This paper describes the anatomical variations observed in the head and neck in eight infants with the 18-trisomy syndrome that were dissected and studied in detail. Of the usual muscles of facial expression, occipitofrontalis and the auricular and nasal muscles were hypolastic in all eight bodies and each subject showed extensive fusion of the muscles around the corner of the mouth. In each body there was a supernumerary muscle band that extended from the region near the corner of the mouth to the occipital attachment of trapezius. The otomandibular region in each body showed a variable spectrum of muscular, skeletal, arterial, and salivary gland variations bilaterally. Three of the bodies had infrahyoid muscle variations. The sum of these observations provides a more complete delineation of the variations that occur in the 18-trisomy syndrome. Tentative pathogenetic hypotheses for several of the defects are discussed, based on observations from human descriptive embryology. Poswillo's teratological model [1973] implicating hemorrhaging as a causal mechanism in human first and second branchial arch malformations is suggested as a possible mechanism to explain the bilateral otomandibular defects.