When the tonus relaxes, the blood precipitates into the enlarged vessels and the cavernous spaces, and an active increase in the amount of arterial blood is the result. Through the increase of the lumen of the arteries and caverns, the veins are compressed and the blood cannot flow out of the cavernous bodies. Thus the active increase of arterial blood serves as a check of the reflux of the blood through the veins by the pressure of the distended arteries and caverns upon these veins. Besides this check, the vena profunda cummunis, by which the blood of the corpora cavernosa penis has to return, passes through the unstriped muscular fibres of the musculus transversus perinei profundus. Now, this muscle contracts synchronously through the same influence of the nervi erigentes which caused the relax of the tonus in the arteries. The contraction of the muscle causes the compression of the vena profunda and prevents the blood from flowing off. Besides the muscular transversus perinei, the musculus ischio-cavernosus which arises from the os ischii and encircles the radix of the penis, as well as the musculus bulbo-cavernosus by compressing the bulbus urethrae will, both, at their contraction, prevent the blood from flowing off. In this way there is not only an active increase in the amount of arterial blood, but also an abrupt decrease in the amount of venous blood, flowing out of the penis. The corpora cavernosa, therefore, become of almost cartilaginous hardness, and the penis reaches the state of rigidity, necessary to the performance of the male sex-act. When erection is complete the contraction of the musculus erector penis draws the organ up against the abdomen and gives it the same direction as that of the vagina. _Colliculus._—With the erection of the corpora cavernosa, the colliculus also swells and almost fills up the entire lumen of the prostatic urethra. In this way the bladder which has already been closed by the contraction of the sphincter, is closed up more tightly, so that not a drop of urine could escape. The erection of the colliculus also causes the orifices of the ejaculatory ducts to take the direction forwards toward the pars membranacea. _Urethra._—At this stage the urethra obtains an actual lumen. Through the net of veins which surround the mucous membrane of the urethra and through the turgescence of the corpus cavernosum urethrae, the canal of the urethra opens and remains gaping through the entire duration of the erection. In this way it is admirably fitted to give a free passage for the semen. In the mean time the urethral glands are constantly secreting their viscid clear fluid, which, together with the secretion of the Cowper’s glands, provide a lubricating and protecting coat for the urethra and neutralize the contents of the urethral walls which are otherwise bathed in acid urine.