The ureters enter the bladder posteroinferiorly. The ureteral orifices are situated on a crescent-shaped ridge and are approximately 2.5 cm apart. The triangular area demarcated by this interureteric ridge and bladder neck is called the trigone (Figure 30-2). As will be discussed later in the chapter, the trigone has a different embryologic origin from the rest of the bladder body, or fundus. The trigone is composed of mesoderm, and the fundus is composed of endoderm.

The bladder and urethra receive their nerve supply from both the sympathetic and the parasympathetic divisions of the autonomic nervous system. The sympathetic fibers, originating mainly from the lower thoracic and upper lumbar segments (T11-T12 and L1-L2), innervate the bladder and urethra as the hypogastric nerves. These sympathetic fibers are distributed more densely in the bladder base and proximal end of the urethra than in the bladder dome. The sympathetic nerves facilitate storage of urine. Studies have revealed differences in the bladder muscle receptors, with cholinergic receptors concentrated in the fundus and adrenergic receptors present in the trigone and proximal end of the urethra (Figure 30-3).

The parasympathetic nerve supply originates from the sacral segments (S2-S4), which proceed to form a plexus surrounding the bladder. In the male, a separate segment will reach the prostate and form the prostatic plexus. From this plexus, nerves emerge to innervate the erectile tissue of the male penis and the clitoris of the female (see Figure 30-3).

Branches of the bladder plexus penetrate the muscular coat of the bladder and become distributed throughout the detrusor. Parasympathetic muscle receptors are cholinergic in nature, and parasympathetic stimulation induces a detrusor contraction that causes bladder emptying.

The male urethra, which extends from the bladder to the external opening (urethral meatus) at the tip of the penis, functions as a conduit for both urinary and genital systems. It is commonly divided into three segments: the prostatic, the membranous, and the penile or spongy urethra (Figure 30-4).

The prostate lies below the bladder and has both a muscular and a glandular component. The normal prostate weighs about 20 g and measures about 3.5 cm transversely and about 2.5 cm in its vertical and anteroposterior dimensions. The prostate is conical and is anterior to the rectum. Its base is continuous with the bladder neck, and the inferior aspect of the prostate gland, or apex, lies adjacent to the urogenital diaphragm (Figure 30-5).

The seminal vesicles are paired organs that lie next to the prostate under the base of the bladder (see Figure 30-5). Their coiled pouches secrete a fluid important to the survival of spermatozoa.

The scrotum (see Figure 30-5) is a pouchlike sac that lies below the penis and pubic symphysis. A septum of connective tissue divides the sac into two compartments. Each compartment contains a male gonad, or testis, with its associated epididymis and the lower portion of the vas deferens protected by the spermatic cord and its coverings. The scrotum not only supports the testes but also, by relaxation and contraction of its muscular layer, helps regulate temperature of the testes.

The scrotal sac consists of several tissue layers. The scrotal skin overlies the dartos muscle layer, whose smooth muscle fibers are embedded in loose connective tissue. The dartos muscle functions to contract the scrotal pouch when cold and expand it when warm. Under the dartos layer are several fascial layers (see Figure 30-5) that are continuous with the muscular layers of the abdominal wall and make up the covering of the spermatic cord. The external spermatic fascia is continuous with the external oblique aponeurosis of the abdominal wall. A few slips of skeletal muscle derived from the internal oblique muscle layer make up the cremasteric muscle, which adds to the upper part of the cord. The internal spermatic fascia is a continuation of the transverse fascia of the abdominal wall, with the transversus abdominis muscle not contributing to the cord layers. Finally, the peritoneum provides the tunica vaginalis layers, which are actually separated from the abdominal cavity by obliteration of the processus vaginalis.

The scrotum receives its blood supply from the external pudendal artery, a branch of the femoral artery. In addition, the scrotum receives blood from portions of the internal pudendal artery (a branch of the hypogastric artery) and the cremasteric and testicular arteries that transverse the spermatic cord.

The testes are the male reproductive organs responsible for sperm production. They average about 4 to 5 cm in length and 2 to 3 cm in thickness. The testes lie within the scrotum and are suspended by the spermatic cord. The testes are covered by a thick fascial layer called the tunica albuginea. This layer invaginates posteriorly to form the mediastinum testis. This fibrous mediastinum sends fibrous septa into each testis that separate it into many different lobules. Each lobule contains one to four seminiferous tubules that if stretched to full length would measure approximately 60 cm. Spermatozoa production occurs within the epithelial lining of the seminiferous tubules (Figure 30-6).

The seminiferous tubules have a basement membrane consisting of elastic and connective tissue that supports the seminiferous cells. The seminiferous cells are either Sertoli cells (supporting cells) or spermatogenic cells. Found between the seminiferous tubules and embedded in connective tissue, the interstitial Leydig cells produce and secrete testosterone, a hormone involved in the development of male sexual characteristics (Figure 30-7). The seminiferous tubules converge on the mediastinum testis. The tubules, which are connected by the straight efferent ducts, drain into the head of the epididymis.

The epididymis is a tightly coiled tube that lies along the top of and behind each testis. It is divided into the head, situated at the upper pole of the testes; the body, lying posterior to the testes; and the tail, which is attached to the inferior pole of the testes (see Figure 30-6). The body and the tail of the epididymis form one continuous tube that serves as a conduit for maturing spermatozoa. In the epididymis, sperm develop the ability to swim.

As the convoluted tube of the tail leaves its testicular attachments, it increases in diameter to become a thick, muscular tube called the ductus deferens, also called the vas deferens. Leaving the spermatic cord, the vas deferens follows an extraperitoneal course and passes caudally and laterally along the pelvic wall. As it passes medial to the distal end of the ureter, it bends caudally to reach the midline and lies on the posterior wall of the bladder just medial to the seminal vesicles. It terminates in a dilated ampulla that courses underneath the base of the prostate. At this point the duct of the seminal vesicle joins with the duct of the ampulla, and the ejaculatory duct is formed. The ejaculatory ducts open in the prostatic urethra at the level of the verumontanum.

The penis is the male organ of copulation and urinary excretion. It is composed of three erectile bodies—two paired corpora cavernosa, which lie dorsally, and the corpus spongiosum, which contains the urethra (Figure 30-8). Grossly, the penis is divided into three segments. The root of the penis consists of the proximal ends of the corpora cavernosa, which attach to the pelvic bones, and the proximal end of the corpus spongiosum, which connects to the undersurface of the urogenital diaphragm. Together these attachments provide fixation and stability to the penis. The shaft or body of the penis consists of all three erectile bodies: the two cavernous bodies lying on the dorsum, and the corpus spongiosum, which occupies a depression on their ventral surface. Finally, the glans of the penis forms the distal segment of the corpus spongiosum (see Figure 30-4).

The arterial blood supply is primarily derived from the paired internal pudendal arteries, which are branches of the hypogastric arteries. Each internal pudendal artery branches several times in the penis. The deep or cavernous artery supplies the entire corpus cavernosum. The urethral artery supplies the corpus spongiosum, and the bulbar artery supplies the bulb of the corpus spongiosum. The dorsal artery continues along the dorsum of the penis and lies below the fascia of Buck and between two dorsal veins. It provides additional supply to the glans (Figure 30-9).

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