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Leonardo da Vinci described the penis as follows: “. . . it disputes with the human intellect, and sometimes has intellect itself, and although the will of a man may wish to stimulate it, it remains obstinate and goes its own way, sometimes moving on its own without the permission or thought of the man. Whether he is asleep or awake, it does what it desires, the penis having a mind of its own.” This organ has been a source of fascination for thousands of years. It facilitates not just urination and sexual activity, but is the seat of masculinity and virility in many cultures. This organ is perfectly designed for its functions and is composed of multiple types of tissues and structures, which, like a good orchestra in concert, function perfectly together in the healthy male.

About two-thirds of the penis is external (that is, outside of the body) and one-third is internal (Figure 1). The erection chambers are paired cigar-shaped structures that travel from the head of the penis into the body and run along the bony structures known as the ischiopubic rami. These bony structures are what we sit on when we sit on a bicycle seat, for example. The paired erection chambers are technically known as the corpora cavernosa (singular: corpus cavernosum). They are composed of two major types of tissue. The outer lining, which is approximately 2mm in thickness, is known as the tunica albuginea, more commonly called, simply, the tunica. Housed within the tunica is the erectile tissue, which itself is composed of numerous types of tissues (Figure 2). This tissue includes muscle, blood vessels, nerves, elastic tissue, collagen, and a tissue very important to erectile function known as the endothelium. I will talk more about this tissue toward the end of this chapter and you will see it pop up again and again in this book.

The erectile tissue is fashioned into a lattice-work of spaces. The simplest way to think of it is that erectile tissue is like Swiss cheese; you have solid tissue interspersed with spaces. In the flaccid (soft) state, these spaces are tiny. However, when men are aroused, these spaces expand, become large, and actually fill with blood to allow an erection to occur (Figure 3). Each of these spaces (known as sinusoids or lacunar spaces) is lined on its inside by endothelium. The endothelium is a major regulator of blood flow into the penis. It produces numerous chemicals, the most important one of which is nitric oxide, which I will discuss in greater detail shortly. Outside of the endothelium is the muscle. This muscle is similar in nature to the bladder and bowel muscle in that it is not under your direct control. In the flaccid state, it is kept contracted to make sure the spaces are tiny, but under arousal, it relaxes greatly and allows these spaces to expand significantly. Coursing outside of the smooth muscle between the spaces are blood vessels and nerves, which, of course, are essential to erection.

The third chamber inside the penis is known as the corpus spongiosum. This sits beneath the paired erectile bodies and houses the urethra (the urine channel). It travels from the opening of the bladder (bladder neck) as the urethra passes through the prostate, all the way out to the head of the penis. In fact, the head of the penis (known as the glans) is in direct continuity with the corpus spongiosum and has no direct connection with the erectile bodies. Strictly speaking, the corpus spongiosum is not a true erectile body as it has very little spongy tissue in contrast to the corpora cavernosa. The corpora cavernosa are tightly joined along their external two-thirds and share an intervening wall known as the septum. This septum has numerous channels passing from right to left side so that there is equalization of blood flow and pressure between the two erectile bodies. For example, with a penile injection (where a patient injects his penis to get an erection; see Chapter 11), an injection on the left side of the penis causes an erection in both right and left erectile bodies.

Figure 1 • Side view of the male pelvis illustrating relationship between the bladder, prostate, and penis (corpora cavernosa).

Outside the erectile bodies, there are numerous layers of tissue, but most importantly, on the top surface of the penis (that is, the surface of the penis that a man looks down on), there is a vein (dorsal vein), a right and a left artery (dorsal artery), and a right- and left-sided set of nerves (dorsal nerves). The deep dorsal vein is a major route of blood flow leaving the penis. The dorsal arteries supply the penile shaft, skin, and subcutaneous tissue, as well as the head of the penis,with fresh blood.They also have some branches that travel into the erectile chambers as the dorsal artery passes from the base to the head of the penis. In some men, these arteries (known as perforators, as they perforate the tunica) are significant contributors to erectile function. The nerves running on the surface of the penis beneath the skin, known as the dorsal nerves, are sensory in nature. That is, they supply sensation and only sensation to the penis. Thus, they are not intrinsically involved in erectile function.

Figure 2 • Cross section of the penis demonstrating the corpora cavernosa and corpus spongiosum. The outer lining of the tunica albuginea is a multi-layered structure. The erectile tissue has a honey-comb appearance.