Each spermatic cord funiculus spermaticus) () is composed of the ductus deferens and its vessels, and the testicular vessels and nerves, along with their serous membrane coverings, the mesoductus deferens and the mesorchium. These structures pass through the inguinal canal during the descent of the testis. The spermatic cord begins at the vaginal ring, the point at which its component parts converge to leave the abdominal cavity via the inguinal canal. The ductus deferens, which arises from the tail of the epididymis, leaves the vaginal ring, runs caudomedially in the deferential fold of peritoneum, and enters the prostate gland before opening into the prostatic part of the pelvic urethra. The ductus deferens is accompanied by the small artery of the ductus deferens, which arises from the prostatic artery and the vein of the ductus deferens, which drains into the internal iliac vein. The testicular artery originates from the ventral surface of the aorta. The testicular arteries arise cranial to the origin of the caudal mesenteric artery. The testicular artery runs laterally and caudally, crossing the ventral surface of the ureter, at which point it is joined by the testicular vein and nerve. The left testicular vein empties into the left renal vein and the right into the caudal vena cava. The peritoneal fold, the proximal mesorchium, enclosing the testicular vessels is attached to the abdominal wall in a line slightly lateral to the junction of the transversus abdominis and psoas muscles. The plexus of the testicular nerves arises from the area of the sympathetic trunk between the third and the sixth lumbar sympathetic trunk ganglia. The testicular lymph vessels pass to the lumbar lymph nodes.
The components of the spermatic cord are joined together by loose connective tissue and are surrounded by the visceral layer of the vaginal tunic. The peritoneal ring formed by the vaginal tunic passing through the deep inguinal ring is termed the vaginal ring (). There is usually an irregular mass of fat at the vaginal ring, covered by peritoneum. It overlaps the cranial border of the ring and probably acts as a valve to decrease the possibility of intestinal or omental herniation. The fat mass may be in two separate parts.
The ductus deferens, with its vessels, is enveloped by one fold of peritoneum at the vaginal ring, the mesoductus deferens and the testicular vessels and nerves are covered by another, the mesorchium. The double layer of peritoneum uniting these two folds to each other and to the edge of the vaginal ring is termed the mesofuniculus. It may be compared to the mesentery, which attaches the intestines to the abdominal wall.
Along the path of the spermatic cord from the deep inguinal ring to the testis, the relationship of the vaginal tunic and the enclosed structures remains constant. The tunic also reflects over the testis as its visceral peritoneum which joins the distal mesorchium along the dorsomedial border of the organ. A small, circumscribed area on the tail of the epididymis is free of tunic, allowing the ligament of the tail of the epididymis (embryonic gubernaculum testis) to attach the epididymis to the spermatic fascia.
The inguinal canal () is a fissure through the abdominal muscles that connects the deep and the superficial inguinal ring (). It is located approximately 1 cm craniomedial to the femoral ring. The femoral ring affords passage for the femoral vessels. The inguinal canal is bounded medially by the rectus abdominis muscle, cranially by the internal oblique muscle, and both laterally and caudally by the aponeurosis of the external abdominal oblique muscle. The superficial ring, located 2 to 4 cm lateral to the linea alba, is merely a slit in the aponeurosis of the external abdominal oblique muscle. It represents where the abdominal wall formed around the gubernaculum in the fetus. The cranial wall of the inguinal canal is made up of the transversus abdominis and internal abdominal oblique muscles, as well as the aponeurosis of the external abdominal oblique muscle. Only the latter forms the caudal wall of the canal.
As the spermatic cord and testis pass through the inguinal canal surrounded by the peritoneum of the vaginal tunic, transversalis fascia (underlying parietal peritoneum) is reflected onto them, and is here known as internal spermatic fascia. The combined superficial and deep abdominal fascia, from the external surface of the external abdominal oblique muscle, is reflected onto the vaginal tunic as it emerges from the inguinal canal. It then lies superficial to the internal spermatic fascia and is known as the external spermatic fascia. The cremaster muscle, a caudal fasciculus of the internal abdominal oblique muscle, lies adjacent to the vaginal tunic between the internal and the external spermatic fascia.
Both scrotal and inguinal hernias may occur in male dogs. In both of these hernias abdominal organs (greater omentum or a loop of jejunum) enter the canal of the vaginal tunic. Inguinal hernias remain in the inguinal canal. The hernia may be bilateral or unilateral. For a description of surface palpation of the superficial inguinal ring, see McCarthy (1976). Inguinal hernia (intestines or omentum pushing into the inguinal canal within the vaginal canal) is recognizable as a soft, fluctuating enlargement to one side of the penis.