The larynx (Fig. 8.13) and the trachea are anterior to the digestive tract in the neck, and can be accessed directly when upper parts of the system are blocked. A cricothyrotomy makes use of the easiest route of access through the cricothyroid ligament (cricovocal membrane, cricothyroid membrane) between the cricoid and thyroid cartilages of the larynx. The ligament can be palpated in the midline, and usually there are only small blood vessels, connective tissue, and skin (though occasionally, a small lobe of the thyroid gland—pyramidal lobe) overlying it. At a lower level, the airway can be accessed surgically through the anterior wall of the trachea by tracheostomy. This route of entry is complicated because large veins and part of the thyroid gland overlie this region.
There are twelve pairs of cranial nerves and their defining feature is that they exit the cranial cavity through foramina or fissures.
All cranial nerves innervate structures in the head or neck. In addition, the vagus nerve [X] descends through the neck and into the thorax and abdomen where it innervates viscera.
Parasympathetic fibers in the head are carried out of the brain as part of four cranial nerves—the oculomotor nerve [III], the facial nerve [VII], the glossopharyngeal nerve [IX], and the vagus nerve [X] (Fig. 8.14). Parasympathetic fibers in the oculomotor nerve [III], the facial nerve [VII], and the glossopharyngeal nerve [IX] destined for target tissues in the head leave these nerves, and are distributed with branches of the trigeminal nerve [V].
The vagus nerve [X] leaves the head and neck to deliver parasympathetic fibers to the thoracic and abdominal viscera.
There are eight cervical nerves (C1 to C8):
C1 to C7 emerge from the vertebral canal above their respective vertebrae.
C8 emerges between vertebrae CVII and TI (Fig. 8.15A).
The anterior rami of C1 to C4 form the cervical plexus. The major branches from this plexus supply the strap muscles, the diaphragm (phrenic nerve), skin on the anterior and lateral parts of the neck, skin on the upper anterior thoracic wall, and skin on the inferior parts of the head (Fig. 8.15B).
The anterior rami of C5 to C8, together with a large component of the anterior ramus of T1, form the brachial plexus, which innervates the upper limb.
Functional separation of the digestive
The pharynx is a common chamber for the digestive and respiratory tracts. Consequently, breathing can take place through the mouth as well as through the nose, and material from the oral cavity can potentially enter either the esophagus or the larynx. Importantly:
The lower airway can be accessed through the oral cavity by intubation.
The digestive tract (esophagus) can be accessed through the nasal cavity by feeding tubes.
Normally, the soft palate, epiglottis, and soft tissue structures within the larynx act as valves to prevent food and liquid from entering lower parts of the respiratory tract (Fig. 8.16A).
During normal breathing, the airway is open and air passes freely through the nasal cavities (or oral cavity), pharynx, larynx, and trachea (Fig. 8.16A). The lumen of the esophagus is normally closed because, unlike the airway, it has no skeletal support structures to hold it open.
When the oral cavity is full of liquid or food, the soft palate is swung down (depressed) to close the oropharyngeal isthmus, thereby allowing manipulation of food and fluid in the oral cavity while breathing (Fig. 8.16C).