Mouth, pharynx and esophagus
- The jaw and tongue muscles are an important source of low grade meat from cattle, pigs and lambs.
- The mouth is lined by a slippery mucous membrane which, in cattle and sheep (ruminants), has posteriorly directed tags of tissue known as papillae. Bovine papillae are stiffened by a core of keratin in their axis.
Lips, cheeks and teeth are, of course, absent in poultry.
- Whereas mammals have a secondary palate that separates the mouth from the nasal cavity, the nostrils of poultry open directly into the roof of the mouth. Thus, when drinking, a bird must use its long neck to keep its head in a horizontal position.
- After the mouth, the alimentary tract leads to the pharynx. The pharynx is a complicated junction because the (1) posterior nares from the nasal cavity open into it, as well as the (2) eustachian tubes balancing the air pressure behind the ear drums, the (3) larynx which tops the windpipe from the lungs and, finally, the (4) esophagus which continues the alimentary canal.
- When a bolus of food is swallowed, the pharyngeal muscles contract to force it down the esophagus. Then it is moved by smooth muscle.
- The esophagus is a long muscular tube that runs to the stomach. It is located dorsally to the trachea so that it appears behind the trachea when the throat is opened ventrally in the abattoir. At this point in the slaughter of ruminants it is desirable to tie off the esophagus to minimize the spread of ruminal contents onto the carcass.
- In the meat trade, the esophagus is known as the gullet or weasand. Weasands from beef and lamb carcasses may be used as sausage casings after they have been cleaned and scraped.
- In poultry, just before the esophagus enters the thoracic cavity, there is a large sack-like expansion on the right side known as the crop. The crop is a temporary storage area for feed.
- The muscular wall of the mammalian esophagus starts with two oblique or spiral layers that then develop into inner circular, and outer longitudinal layers farther down. In ruminants, all the muscle tissue of the esophagus is striated. Smooth muscle replaces striated muscle at the level of the diaphragm in pigs. Smooth muscle occurs along the remainder of the alimentary tract and in other organs such as the uterus.
- Microscopically, smooth muscle is formed from thick layers of elongated cells, each with a single nucleus. The contractile elements of smooth muscle cells do not form microscopically striated fibrils as they do in heart and skeletal muscles.