from 0.28 to 2.85 mm, with a mean of (1.72±0.81) mm. the outer diameter of the inferior rectal artery when it reached the rectum ranged from 0.28 to 2.81 mm, with a mean of (1.63±0.78) mm. the linear length of the inferior rectal artery from its origin to its end point ranged from 29.89 to 78.12 mm, with a mean of (59.10±13.02) mm; its actual length was 31.71 to 95.12 mm; mean (63.90±16.90) mm. (2) Distribution of the inferior rectal artery in the pelvic cavity in terms of its course In three specimens, there was one inferior rectal artery on both sides, and two of them were female. In the other 6 specimens, only one side of the inferior rectal artery was present, including 5 cases on the left side and 1 case on the right side. There were three inferior rectal arteries entering the anterior rectum; one entering the rectum in the left anterior quadrant; four vessels entering the rectum in the left posterior quadrant; three inferior rectal arteries entering the rectum in the right posterior quadrant; and one entering the rectum in the left median rectum. Of the 12 inferior rectal arteries, 6 did not pass through the rectal mesentery and 6 passed through the rectal mesentery into the rectum. The 6 vessels that did not pass through the rectal mesentery were all specimens with 1 inferior rectal artery on each side, all originating from the internal pubic artery. The vessels of this group started and traveled at the inferiormost edge of the inferior ventral fascia of the bladder without breaking through the medial layer of the inferior ventral fascia of the bladder (see Figure 1a, b). The six subrectal arteries entering the rectum via the rectal mesentery were all unilateral specimens entering the rectum via the rectal mesentery The distribution of the six subrectal arteries in the mesentery was in two ways: one was to anastomose with the ipsilateral superior rectal artery branch immediately after entering the rectal mesentery; the other was to cross the medial layer of the subdominal fascia of the bladder into the rectal mesentery and then distribute in the anterior part of the rectum and penetrate into the rectal musculature. For the rectum, which is above the pelvic diaphragm, the main sources of arterial blood supply are the superior rectal artery and the middle rectal artery (MRA). Although the superior rectal artery and the MRA are both located above the pelvic diaphragm, they travel at different levels. The superior rectal artery is the main arterial structure in the rectal mesentery. From our anatomical findings, it appears that the inferior rectal artery travels in the lowest part of the inferior ventral fascia of the bladder and is distributed over the surface of the anal raphe. As to whether there are inferior rectal arteries that enter the rectal mesentery, only Jones mentioned that some of the “middle rectal arteries cross the rectal mesentery”. However, there is no further description of how it crosses, nor is there a corresponding diagram. (3) The relationship between the level of the inferior rectal artery and the rectal mesentery The level of the inferior rectal artery The literature discussing the inferior rectal artery mainly describes the origin, frequency, length, and outer diameter of the vessel, but records of its course before it enters the rectum are scarce. It has long been clinically and anatomically believed that the inferior rectal artery travels in the lateral rectal ligament before entering the rectum. In our experiments, the presence of the inferior rectal artery was found in 9 of 10 specimens, and the inferior rectal artery was not found to enter the rectum through the lateral rectal ligament. The inferior rectal artery travels in a tunnel-like structure on the surface of the levator ani muscle. The tunnel-like structure consisted of two layers of dirty fascia, the lateral fascia of which was continuous with the pelvic fascial tendon arch. The pelvic fascial tendon arch is composed of the pelvic visceral fascia and the pelvic wall fascia that migrate from each other, starting at the point where the inferior rectal artery emanates from the internal pubic artery and continuing to the narrowest part of the pelvis. Such a structure made of a double layer of visceral fascia is present in all specimens found to have a subrectal artery. This structure, which we believe to be part of the inferior ventral fascia of the bladder, anchors the inferior rectal artery to the surface of the levator ani muscle. The relationship between the inferior rectal artery and the rectal mesentery From the point of view of the structure in which the inferior rectal artery travels, the inferior ventral fascia of the bladder and the rectal mesentery should be two separate levels. If the inferior rectal artery enters the rectal mesentery, these two levels should be somehow connected. Nano et al. reported that in rectal cancer surgery patients who underwent radical total rectal mesenteric resection followed by angiography, the presence of the inferior rectal artery was still found in 80% of the patients. This result suggests that the inferior rectal artery does not cross the rectal mesentery into the rectum. the study by Sterk et al. showed that only the superior rectal artery is present in the rectal mesentery, and Sterk’s conclusion further validates Nano’s findings. In our anatomic specimens, we did find that the inferior rectal artery did not enter the rectum through the rectal mesentery in three specimens. The inferior rectal artery traveled in the lowermost part of the inferior ventral fascia of the bladder to the intersection of the rectum and the pelvic diaphragm and penetrated into the rectal musculature. However, at the same time, we found that the inferior rectal artery in the remaining 6 specimens was passing through the rectal mesentery before reaching the rectum. The entry of the inferior rectal artery into the rectal mesentery was manifested by the fact that the direction of entry into the rectal mesentery was only possible from the lateral side (due to its anatomical restriction), and the entry was dependent on whether it could break through its medial fascia or not. The direct connection between the inferior rectal artery and the rectal mesentery is determined at the moment of penetration through the fascia of the rectal mesenteric surface. In five specimens, the inferior rectal artery crossed the fascia and immediately entered the rectal mesentery. Once the inferior rectal artery entered the rectal mesentery, it was immediately distributed anteriorly or laterally in the rectal mesentery of the lower rectum. Only little adipose tissue could be found in these areas. In two cases, the inferior rectal artery entering the lateral mesentery of the rectum sent small branches to the adjacent organs, either the prostate or the vaginal wall, before entering the mesentery, where it often chose the shortest route to anastomose with the superior rectal artery and together complete the blood supply to the rectum. Three cases were distributed anterior to the lowermost part of the rectum, which is a rather fixed position of the pelvic organs anatomically, i.e., in the pelvic cavity where there is the least mobility: the posterior lower part of the prostate (posterior to the vagina) and the lower part of the rectum, which, from the anatomical point of view, minimizes the compression of its vessels by the volume changes of the cavernous organs. The six inferior rectal arteries that did not enter the rectal mesentery entered the rectum in the right posterior or left posterior quadrant. one inferior rectal artery that entered the rectal mesentery from the left posterior quadrant showed a clear restriction by fascial structures during its course, and it anastomosed with the left branch of the superior rectal artery immediately after entering the mesentery. Clinical considerations on the relationship between the rectal mesentery and the inferior rectal artery The possibility of injury to the inferior rectal artery by freeing the middle and lower rectum According to our anatomical findings, the inferior rectal artery travels along the surface of the anal raphe, and even though there are cases in which the inferior rectal artery enters the mesentery and anastomoses directly with the superior rectal artery, the inferior rectal artery before the anastomosis remains fixed to the surface of the pelvic diaphragm by the inferior ventral fascia of the bladder. The site of the inferior rectal artery entering the rectal mesentery is close to the plane of the sphincter ring, so it is highly unlikely that the inferior rectal artery is going to be injured during surgery as long as the rectum is not freed to a very low position to reach the surface of the anal levator muscle during surgery related to the rectum. How should we understand the phenomenon of bleeding during lateral rectal freeing? (1) First of all, we cannot completely exclude the possibility that there is a direct entry of the inferior rectal artery into the lateral rectal ligament, but from our current observations, this possibility is extremely small. The lateral rectal ligament has a different pathway from the distribution of the inferior rectal artery. Our dissection revealed that the lateral rectal ligament is located in the lateral posterior aspect of the rectum, whereas the inferior rectal artery is in the anterolateral aspect of the rectum. (2) From anatomical and clinical point of view, the rectal mesentery is under the retroflexion of the peritoneum, and the superior rectal artery is graded to the sides in a position where it appears winged or ear-shaped, which may be damaged in surgical misuse. In this case, the surgery can hardly be considered to be performed in a level where there is no vascularity around the rectal mesentery. Relationship between the inferior rectal artery and hemorrhoids in different states We found that the inferior rectal artery distributed outside the rectal mesentery is directly into the rectal muscle layer at the point where the rectum exits the pelvic diaphragm and further into the submucosal layer. Based on Thomson’s [10] theory of hemorrhoids in the inferior anal cushion theory, Longo proposed supra-hemorrhoidal mucosal circumferential hemorrhoidectomy for the treatment of cricoid prolapsed hemorrhoids (PPH). “The PPH “cut off” should not only block the blood supply from the superior rectal artery to the hemorrhoid area, but also cut off the blood supply from the inferior rectal artery to the hemorrhoid area. The PPH “cut off” should not only block the blood supply from the superior rectal artery to the hemorrhoid area, but also cut off the blood supply from the inferior rectal artery to the hemorrhoid area, otherwise the “cut off” is not complete. We found that part of the inferior rectal artery enters the rectum at the point where the rectum penetrates the pelvic diaphragm. In this state, it is difficult to block the blood supply from the inferior rectal artery by PPH, which will definitely affect the treatment effect. Therefore, we believe that it is clinically significant to be able to determine the status of the inferior rectal artery before performing PPH surgery for hemorrhoids. The “middle rectal artery” should be called the inferior rectal artery According to our anatomical observation, the inferior rectal artery is located in the lowermost part of the rectum above the pelvic diaphragm. The upper part of the rectum is the part of the rectum that is surrounded by peritoneum on the front and both sides, the middle part of the rectum is the part that is covered by peritoneum only in the front, and the part below the peritoneal reflex is the lower part of the rectum. On the other hand, our anatomical findings show that the inferior rectal artery does not enter the rectum through the lateral rectal ligament. Based on this, we believe that the naming of the inferior rectal artery is more consistent with the actual situation of the distribution of this artery in the rectum. 4. Conclusion The inferior rectal artery is a relatively constant rectal blood supply artery. The inferior rectal artery enters the rectum in two ways: one does not pass through the rectal mesentery and enters the rectum directly at the junction of the rectum and the anal raphe, and the other crosses the subperitoneal fascia of the bladder and enters the rectal mesentery. The inferior rectal artery, which does not enter the rectal mesentery, is not damaged during rectal-related surgery. An understanding of the anatomical relationship between the rectal mesentery and the inferior rectal artery contributes to a deeper understanding of rectal surgery.