A temporary depression caudal to the second pharyngeal arch, containing the succeeding pharyngeal arches; it is overgrown by the second pharyngeal arch and closes off as the cervical vesicle. This sinus is bounded in front by the hyoid arch, and behind by the thoracic wall. [ http://en.wikipedia.org/wiki/Cervical_sinus http://medical-dictionary.thefreedictionary.com/cervical+sinus ]