The horse has a capacious gastrointestinal tract and a correspondingly bulky abdomen.
However, the extent of the abdomen is not immediately apparent because a large part is concealed within the rib cage. The olecranon and the lower end of the sixth rib are handy guides to the most cranial extent of the diaphragm (see Fig.
20.3). The flank is reduced in size by the caudal inclination of the ribs, the last of which may be within a few fingerbreadths of the coxal tuber (see Fig. 22.23A/1" and 3).Abdominal conformation varies much with age, condition, and the amount and nature of the rations. The ventral contour is especially variable; it slopes gradually between the sternum and the pubic brim in animals in hard condition but dips to reach its lowest point behind the xiphoid process in those in softer condition, in pregnant mares, and in ponies generally. In the latter groups the most caudal part of the floor ascends very steeply. These differences are not always visible because the most caudal part of the abdomen is covered laterally by the fold of skin that passes between the flank and the thigh (see Fig. 22.23A/6) and ventrally by the prepuce or udder.
The trunk is broadest at the last ribs. The upper part of the flank sinks in to form a paralumbar fossa, but it is much less obvious than in cattle. The lower part of the belly is rounded from side to side, except in foals, in which the whole abdomen is slab-sided and shallow (see Fig. 23.2). The usual symmetry may be disturbed in late pregnancy or by accumulation of gas in parts of the gastrointestinal tract.
The position of the last rib is often visible, but most other skeletal boundaries of the flank and floor are less easily found. The transverse processes of the lumbar vertebrae are usually too deeply buried under muscle to be palpable. The dorsal part of the coxal tuber is very conspicuous, but the ventral part, which gives origin to the internal oblique and tensor fasciae latae muscles, is not visible, although it is easily palpable.
Soft features that may be recognized include the internal oblique muscle, which raises a ridge along the caudoventral boundary of the paralumbar fossa (Fig. 21.1B/5), and the superficial thoracic ("spur") vein, which runs over the ventral part of the abdominal wall toward the axilla, following the dorsal border of the deep pectoral muscle. One can identify and palpate the subiliac lymph nodes arranged in a spindle at the cranial margin of the thigh, midway between the coxal tuber and patella. They are more easily found if drawn forward. The superficial ring of the inguinal canal can be found on deep palpation of the groin, which is a procedure sometimes resented by the horse and therefore to be performed with care (Fig. 21.1A/3).