Mouth, pharynx, larynx and neck, continued

Here a longitudinal incision has been made through the midline of the soft palate to expose the nasal pharynx. Look for a small pair of slits, the orifices of the auditory (eustachian) tubes, in the dorsolateral walls of the nasal pharynx. They develop from the first pair of gill pouches of the embryo, lead to the tympanic cavities, and help equalize air pressure on the ear drums. These are quite far forward on the pig.

Next, tissue was cut away from the lateral and ventral surfaces of the larynx, facilitated by making a midventral incision down the neck. During this dissection a large, gland-like mass on each side of the neck is exposed. This is the thymus. It is relatively large in the fetus and infant, but regresses with age. It is an important source of one type of white blood cell, the lymphocyte, most of which are eventually localized in lymph nodes. Some lymphocytes are involved in the production of antibodies, and hence with the body's defense mechanism.

If you open the larynx by making a longitudinal, middorsal incision, as shown above, and spread it open, you will see a pair of small, whitish folds on the lateral walls. The vocal cords are much better developed in an adult and may not be seen in the fetus at all.

A further incision reveals the esophagus, a collapsed tube dorsal, or deep, to the trachea. In life it is pushed open by the food that is being swallowed. Because the esophagus is dorsal to the trachea, air and food must cross en route to the trachea and esophagus, respectively. Food is prevented from entering the trachea by a trough shaped flap of tissue, the epiglottis. The epiglottis covers the larynx during swallowing. As you dissect in this region you may also see a small, dark, compact, glandular mass partly surrounded by the thymus. This thyroid gland produces thyroxin, an endocrine hormone that plays an important role in regulating the metabolic level of the body.