Page 413 - Anatomy and Physiology of Farm Animals, 8th Edition

398 / Anatomy and Physiology of Farm Animals

in the rumen so that the pH in the rumen wall coupled with regional contraction of
longitudinal muscles in the area of the
does not become too acidic.
VetBooks.ir limbs of neural reflexes that regulate bolus (Fig. 21‐2).
Parasympathetic nerves are the efferent
Peristalsis carries solid and semisolid
salivary secretion. Afferent inputs that food through the esophagus of the horse at
stimulate salivary secretion include sight 35 to 40 cm/s. Liquids travel about five
and smell of food, presence of food in the times as fast by a squirting action of the
oral cavity, and conditioned reflexes, where mouth and the pharynx.
some event is associated with food and Vomiting (emesis) is a protective
feeding. Conditioned reflex control of sali- response to remove potentially harmful
vation was the subject of the classic studies ingesta from the stomach and upper
by Pavlov, who conditioned dogs to salivate small intestine. Vomiting is a highly
at the sound of a bell. coordinated reflex that is controlled by a
reflex center in the brainstem. Drugs
Swallowing that stimulate this center to produce
vomiting are termed emetics. The pro-
Deglutition, the act of swallowing, is arbi- cess begins with relaxation of the sphinc-
trarily divided into three stages. The first ter between the stomach and upper small
stage is passage of food or water through intestine and reverse peristalsis to move
the mouth; the second is passage through intestinal contents to the stomach. The
the pharynx; and the third consists of movement of stomach contents into the
passage through the esophagus into the esophagus and out of the mouth requires
stomach. relaxation of the upper and lower esoph-
The first stage of swallowing is under ageal sphincters together with an inspir-
voluntary control. After the food is chewed atory movement against a closed glottis
and mixed with saliva, a bolus (rounded and forceful contraction of abdominal
mass of food) forms and is moved to the muscles. Closure of the glottis and move-
upper surface of the tongue. The tongue is ment of the soft palate prevent regurgi-
raised against the hard palate (tip first) to tated food from entering the trachea and
push the bolus toward the pharynx. At the nasal cavity, respectively.
same time the soft palate is raised, closing
the caudal nares. The base of the tongue
then acts as a plunger, forcing the bolus Ruminant Forestomach
into the pharynx.
As the bolus enters the pharynx, it stim- Fermentative Digestion
ulates pressure receptors in the walls,
which reflexively initiates the second stage, No mammal can directly digest the com-
passage of the bolus through the pharynx. plex carbohydrates that constitute plant
Respiration is reflexively inhibited, and the cell walls (cellulose and hemicellulose),
larynx reflexively closes and pulls up and because mammals do not produce the
forward. The base of the tongue folds the enzyme cellulase, which is necessary to
epiglottis over the laryngeal opening as it break the unique chemical bonds in these
moves back. The pharynx shortens, and a compounds. The ruminant forestomach
peristaltic (milking) action of the pharyn- provides an excellent environment for the
geal muscles forces the bolus into the growth of bacteria, protozoa, and possi-
esophagus. bly other microbes that do produce cel-
The third stage of deglutition consists of lulase. The action of cellulase on cellulose
reflex peristalsis of the esophagus initiated and hemicellulose produces monosac-
by the presence of food in the esophagus. charides and simple polysaccharides,
Peristalsis consists of alternate relaxation which are available for further microbial
and contraction of rings of muscle in the digestion.