Accessory Sex Glands

Prostate

The prostate gland completely surrounds the neck of the bladder and the beginning of the pelvic urethra in the dog.

Note that the prostate of the male cat does not completely surround the pelvic urethra and does not enlarge very much with age.

The normal size and weight of the prostate vary greatly. The organ generally lies at the pelvic inlet. It is larger and extends farther into the abdomen in older dogs. The prostate is flattened dorsally and rounded ventrally and on the sides. It is heavily encapsulated. Muscle fibers from the bladder run caudally on its dorsal surface. A longitudinal septum leaves the ventral part of the capsule and extends dorsally to reach the prostatic part of the pelvic urethra, thus partially dividing the gland ventrally into right and left lobes. This is indicated on the ventral surface by a shallow but distinct longitudinal furrow.

Again notice that the pelvic (prostatic) urethra runs through the center of the gland.  The prostatic urethra is simply a subpart of the pelvic urethra surrounded by the prostate gland.

On the dorsal surface of the bladder, locate the two ductus deferentes (singular:  ductus deferens) where they are joined by the genital fold.   The genital fold is where the left and right mesoductus deferens combine to connect the left and right ductus deferens.  The mesoductus deferens is the connecting periotoneum of the ductus deferens.  It covers the ductus deferens itself and connects it to the abdominal wall and to the other ductus deferens by the genital fold.

Bulbourethral Glands

Tomcats only:  The bulbourethral glands, located dorsolateral to the pelvic urethra at the ischiatic arch should be identified.

Prepuce

The prepuce is a tubular sheath or fold of integument that is continuous with the skin of the ventral abdominal wall and is reflected over the glans. It has a smooth internal lamina and a haired external lamina, which meet at the preputial orifice. At its deepest recess, the fornix of the prepuce, the internal layer is reflected onto the glans as the skin of the glans. In the erect state the fornix is eliminated, because the external layer of the prepuce is closely applied to the body of the penis.

The penis is composed of a root, a body, and a glans. The dorsal surface of the penis faces the pelvic symphysis and the abdominal wall. In the nonerect state the glans is entirely withdrawn into the prepuce.

Root of the Penis

The root of the penis is formed by right and left crura and the bulb of the penis.  The crura originate on the ischiatic tuberosity of each side. The root ends where the crura blend with each other on the midline to form the body. Each crus is composed of vascular cavernous tissue, corpus cavernosum (penis), supplied by the deep artery of the penis and surrounded by a thick fibrous tunic, the tunica albuginea penis.

At the root of the penis between the crura is the bulb of the penis covered by the bulbospongiosus muscle. This bilobed dorsal expansion of the corpus spongiosum (penis) surrounds the urethra and is located at the ischial arch. It is supplied by the artery of the bulb. A thin layer of corpus spongiosum penis surrounds the entire length of the urethra in the penis.  FYI:  Corpus spongiosum also surrounds the pelvic urethra, but for our purposes, we will focus on the corpus spongiosum as part of the penis. 

The artery of the bulb of the penis arborizes in the bulb and continues to supply the corpus spongiosum penis and penile urethra.  While we will not be dissecting the origin of this vessel, knowing what part of the penis it vascularizes is required.

The deep artery of the penis arises close to the artery of the bulb and enters the corpus cavernosum penis at the crus. This is at the level of the ischial arch lateral to the penile bulb.  Like the artery of the bulb, we will not be dissecting the origin of this vessel.  However, naming the part of the penis it vascularizes is required.

The retractor penis muscle is an elongated slip of mixed smooth and striated muscle fibers. It originates from the ventral surface of the sacrum, or first two caudal vertebrae, blends with the external anal sphincter, and extends distally on the ventral surface of the penis to the level of the glans, where it inserts. In the region of the anal sphincter, there is muscle fiber exchange between the retractor penis muscle and the external anal sphincter.

The bulbospongiosus muscle covers the bulb of the penis and thus bulges between the ischiocavernosus muscles (which cover the crura), ventral to the external anal sphincter. The fibers of the bulbospongiosus are transverse proximally, where they cover the bulb of the penis, and longitudinal distally, where they pass onto the body of the penis.

Body of the Penis

The body of the penis extends from the root, where the crura blend with each other, to the glans penis covering the os penis in the caudal portion of the prepuce.

Note that the region at the beginning of the body of the penis is compressed from side to side and wrapped by a thick tunic.  It is capable of being bent without twisting when the male dismounts during coitus and remains “locked” for a variable period.

The corpus cavernosum penis of each crus converges toward the midline and the two corpora extend side by side throughout the body to the os penis. A median septum completely separates the two corpora in the carnivore, and each is covered by a thick white capsule (tunica albuginea penis-differentiate this term from tunica albuginea teste) throughout its length. The two corpora cavernosa form a groove ventrally that contains the urethra and the thin corpus spongiosum that surrounds the urethra.

Glans Penis

The glans of the penis is composed of corpus spongiosum glandis erectile tissue.  In the dog, two subparts of the corpus spongisosum glandis are identified—the proximal bulbus glandis and the distal, more elongate pars longa glandis. The bulbus glandis, surrounding the proximal end of the os penis, is a dorsal extension of the corpus spongiosum. It is an expansile vascular structure that is largely responsible for retaining the penis within the vagina during copulation.

The pars longa glandis is a cavernous tissue structure that overlaps the distal half of the bulbus glandis and continues to the distal end of the penis, partially encircling the os penis and the urethra. The pars longa glandis has no direct arterial communication with the corpus spongiosum penis and is separated from the bulbus glandis by a layer of connective tissue. Venous channels drain the pars longa glandis into the bulbus glandis through this layer.

Note that in the tomcat, the glans penis of the sexually mature penis is covered with numerous highly keratinized, proximally directed penile spines. These are testosterone dependent and will atrophy following castration as will the accessory sex glands (prostate and bulbourethral glands).  The cat’s corpus spongiosum glandis is not differentiated into two parts like in the dog. 

 

The os penis is a long, ventrally grooved bone that lies almost entirely within the glans penis. It forms about a month after birth as an ossification of the fused distal ends of the corpora cavernosa penis. The body of the os penis extends through the glans penis. The base and body are grooved ventrally by the urethral groove, which surrounds the urethra and the corpus spongiosum on three sides. The bone ends as a long, pointed fibrocartilage in the tip of the glans, dorsal to the urethral opening.

At the level of the collar-like bulbus glandis of the glans of the penis, there is a communication between the corpus spongiosum penis and the bulbus glandis. The dorsal artery of the penis courses to the glans, where it supplies the prepuce, corpus spongiosum penis, and pars longa glandis.

 

Male Inguinal structures and Testicles

Review the anatomy of the inguinal canal now.  Understanding of the inguinal canal will inform the understanding of the layers of tissue surrounding the descended testicle: The inguinal canal  is a short fissure filled with connective tissue between the abdominal muscles. It extends between the deep and superficial inguinal rings. It is bounded laterally by the aponeurosis of the external abdominal oblique.  It begins at the deep inguinal ring, the bordered cranially by the caudal border of the internal abdominal oblique; caudally by the caudal border of the aponeurosis of the external abdominal oblique (inguinal ligament); and medially, in part, by the superficial surface of the rectus abdominis. It ends at the superficial inguinal ring, which is a slit in the aponeurosis of the EAO.  The transversalis fascia and parietal peritoneum pass into the inguinal canal and change name to the internal spermatic fascia and parietal vaginal tunic, respectively.  The spermatic cord  in the male and the vaginal process and round ligament of the uterus in the female dog pass obliquely caudoventrally through the canal.  In the queen, the vaginal process does not extend into the inguinal canal.  In both sexes in domestic species, the external pudendal a/v and genitofemoral nerve traverse the canal.

The external pudendal artery and vein leave the superficial inguinal ring caudal and medial to the structures that extend to the testis.

The genitofemoral nerve arises from the ventral branches of the third and fourth lumbar nerves. It is bound by fascia to the external pudendal vein medial to the spermatic cord. It passes through the inguinal canal adjacent to the spermatic cord and innervates the cremaster muscle and the skin covering the inguinal region and proximal medial thigh of both sexes and part of the prepuce in the male.

Recall that although they lack a spermatic cord, the genitofemoral nerve and external pudendal artery still DO extend through the inguinal canal in females of the domestic species.

The internal spermatic fascia, a continuation of the transversalis fascia (which lines the abdominal cavity external to the parietal peritoneum) invested with the parietal vaginal tunic, surrounds the structures emerging from the superficial inguinal ring, including the anatomic spermatic cord.  The cremaster m., external pudendal a/v, and genitofemoral n pass through the inguinal canal adjacent to but external to the internal spermatic fascia.

The cremaster muscle is surrounded by the external spermatic fascia (continuation of the fascia of the external abdominal oblique) as it courses along the external aspect of caudal part of the internal spermatic fascia. The cremaster muscle arises from the caudal free border of the internal abdominal m. and attaches to the testes distally. It functions to draw the testicle toward the body.

The external spermatic fascia is a continuation of the loose, subcutaneous connective tissue just exterior to the abdominal muscle walls layers into the inguinal region and then into the testicle.

The internal spermatic fascia is a continuation of the transversalis fascia one it has been pulled through the deep inguinal ring by the descending testicle.  The parietal vaginal tunic is adhered to the interior surface of the internal spermatic fascia and is a continuation of the parietal peritoneum starting at the vaginal ring.  Recall that the vaginal ring is where the parietal peritoneum turns on itself to enter the deep inguinal ring, where it also name changes to parietal vaginal tunic.  The testicle itself, as well as the testicular a./v./n./l., are covered in connecting peritoneum called the mesorchium in the abdominal cavity.  Once through the inguinal ring, the mesorchium changes name to the visceral vaginal tunic, the thin serous membrane covering the testicle and accompanying nerves and vasculature outside of the abominal cavity.

The spermatic cord is carried through the inguinal canal by the descent of the testis and is composed of two distinct parts: the ductus deferens and the testicular artery and vein.

The testicular artery and vein, as well as the testicular lymph vessels and the testicular plexus of autonomic nerves, are closely associated with each other. These vessels and nerves are covered by a fold of peritoneum, the mesorchium, that is continuous with the parietal and visceral vaginal tunics. The artery is tortuous, and woven around it are the nerve plexus and the venous plexus. The venous plexus is the pampiniform plexus, which will be observed with the testicle. The testicular artery and vein are branches of the aorta and caudal vena cava, respectively. They enter the testis at its cranial end. The nerve plexus is autonomic and sensory and contains postganglionic sympathetic axons, which arise from the third to fifth lumbar sympathetic ganglia.

The ductus deferens carries the spermatozoa from the epididymis to the urethra. It arises from the tail of the epididymis at the caudal end of the testis and is attached to the mesorchium by the mesoductus deferens. The mesoductus deferens is the connecting mesentery that attaches the ductus deferens to the body wall proximally and the mesorchium distally. It contains the artery, vein, and nerve of the ductus deferens.  Recall that the right and left ductus deferentas are connected to each other by the genital fold.

Scrotum and Testicles

The scrotum is a pouch divided by an external scrotal raphe and an internal median septum into two cavities, each of which is occupied by a testis, an epididymis, and the distal part of the spermatic cord.  The tunica dartos, muscle fibers located deep to the skin of the scrotum act in temperature regulation of the testicle.  The internal spermatic fascia is connected to the scrotum via the scrotal ligament.

The testis and the associated epididymis and ductus deferens are intimately covered by the visceral vaginal tunic. At the caudal extremity of the epididymis, the visceral vaginal tunic leaves the tail of the epididymis at an acute angle and is continuous with the parietal vaginal tunic. The connective tissue that attaches the epididymis to the parietal vaginal tunic and internal spermatic fascia at this point is the ligament of the tail of the epididymis.  The testicular parenchyma is covered by a thick fibrous capsule called the tunica albuginea teste, which itself is covered in visceral vaginal tunic.

The epididymis lies more on the lateral side of the testis than on its dorsal border. For descriptive purposes it is divided into a cranial extremity, or head, where the epididymis communicates with the testis; a middle part, or body; and a caudal extremity, or tail, which is continuous with the ductus deferens. The tail is attached to the testis by the proper ligament of the testis and to the parietal vaginal tunic and spermatic fascia by the ligament of the tail of the epididymis. The ductus deferens passes cranially over the testis medial to the epididymis.