408 A MANUAL OF ANA TOMY. 



The Hemorrhoidal Plexus. 



This venous plexus begins around the anus and extends 

 upward upon the rectum. It empties the blood by the in- 

 ferior hemorrhoidal vein into the internal pudic vein ; by 

 the middle, into the internal iliac and superior hemor- 

 rhoidal ; and by the superior, into the inferior mesenteric, 

 which opens finally into the portal vein. This establishes 

 a free venous communication from the anus to the portal 

 vein, and so to the liver. There are no valves in these 

 veins, so the blood can be dammed back from the liver or 

 from the heart. 



The Inferior Hemorrhoidal Artery. See page 412. 

 The Inferior Hemorrhoidal Nerve. See page 413. 



DISSECTION. 



Carefully divide the perineal fascia along the median line and across its 

 posterior attachment and reflect the two portions. If the fascia has been par- 

 tially destroyed in the dissection remove any parts left and complete the clean- 

 ing of the structures forming and contained within the perineal triangle. 



The Perineal Triangle 



Is formed by the erector penis, accelerator urinse, and 



transversus perineae muscles. 



It contains the superficial and transverse perineal vessels 

 and nerves. 



Erector Penis. (Isckiocavernostis.) Fig. 82. 



Origin. From the inner surface of the tuberosity and 

 ramus of the ischium. 



Insertion. Into the outer and under surface of the crus 

 penis, and by a small slip around the crus penis, to pass 

 over the dorsal vein of the penis (situated posterior to the 

 anterior slip of the accelerator urinae). This small slip is 



