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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
mandibular arch, and laterally by the maxillary processes (Fig. 978). With the inward growth and fusion of the palatine processes (Figs. 50, 51), the stomodeum is divided into an upper nasal, and a lower buccal part. Along the free margins of the processes bounding the mouth cavity a shallow groove appears; this is termed the primary labial groove, and from the bottom of it a downgrowth of ectoderm takes place into the underlying mesoderm. The central cells of the ectodermal downgrowth degenerate and a secondary labial groove is formed; by the deepening of this, the lips and cheeks are separated from the alveolar processes of the maxillæ and mandible.

The Salivary Glands.—The salivary glands arise as buds from the epithelial lining of the mouth; the parotid appears during the fourth week in the angle between the maxillary process and the mandibular arch; the submaxillary appears in the sixth week, and the sublingual during the ninth week in the hollow between the tongue and the mandibular arch.


FIG. 978– Head end of human embryo of about thirty to thirty-one days. (From model by Peters.) (See enlarged image)



FIG. 979– Floor of pharynx of human embryo about twenty-six days old. (From model by Peters.) (See enlarged image)



FIG. 980– Floor of pharynx of human embryo of about the end of the fourth week. (From model by Peters.) (See enlarged image)


The Tongue (Figs. 979 to 981).—The tongue is developed in the floor of the pharynx, and consists of an anterior or buccal and a posterior or pharyngeal part which are separated in the adult by the V-shaped sulcus terminalis. During the third week there appears, immediately behind the ventral ends of the two halves of the mandibular arch, a rounded swelling named the tuberculum impar, which was described by His as undergoing enlargement to form the buccal part of the tongue. More recent researches, however, show that this part of the tongue is mainly, if not entirely, developed from a pair of lateral swellings which rise from the inner surface of the mandibular arch and meet in the middle line. The tuberculum impar is said to form the central part of the tongue immediately in front of the foramen cecum, but Hammar insists that it is purely a transitory structure and forms no part of the adult tongue. From the ventral ends of the fourth arch

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