This chapter will be concerned with a consideration of gastrointestinal physiology in the horse and the pig, concentrating on those features of the two species not normally discussed in textbooks of medical physiology.
Gastrointestinal physiology is less well understood in the horse than in the ruminant.
Whereas ruminants cope with the cellulose in their diets with the help of the microorganisms in the rumen, in herbivores like the horse, it is the large intestine that becomes enlarged to serve as a fermentation vat. The pig occupies a somewhat intermediate position, the capacity of its stomach being nearly 30 percent, while the capacity of its caecum, colon and rectum adds up to 37 percent of the total capacity of its GI tract. The nonsecretory mucosa is a stratified squamous epithelium which constitutes the lining of the entire ruminant forestomach. Cardiac glandular mucosa occupies the cranial half of the pig stomach, while a belt of cardiac mucosa lies between the cranial nonglandular and caudal glandular parts of the horse stomach. In the pig, this mucosa secretes a fluid with ionic composition similar to that of plasma, except that the bicarbonate concentration is about four times higher - so that the fluid is a powerful biffer.