PG Diploma in Sexology and Psychosexual Counseling. For More Info WhatsApp- 9604715783/9359863878. Penis consists of three parallel cylinders of spongy tissue bound in a thick membranous sheath. The upper two are corpora cavernosa and the one on the underside of penis through which urethra passes is known as corpus spongiosum. Penis has numerous blood vessels. During sexual arousal, the blood rushes in to cause erection of penis.