THE ABDOMINAL WALL (See also pp. 52-55.)

The ventrolateral abdominal wall is constructed accord­ing to the common pattern and has only a few features of distinction; because abdominal surgery is so fre­quently performed in dogs and cats, it is necessary to be familiar with the details and it may be prudent to review the description previously given.

The linea alba is the fibrous seam in which the apo­neuroses of the right and left oblique and transverse abdominal muscles come together. It extends from the xiphoid process to the pubis and includes the umbilicus at about the level of the third lumbar vertebra. The linea alba is about 1 cm wide cranial to the umbilicus but gradually narrows behind this point and is reduced to a barely visible line in its caudal third (see Figure 2-26). Incisions through the linea alba spare the muscles, vessels, and nerves; there is the additional advantage that the parietal peritoneum does not retract from the edges of a median incision as happens elsewhere. The falciform ligament (see further on) and the median liga­ment of the bladder attach to the dorsal surface of the linea alba, cranial and caudal to the umbilicus, respec­tively. Umbilical hernias, often associated with an over­wide linea alba and hypoplastic rectus muscles, are common.

The rectus sheath is formed by the aponeuroses of the oblique and transverse abdominal muscles. In the dog and cat the aponeuroses of the external and internal oblique muscles pass ventrally (externally) to the rectus muscle over the length of the linea alba. However, the most cranial portion of the internal oblique muscle also detaches an additional lamina that passes dorsally (internally), on the deep surface of the rectus (Figure 2-26, A).

The inguinal canal is a potential space between the external and internal abdominal oblique muscles that extends between deep and superficial openings (rings). The deep ring leads from the canal into the abdominal cavity, and the superficial ring leads from the canal to the subcutaneous tissues of the groin. In both sexes the canal conveys the external pudendal vessels and the genitofemoral nerve; it also conveys the spermatic cord in the dog and tom and the vaginal process in the bitch and queen. These all emerge at the superficial inguinal ring, a nearly sagittal slit in the external abdominal oblique aponeurosis about 3 cm lateral to the linea alba, close to where this attaches on the pubis (see Figure 2-27, AZ4'). Only the caudal end of the ring is palpable. The narrow strip of aponeurosis (Figure 2-27, A between 4' and 6) lateral to the ring forms the only barrier between the structures issuing from the canal and the large femoral vessels and saphenous nerve as they enter the thigh through the vascular lacuna (Figure 2-27Z6).

The deep inguinal ring is visible only from within the abdomen. It is bounded caudolaterally by the caudal border of the external abdominal oblique aponeurosis (inguinal ligament), cranially by the unattached border (caudal edge) of the internal abdominal oblique muscle, and medially by the rectus muscle (see Figure 2-27, B). None of these boundaries is palpable in the intact animal. The parietal peritoneum that covers the ring evaginates through the inguinal canal and, named as the vaginal tunic, accompanies the spermatic cord into the scrotum. In the bitch and queen it envelops the round ligament of the uterus and is known as the vaginal process; the process is not present in females of other domestic species and is the occasional recipient of her­niated abdominal organs (p. 461).