href="#ulink_91c3f1d1-c112-56db-9c1e-8bdeed4c9be8">Figure 1.7 Regions of the penis: R = root of penis, Bb = bulb of penis, Bd = body of penis, Fp = free part of penis, G = glans penis.
The erectile tissue is enclosed in the dense outer covering of the tunica albuginea. The tunica albuginea is a dense covering that consists of an inner circular layer and outer longitudinal layer of fibers. The inner circular layer sends trabecular scaffolds throughout the corpus cavernosum for the attachment of the cavernous endothelium.
Located caudal to the root of the penis are the muscles of the penis: the ischiocavernosus, bulbospongiosus, and retractor penis muscles (Figure 1.8). The paired ischiocavernosus muscles originate on the medial surfaces of the ischiatic tuberosities overlying the crura; the muscle fibers pass ventromedially in a “V” fashion until ending a short distance on the body of the penis [1]. During erection the ischiocavernosus muscle contracts, pushing blood from the cavernous spaces of the crura into the body of the penis [7]. The bulbospongiosus muscle lies caudal to the bulb of the penis, originating along the ischiatic arch and continuing until the junction of the crura [1]. The bulbospongiosus muscle fibers run transversely across the bulb of the penis and contraction of this muscle results in propulsion of the ejaculate through the urethra [7]. The retractor penis muscle extends from the caudal vertebrae and internal anal sphincter to insert distal to the sigmoid flexure [8]. These paired muscles relax during erection, allowing the penis to extend from the prepuce and contract during quiescence, retracting the penis into the sheath [8].
Figure 1.8 Muscles of the penis: Bs = bulbospongiosus muscle, Ic = ischiocavernosus muscle, Rp = retractor penis muscle.
The body of the penis begins where the two crura meet distally to the ischiatic arch; it extends craniad, along the ventral body wall to become at the mid‐ventral abdomen the free part of the penis (Figure 1.9). The body of the penis is bent in an “S” shape called the sigmoid flexure. The proximal bend of the sigmoid flexure opens caudally and is located near the scrotum. The distal bend is opened cranially and the short suspensory ligaments of the penis attach the penis to the ventral surface of the ischiatic arch. The glans penis is a small restricted region at the tip of the free part of the penis [8] (Figure 1.10). The free part of the penis is the distal extent from the attachment of the internal lamina of the prepuce to the glans penis [8]. The free end of the penis is twisted in a counterclockwise direction as viewed from the right side, illustrated by the oblique direction of the raphe of prepuce continued as the raphe of the penis to the urethral process (Figure 1.11). The twist of the free end of the penis is due to the attachment of the apical ligament. The apical ligament of the penis is formed by the longitudinal fibers of the tunica albuginea leaving the body of the penis just distal to the sigmoid flexure and reattaching near the apex of the penis [9].
Figure 1.9 Sigmoid flexure of the penis: Bd = body of penis, Sf = sigmoid flexure.
Figure 1.10 Glans penis and associated structures: G = glans penis, Fp = free part, Ip = internal lamina of prepuce.
Figure 1.11 Prepuce and associated structures: Rp = raphe of penis, Ip = internal lamina of prepuce, G = glans penis, Up = urethral process.
The prepuce of the penis is composed of an external and internal fold or lamina [8] (Figure 1.12). The external lamina is the haired outer fold of skin attached to the ventral abdomen. The haired skin terminates at the preputial orifice where the external fold turns inward to line the preputial cavity as the internal lamina. The internal lamina serves to attach the external lamina to the penile epithelium.
Figure 1.12 Components of the prepuce: Fp = free part of penis, G = glans penis, Ip = internal lamina of prepuce, Ep = external lamina of prepuce, Po = preputial orifice.
Blood Supply
Before ejaculation can occur the testis must produce spermatozoa. This requires an adequate blood supply for the metabolic demands of cellular division for spermatogenesis and steroidogenesis. The arterial blood supply to each testis is provided by a testicular artery, a direct branch of the abdominal aorta arising caudal to the renal arteries. The testicular artery crosses the lateral abdominal wall and then passes ventrally through the inguinal canal [10]. As the testicular artery approaches the testis it begins to spiral, with the nearby tortuous pampiniform plexus of the testicular vein forming a vascular cone. This arterial/venous arrangement is an effective thermoregulatory apparatus [11].
An adequate blood supply to the penis and associated muscles is required for the processes of erection, ejaculation, and tissue maintenance. This comes by way of the internal iliac artery. The internal iliac artery is a direct continuation of the abdominal aorta at the entrance to the pelvic cavity. The umbilical artery, a branch of the internal iliac, supplies the ductus deferens and the bladder [4]. The prostatic artery leaves the internal iliac and supplies the prostate, vesicular glands, ductus deferens, ureter, and urethra [4]. As the internal iliac continues through the pelvic cavity it divides into the caudal gluteal and internal pudendal [10]. The internal pudendal gives off the ventral perineal artery and urethralis artery, and continues as the artery of the penis [10]. The artery of the penis gives off the artery of the bulb of the penis, which supplies the bulbospongiosus muscle and the cavernous spaces of the corpus spongiosum [12] (Figure 1.13). The deep artery of the penis is another branch of the artery of the penis that enters the crus of the penis and supplies the erectile tissue, the corpus cavernosum [12]. After the deep artery branches off, the artery of the penis continues as the dorsal artery of the penis which passes along the dorsal aspect of the penis toward the glans penis and prepuce. It is responsible for maintenance of penile tissue during quiescence [13].
Figure 1.13 Arterial supply to the penis: Ip = internal pudendal artery, Ab = artery of the bulb of the penis, Dpa = deep artery of the penis, Dn = dorsal nerve, Da = dorsal artery of the penis, Rp = retractor penis muscle, Sc = spermatic cord, Sf = sigmoid flexure.
Nervous Supply
The innervation of the external genitalia of the bull consists of the pudendal nerve and its branches. The pudendal nerve carries motor,
