abdominal cavity.
1.3.1 The skin
The skin, or common integument, varies in thickness between species and bodily location. The abdominal skin is very thick (4–5 mm) in the ox but is quite delicate and thin (1–3 mm) in the other domestic species. Hair grows from the skin in all of the species but is much less in the pig. In all species there is much less hair on the ventral abdomen than elsewhere. Most of the hair of the sheep has a specific structure and is termed wool. In all species except the pig a principal function of the hair/wool is to reduce heat loss; the pig relies on a large amount of subcutaneous fat for this function.
The domestic species vary in regard to the number and distribution of the mammary glands. The mare has only two mammary glands, and these are located either side of the midline on the ventral abdomen in a prepubic position. The cow usually has four mammary glands, collectively known as the udder; it is located mainly ventral to the caudal abdomen but with its caudal part ventral to the pelvis. The udder is suspended by strong elastic tissue extending essentially from the linea alba and the symphyseal tendon.
There are seven pairs of mammary glands in the sow, although only 8–10 are usually functional depending on litter size. In this species the mammary tissue extends in the body wall from the axilla to the level of the stifle.
The udder of small ruminants comprises two glands and is situated in the inguinal region. In the bitch there are usually five pairs of mammary glands; in the cat there are generally four pairs.
1.3.2 The subcutaneous fascia
Superficial fascia: In the pig this layer is adipose over most of its area and functions as an insulating layer promoting heat retention. However, in most other species this adipose tissue is not complete except in the inguinal region. In horses and cattle the cutaneous muscle is well developed in the superficial fascia layer and serves to twitch the skin to dislodge flies.
Deep fascia: In the horse and ox the deep fascia is developed as a thick sheet of fibroelastic tissue covering most of the external abdominal oblique muscle, the ribs and the tuber coxae. This is termed the yellow elastic tunic providing support for the abdominal contents and contributing to the suspensory apparatus of the udder in the cow.
1.3.3 The rectus abdominis muscle (Figures 1.1 and 1.2)
Figure 1.1 Lateral view of inguinal region of horse showing the rectus abdominis muscle. The peritoneum of the vaginal tunic is strongly reinforced by fusion with the internal spermatic fascia (derived from the transverse abdominal muscle).
Figure 1.2 Ventral view of the inguinal region of horse showing the rectus abdominis muscle. The left side of the diagram shows the relative positions of the superficial inguinal ring and the vaginal ring. On the right side of the diagram the vaginal tunic is shown wending its way from the deep inguinal ring, through the inguinal canal and out through the superficial inguinal ring.
Origin: The ventral surfaces of the sternal ribs and sternum.
Insertion: The cranial border of the pubis with the prepubic tendon. The prepubic tendon is the tendon of insertion of the two rectus abdominis muscles, although most of its fibres extend between the iliopubic eminences.
Structure: The left and right muscles are separated longitudinally by the linea alba, a band of fibrous tissue extending from the xiphoid cartilage to the prepubic tendon. A series of three to six transverse tendinous inscriptions cross each muscle belly, but the resulting muscle segments are not correlated with the nerve supply.
Species variations: In the ox there is wide separation of the medial borders of the rectus abdominis muscles. caudally. In the immature animal the linea alba is perforated by the umbilicus.
1.3.4 External abdominal oblique muscle (Figures 1.3–1.5)
Figure 1.3 Lateral view of the inguinal region of the horse left external oblique abdominal muscle.The spermatic sac is seen emerging from the left superficial inguinal ring.The spermatic sac contains the testicle and the spermatic cord (See Figure 15.4).For a definition of the spermatic sac see Section 16.4
Figure 1.4 Ventral view of inguinal region of the horse showing the left and right external abdominal oblique muscles. The arrows indicate the left and right femoral canals, providing exit for the femoral arteries and veins.
Figure 1.5 Lateral view of the abdomen of the ox showing the left external abdominal oblique muscle.
Origin: The lateral surfaces of the ribs caudal to the fourth rib and the lumbodorsal fascia.
Insertion: The linea alba and prepubic tendon.
Structure: Most of the muscle fibres run caudoventrally. At its origin it consists of muscle fibres but towards its insertion caudoventrally it becomes a tendinous aponeurosis. Towards its insertion in the prepubic tendon there is a slit in the aponeurosis; this is the superficial inguinal ring. The slit divides the tendon into an abdominal part cranially and a pelvic part caudally. The caudal edge of the pelvic part of the tendon is the inguinal ligament.
Species variations: The external abdominal oblique muscle of the dog and pig is mainly muscular almost to the dorsal edge of the rectus abdominis muscle. In ruminants there is no origin from the lumbodorsal fascia, but there is an insertion on the tuber coxae. In the ox the aponeurosis of this muscle is extensive. In the horse the external abdominal oblique muscle is very large and inserts onto the femoral fascia, linea alba, tuber coxae and the prepubic tendon.
1.3.5 Internal abdominal
