The _membrum virile_ or _penis_ is visible externally, and behind it is situated the scrotum. Within this latter are two ovoid structures, named _testicles_ or _testes_. Each testicle is enveloped in a fibrous capsule, known as the _tunica albuginea_, from which fibrous _septa_ pass into the interior of the organ, thus dividing it into a number of separate _lobules_. Each lobule is composed of _seminiferous tubules_, which are greatly convoluted and likewise branched, the branches being continuous with those of neighbouring tubules, both within the same lobule, and (by perforating the fibrous septa) in adjoining lobules. In the walls of the seminiferous tubules the _spermatozoa_ are formed. The seminiferous tubules unite to form the efferent ducts (_vasa efferentia_), about a dozen in number for each testicle; immediately passing out of the testicle, these efferent ducts make up the _epididymis_, situated at the upper and back part of the testicle. After numerous convolutions, these unite at length on each side to form a single canal, which leaves the epididymis under the name of the _vas deferens_; this is the excretory duct of the testicle, conveying the secretion of that organ to the exterior. The vas deferens traverses the inguinal canal into the abdominal cavity, and therein passes downwards to the prostatic portion of the urethra (_vide infra_). The anterior portion only of the _penis_ is visible externally, dependent in front of the scrotum; the posterior portion is concealed by the scrotum and the skin of the perineum. The terminal segment of the penis is formed by the _glans_, which is covered by the _foreskin_ or _prepuce_. This last is sometimes artificially removed: either on ritual grounds, as, for instance, among the Jews; or for medical reasons, for example, when the preputial orifice is greatly constricted. At the anterior extremity of the glans penis is the orifice of the urethra (_meatus_). The _urethra_ is a canal running through the entire length of the penis, opening by its proximal extremity into the urinary bladder, and serving for the passage of the urine from the bladder to the exterior of the body. The main substance of the penis is composed of three cavernous bodies, the paired _corpora cavernosa penis_, and the single _corpus spongiosum_, or _corpus cavernosum urethrae_. These consist of what is known as _erectile tissue_, a spongy mass within whose lacunar spaces a large quantity of blood can, in certain conditions, be retained. When this occurs, the penis becomes notably thicker and longer, and simultaneously hard and inflexible. This process is known as _erection_ of the penis, and is requisite to render possible the introduction of the organ into the genital canal of the female.