ERECTION
1. How is erection of the penis accomplished?
2. Does erection accomplish straightening of the sigmoid flexure?
3. What is an approximate blood pressure within the corpus cavernosum penis of the bull.during coitus? What is hematoma of the penis?
An increase in the turgidity of the penis is known as erection.
It is caused by an increase of blood pressure within the cavernous sinuses of the penis as a result of greater blood inflow than outflow. The inflow of blood is increased via vasodilation of the arteries caused by parasympathetic stimulation. The outflow of blood is decreased via compression of the dorsal veins of the penis against the pelvis when the ischiocavernosus muscles contract. Contraction of the ischiocavernosus muscles also compresses the blood in the cavernous sinuses (now a closed system), which also assists erection by increasing blood pressure in the cavernous sinuses (see Figure 14-13).Complete erection of the glans penis of the horse is delayed until after introduction of the penis into the vagina of the mare. Mounting of the mare compresses the prepuce against the vulva and venous drainage from the prepuce is impaired. Complete erection of the glans is then possible because venous drainage from the glans is directed to the prepuce, which is blocked.
In animals with a sigmoid flexure, the filling of the cavernous sinuses, coupled with relaxation of the retractor penis muscles, causes the sigmoid flexure to be eliminated and the penis to be straightened. Although animals with a sigmoid flexure have a higher ratio of connective tissue to cavernous tissue (see Figure 14-10), the length and diameter of the penis increase somewhat as a result of erection, in addition to penis straightening. As compared with the bull, ram, and boar, the penis of the horse has a lower ratio of connective tissue to cavernous tissue, and a relatively greater increase in the length and diameter of its penis occurs during erection.
Blood pressure within the corpus cavernosum penis of the bull has been measured during coitus. A pressure of approximately 14,000 mm Hg was associated with peak activity, and peak activity was correlated with an increased intensity of ischiocavernosus muscle contraction that furthered compression of blood in the cavernous tissue. Higher pressures have been recorded. It is believed that these high pressures, coupled with cavernous tissue capsule weakness, might be the cause of rupture of the corpus cavernosum penis (hematoma of the penis) in some bulls. The usual rupture site is on the dorsal surface of the distal curve of the sigmoid flexure (see Figure 14-12).
■