» The Roof of the Abdomen
The bodies of the lumbar vertebrae, the sublumbar muscles, and the diaphragm furnish the roof of the abdomen. The aorta and caudal vena cava lie to the left and the right within the cleft between the two psoas minor muscles (Fig.
21.22/6 and 7). The branches of the aorta and the tributaries of the vein are, in principle, the same as in other species.The autonomic nerves and ganglia show some equine-specific features, although not of much substance. The general pattern is shown in Fig. 21.24. The fused celiac and cranial mesenteric ganglia lie ventral to the aorta, to each side of the celiac and cranial mesenteric arteries. The right and left ganglia are joined by bridges cranial and caudal to the latter artery. They are sizable structures, 5 cm or so long, and are generally unequal, with the left being larger and more regular (Fig. 21.24/18). Each is joined by a major splanchnic nerve and, in varying fashion, by parasympathetic fibers from the dorsal vagal trunk. The nerves that leave the ganglia follow the branching pattern of the arteries and form a dense plexus in which the sympathetic and parasympathetic contributions mingle. The whole plexiform arrangement that radiates from the major ganglia is known as the Celiacomesenteric (solar) plexus. Additional small renal ganglia occur on the nerves about the renal arteries.
The celiacomesenteric complex is joined to the caudal mesenteric plexus by a plexus on the aorta and an additional trunk that runs at a more ventral level within the colic mesentery. The caudal mesenteric ganglion lies cranial to the origin of the like-named artery (Fig. 21.24/9 and 20). It gives rise to nerve plexuses that follow this vessel and the gonadal vessels to the small colon and reproductive organs, respectively, and to the hypogastric nerves (Fig. 21.24/21) that pursue a retroperitoneal course on the roof of the pelvis.
Lumbar splanchnic nerves join the major ganglia and the aortic plexus in an erratic fashion.There is usual direct detachment of preganglionic fibers from the splanchnic nerves to the medullary parts of the adrenal glands.
Comprehension Check
Working in a group, discuss and demonstrate the embryologic origins of the arrangement of the gastrointestinal tract of the horse. Develop a model to demonstrate the arrangement of the large intestine of the horse.
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* Although all agree that the prepubic tendon is the means by which the abdominal muscles obtain a principal attachment to the pelvic skeleton, opinions are divided on what constitutes the essential elements of this structure (and what are to be regarded as secondary augmentations). We adhere to the view that it is primarily formed of the linea alba and rectus tendons and secondarily by the incorporation of other elements, especially the decussation of the pectineus tendons. Others have regarded it as primarily a transverse structure attaching to and lying in front of the right and left pubic bones and strengthened by giving attachment to the linea alba and recti (and other components). However, the details are not of major relevance to most of the readers of this book.