The major external structures are the penis, penile urethra and scrotum. Embryologically, the cloacal folds fuse ventrally to form the genital tubercle. The genital tubercles elongate and the accompanying urogenital folds fuse to convert the groove in between the folds to a tube which becomes the urethra. The surrounding genital tubercles form the penis. Proliferating tissue lateral to the urogenital folds called the genital swellings form the scrotum. The midline region of fusion between the genital swellings becomes the medial raphe which separates the testes. The prepuce is formed from loose connective surrounding the penis.
The urethra is divided into the pelvic and penile portions. Both portions are lined by transitional epithelium. Under the lamina propria, portions of the tunica muscularis is formed from striated muscle rather than smooth muscle. A tunica adventitia is present in this region. The mucosa of the penile urethra changes to squamous epithelium at the external opening. Glands are present in the lamina propria, particularly in the stallion and boar. The tunica muscularis is composed of smooth muscle in the penile urethra.
The anatomy of the penis can vary tremendously by tissue composition and organization between species. In general terms, the penis can be divided into the glans, the body and the roots. The glans is covered by a reflection of preputial skin. The tissue of the glans can be composed of vascular erectile tissue, cartilage in bulls or in the case of the carnivores, bony tissue. The body is enclosed by a capsule which is thick and prominent in swine and ruminants (fibrous penis) and thin in carnivores (vascular penis). The subcapsular tissue encloses vascular erectile tissue (corpora cavernosa and corpus spongiosum). This tissue is composed of collagenous tissue and vascular spaces which can expand due to the presence of elastin fibers in the connective tissue. The roots are the most proximal aspects of the penis and are similar in structure to the body.
The scrotum is enclosed in haired skin with an underlying layer of smooth muscle (tunica dartos). The scrotum contains invaginations of peritoneal mesentery (the parietal and visceral vaginal tunics) which enclose the testes, epididymus, ductus deferens and associated vascular structures.
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