Fascial spaces (also termed fascial tissue spaces or tissue spaces) are potential spaces that exist between the fasciae and underlying organs and other tissues. In health, these spaces do not exist; they are only created by pathology, e.g. the spread of pus or cellulitis in an infection. The fascial spaces can also be opened during the dissection of a cadaver. The fascial spaces are different from the fasciae themselves, which are bands of connective tissue that surround structures, e.g. muscles. The opening of fascial spaces may be facilitated by pathogenic bacterial release of enzymes which cause tissue lysis (e.g. hyaluronidase and collagenase). The spaces filled with loose areolar connective tissue may also be termed clefts. Other contents such as salivary glands, blood vessels, nerves and lymph nodes are dependent upon the location of the space. Those containing neurovascular tissue (nerves and blood vessels) may also be termed compartments. Generally, the spread of infection is determined by barriers such as muscle, bone and fasciae. Pus moves by the path of least resistance, e.g. the fluid will more readily dissect apart loosely connected tissue planes, such the fascial spaces, than erode through bone or muscles. In the head and neck, potential spaces are primarily defined by the complex attachment of muscles, especially mylohyoid, buccinator, masseter, medial pterygoid, superior constrictor and orbicularis oris. Infections involving fascial spaces of the head and neck may give varying signs and symptoms depending upon the spaces involved. Trismus (difficulty opening the mouth) is a sign that the muscles of mastication (the muscles that move the jaw) are involved. Dysphagia (difficulty swallowing) and dyspnoea (difficulty breathing) may be a sign that the airway is being compressed by the swelling.