The penis consists of three identifiable parts: the roots, the body or the shaft, and the glans penis. In the resting position the penis lies retracted and held within its protective sheath, with the prepuce surrounding the glans penis, which is out of sight (Fig. 3.4).
The penis is held in this retracted position by the retractor penis muscle and the bulbospongiosus muscle, which run ventral to the penis along its length. Innervation by the sympathetic nervous system of these two muscles
results in their contraction, drawing the penis up within the sheath. The stallion's penis is classified as haemodynamic (reacting to an increase in blood pressure) and is of a musculocarvenosus type (muscle tissue surrounding air cavities), in common with humans. During sexual excitation the penis undergoes considerable enlargement in both width and length as a result of engorgement of the areas of erectile tissue with blood. This is unlike the bull, ram and boar, whose penises have a sigmoid flexure ('S'-shaped bend) which straightens at sexual excitation, giving rise to an increase in penis length.
The glans penis, sometimes referred to as the rose penis, is the outer portion of the penis, normally retracted out of sight and held within its protective sheath or prepuce. The whole of the area is richly innervated and hence highly sensitive. The area becomes highly engorged and erect during sexual excitation, increasing in size three- to fourfold. Protruding from the end of the glans penis by approximately 5 mm is the exit to the urethra. Surrounding this protruding exit are the folds of the urethral fossa. This area is of particular significance as it is often filled with smegma (red-brown secretions
of the glans penis's epithelial glans plus sloughed cellular debris). This provides an environment ideal for the harbouring of bacteria, especially those responsible for venereal disease.
In cross-section, as illustrated in Figs 3.5 and 3.6, the glans penis can be seen to consist primarily of two areas of erectile tissue: the corona glandis, found dorsal to the urethra and outside the fibrous tunica albuginea; and the corpus carvenosus penis, also found dorsal to the urethra but within the tunica albuginea. The major area of erectile tissue is the corona glandis, which, as it is outside the tunica albuginea, has no fibrous capsule restricting the extent to which it can expand. Closer towards the end of the penis this area becomes increasingly more significant, at the expense of the corpus carvenosus penis. The significant expansion of the glans penis apparent at ejaculation is achieved
by engorgement of the unrestricted corona glandis tissue rather than the corpus carvenosus penis tissue, which is primarily responsible for the increase in size seen with the body of the penis.
The body of the penis is the major section running from the glans penis, externally, to the roots, internally. The major erectile tissue of the penis is situated in this body section, the main area of which is the corpus carvenosus penis, which runs along the dorsal aspect of the penis above the urethra and is enclosed by the tunica albuginea. This tough, fibrous sheath confers some rigidity to the penis, giving it support at full erection but, due to its elastic properties, also allowing for expansion of the penis, by up to twofold (Figs 3.7 and 3.8).
The corpus carvenosus penis is made up of a dense network of trabeculae with associated muscle tissue forming cavities, which become engorged at erection. A further, much smaller area of erectile tissue runs ventrally to the corpus carvenosus penis and is termed the corpus carvenosus urethra; it lies ventral to and around the urethra. This area, though not enclosed within, nor restricted by, the tunica albuginea, plays only a minor role in erection of the body of the penis. However, it is this area that extends to form the corona glandis of the glans penis. A third major area can be identified: the bulbo-spongiosus muscle, running along the ventral side of the penis, enclosing the corpus carvenosus urethra. This muscle and the retractor muscle (which can be identified as a relatively small, self-contained muscle running externally along the ventral side of the penis) are responsible for retraction of the penis and its retention within its sheath.
The base of the penis consists of two root areas, emerging laterally. These two root areas are attached to the pelvic tuber ischii via the paired ischiocarvenosus muscles running along either side of the penis. The root of the penis is stabilized by paired suspensory ligaments, again running along either side of the root from the base of the ischiocarvenosus muscle to the base of the pelvis.
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