Erection in the dog results from the filling of the spaces of the cavernous bodies with blood. The phenomenon of delayed erection in the dog is due to slow engorgement of the bulbus glandis and the pars longa glandis. There are two phases in the process of filling the erectile tissue of the penis with blood. One phase occurs prior to intromission and one post intromission. For intromission to occur the penis must be stiffened. This primarily involves the corpus cavernosum penis in the body of the penis. Presumably olfactory stimuli initiate activity in the brain that in turn activates sacral spinal cord segments. Pelvic nerve stimulation increases penile blood pressure, partial inhibition of venous drainage () and dilation of arteries in the penis (). Activation of sacral parasympathetic neurons inhibits the smooth muscle in the helicine branches of the deep arteries and the arteries of the bulb of the penis, allowing them to relax, which results in increased blood flow to the cavernous bodies. Activation of sacral somatic efferent neurons results in rhythmic contractions of the ischiocavernous muscles to help force blood into the corpus cavernosum penis.
Arterial blood pressure in the penis rises, but venous outlets are still sufficient to accommodate the increased inflow of blood. The corpus spongiosum receives the greater share of the blood through the artery of the bulb of the penis. Venous blood continues to flow into the bulbus glandis and into the vein of the bulb. Arterial blood is shunted from the artery of the bulb into the corpus cavernosum via anastomotic branches. The deep vein of the penis is not of sufficient diameter to drain the increased amount of arterial blood emptying into the cavernous spaces from the helicine arteries. Internal pressure against the tunica albuginea causes a stiffening of the corpus cavernosum. The intrinsic veins tend to be compressed. Intromission occurs prior to engorgement of the bulb of the glans.
In the second phase of penile erection that follows intromission, the process of intromission stimulates sacral somatic afferent dendritic zones in the skin of the penis (). This results in reflex activation of sacral somatic efferent neurons that innervate penile muscles. A rhythmic contraction of the bulbospongiosus muscles occurs and is visible in the copulating male. This forces the blood in the expanding bulb of the penis distally in the corpus spongiosum. At the level of the caudal portion of the os penis the vascular spaces in the corpus spongiosum surrounding the urethra are continuous around the os penis into the bulb of the glans. This permits the expanding blood volume in the corpus spongiosum to expand into the bulb of the glans. A tonic contraction of the two ischiourethralis muscles contributes to maintaining stasis in the erectile tissue by restricting outflow of blood in the dorsal vein of the penis where they form a common trunk.
When the paired ischiourethral muscles contract, the fibrous ring (encircling the common venous trunk) () is pulled caudally and laterally, narrowing the lumen of the ring and partially squeezing the vein. The fibrous ring is anchored to the symphysis pelvis by a short, thick ligament. Muscular contraction does not affect the arteries, which lie outside the encircling fibrous ring. The ischiocavernosus and bulbospongiosus muscles, upon contracting, slow down egress of blood through the deep vein, the vein of the bulb of the penis, and their union as the vein of the penis. Before intromission, the superficial veins of the glans, as well as the dorsal veins of the penis, allow free flow of venous blood away from the glans. With constriction of the veins leaving the two cavernous bodies, venous blood is further directed into the bulbus glandis via shunts from the corpus spongiosum. More arterial blood enters the capillaries of the pars longa glandis through the branches of the dorsal artery of the penis and the artery of the bulb.
Intromission also stimulates sacral somatic afferent neuro-nal dendritic zones in the skin and mucosa of the vulva and vestibule of the female, resulting in a reflex tonic contraction of the constrictor vulvae and vestibuli muscles. This contraction is on the body of the penis caudal to the engorged bulb of the glans restricting outflow of blood in the dorsal veins of the penis (). This contraction of the females constrictor vestibuli muscle also prevents blood from leaving the pars longa glandis through the superficial vein of the glans. Because of their relatively thick muscular walls and greater intrinsic pressure, the arteries of the penis are not occluded by extrinsic muscular contraction.
Because the superficial veins of the glans no longer permit outflow of blood, all blood in the pars longa is directed toward the deep veins of the glans. The spaces of the bulbus glandis receive venous blood from two sources: deep veins of the glans and the corpus spongiosum. When the dorsal veins of the penis permit free exit of blood from the bulbus, the blood traverses the ventral shunt in the bulbus without expanding the erectile tissue. With the dorsal veins partially occluded, and the inflow of blood greatly increased, blood entering the bulbus through the deep vein of the glans is forced into the dorsal bulbar shunt (). Because of the abundance of openings in the dorsal shunt, excess venous blood is permitted to enter and engorge the spaces of the bulbus glandis. The caliber of the dorsal veins of the penis is not sufficient for them to accommodate this increased amount of blood. Valves in the deep veins of the glans and in the venous connections with the corpus spongiosum prevent blood from leaving the bulbus by any route except through the now inefficient dorsal penile veins.
The engorgement of the bulbus glandis and pars longa glandis is facilitated by relaxation of the smooth muscles of the intersinusoidal trabeculae and by stretching of the elastic fibers in the trabeculae.
The branches of the dorsal artery of the penis lose their helicine-like appearance as the pars longa glandis is distended. The deep branch of the dorsal artery uncoils and straightens out during the height of erection. As the glans penis becomes engorged and firm, less arterial blood can be accommodated, and it is shunted from the dorsal artery to the corpus caverno-sum, increasing its stiffness. After ejaculation, the extrinsic penile muscles relax and arterial blood pressure drops to normal. Venous pressure declines as the erectile bodies shrink. The bulbus glandis decreases in diameter sooner than the pars longa glandis, owing to its greater venous drainage. Elastic recoil of the intersinusoidal trabeculae helps force blood out of the glans.
For a historical discussion of the mechanism of erection, refer to Eckhard (1863), Francois-Franck (1895), and Langley (1896). Hart (1972) recorded tonic contractions of the ischiourethralis muscle, which inhibits venous return in the dorsal vein, and rhythmical contractions of the bulbospongiosus, which forces blood distally. Purohit and Beckett (1976) found high arterial pressures in the corpora cavernosa that were correlated with contractions of the ischiocavernosus muscles. This indicates a more active role for the corpora cavernosa than was previously believed, although their small blood volume and inexpansible tunica albuginea would not indicate it.
Grandage (1972) divides coitus into a first stage (mounted) and a second stage of longer duration, when the male dismounts with the engorged penis still within the vestibule and vagina and faces in the opposite direction during the “tie” or “lock.” During the second stage there is a 180-degree bend in the middle of the body of the penis, which occludes the emissary veins and prevents detumescence. Using radiographic techniques, Grandage was able to determine that the penis displaced the cervix to the level of the sacral promontory. The glans and approximately 3 cm of the body enter the vagina. During this paradox of flexible rigidity, the dorsal surface of the penis remains dorsal, because it is a bend of the corpus penis and not a twist. Although the sperm-rich fraction of the ejaculate is passed within 80 seconds of first-stage coitus (following intromission), another 30 mL of seminal fluid is produced during second-stage coitus, which probably aids passage into the uterus and possibly stimulates peristaltic contractions of the uterine tubes.
The “tie,” or “lock,” depends on the reflex contraction of the ischiourethral muscle by stimulation of deep receptors caudal to the bulbus glandis and by the contraction of the constrictor vestibuli muscle of the female during copulation. Anesthesia of the receptors caudal to the bulbus glandis will result in a partial erection followed by copulation and ejaculation but no locking. Section of the ischiourethral muscle prevented enlargement of the bulbus glandis and locking, but did not interfere with copulation or ejaculation ().
The flaccid penis varies greatly in length and diameter, depending on individual variations as well as on breed and size of the animal. Physiologic changes caused by temperature, urination, and sexual excitement also contribute to marked variations of size.