1.1 The Cavities of the Head

The vertebrate body has several cavities, mostly in the trunk and head. These are well delimited spaces containing fluids or organs. The largest body cavities are found in the trunk. The head contains the cranial cavity housing the brain and the neck contains the vertebral cavity which lodges the spinal cord.

Several smaller cavities are also present in the head, however, and these make the subject of this book. The mouth contains the oral cavity. Immediately superior to it, one finds the nasal cavity. Both oral and nasal cavities connect to the pharynx (throat), which connects to the larynx (voice box) and to the ears. This physical connection between organs reflects evolutionary, developmental or functional interactions between them. It also links them from a clinical perspective, as the connection facilitates the spread of a same malignancy from one region to another.

The mouth is the first portion of the alimentary canal that receives food and saliva. It contains of two spaces, the vestibule (between the teeth and the lips or cheek) and the oral cavity. The mouth, normally moist, is lined with a mucous membrane. The lips mark the transition from mucous membrane to skin, which covers most of the body. After food is processed in the mouth, it is passed into the pharynx, which is a muscular tube that is shared with the respiratory system. Food in the pharynx triggers the swallowing reflex, which is an elaborate behavior involving more than a dozen muscles. Swallowing pushes the food into the esophagus which then transports it down to the stomach.

In addition to its primary role as the beginning of the digestive system, in humans the mouth also plays a significant role in communication. While primary aspects of the voice are produced by focal folds in the larynx. Muscles in the tongue and lips are also needed to produce the range of sounds included in human language. The voice is also modified through resonance within the nasal cavity and sinuses that are connected to it. The roof of the nasal cavity houses the olfactory epithelium, containing sensory receptors responsible for the sense of smell.

The mouth is also connected to the ears by the auditory (Eustachian) tubes. The ears are essential to hearing and balance. The connection with the mouth allows for the air pressure behind the eardrum the be equalized with that of the external environment but this connection also facilitates the access of germs to the ears, causing middle ear infections.

This book focuses on the mouth and associated cavities of the head, including the ears, nose and throat (ENT) in addition to some smaller spaces. It discusses the anatomy and physiology of these structures in humans but also in animals. The extension to animals is justified because the evolutionary history and diversity of the mouth and ENT can help the students understand the design principles of these structures. Similarly, the book presents some major clinical issues and treatments in animals and humans.

Summary

The mouth, ears, nose and throat are have cavities that are physically interconnected in the head. These structures contain organs that may have very distinct function and yet interact during development, through proximity or action.

Key terms

Body cavity, ears, larynx, mouth, nasal cavity, nose, oral cavity, pharynx, throat

Figure credits

Figure 1 by Connexions - https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=29624322

Figure 2 by OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=30148430