|Henry Gray (18251861). Anatomy of the Human Body. 1918.|
|there arises a second and larger elevation, in the center of which is a median groove or furrow. This elevation was named by His the furcula, and is at first separated from the tuberculum impar by a depression, but later by a ridge, the copula, formed by the forward growth and fusion of the ventral ends of the second and third arches. The posterior or pharyngeal part of the tongue is developed from the copula, which extends forward in the form of a V, so as to embrace between its two limbs the buccal part of the tongue. At the apex of the V a pit-like invagination occurs, to form the thyroid gland, and this depression is represented in the adult by the foramen cecum of the tongue. In the adult the union of the anterior and posterior parts of the tongue is marked by the V-shaped sulcus terminalis, the apex of which is at the foramen cecum, while the two limbs run lateralward and forward, parallel to, but a little behind, the vallate papillæ.|
FIG. 981 Floor of pharynx of human embryo about thirty days old. (From model by Peter.) (See enlarged image)
The Palatine Tonsils.The palatine tonsils are developed from the dorsal angles of the second branchial pouches. The entoderm which lines these pouches grows in the form of a number of solid buds into the surrounding mesoderm. These buds become hollowed out by the degeneration and casting off of their central cells, and by this means the tonsillar crypts are formed. Lymphoid cells accumulate around the crypts, and become grouped to form the lymphoid follicles; the latter, however, are not well-defined until after birth.
FIG. 982 Sketches in profile of two stages in the development of the human digestive tube. (His.) A X 30. B X 20. (See enlarged image)
The Further Development of the Digestive Tube.The upper part of the fore-gut becomes dilated to form the pharynx (Fig. 977), in relation to which the branchial arches are developed (see page 65); the succeeding part remains tubular, and with the descent of the stomach is elongated to form the esophagus. About the fourth week a fusiform dilatation, the future stomach, makes its appearance, and beyond this the gut opens freely into the yolk-sac (Fig. 982, A and B). The opening is at first wide, but is gradually narrowed into a tubular stalk, the yolk-stalk or vitelline duct. Between the stomach and the mouth of the yolk-sac the liver diverticulum appears. From the stomach to the rectum the alimentary canal is attached to the notochord by a band of mesoderm, from which the common mesentery of the gut is subsequently developed. The stomach has an additional attachment, viz., to the ventral abdominal wall as far as the umbilicus by the septum transversum. The cephalic portion of the septum takes part in the formation of the diaphragm, while the caudal portion into which the liver grows forms the ventral mesogastrium (Fig. 984). The stomach undergoes a further dilatation, and its two curvatures can be recognized (Figs. 983, B, and 984), the greater directed toward the vertebral column and the lesser toward the anterior wall of the abdomen, while its two surfaces look to the right and left respectively. Behind the stomach the gut undergoes great elongation, and forms a V-shaped loop which projects downward and forward; from the bend or angle of the loop the vitelline duct passes to the umbilicus (Fig. 984). For a time a considerable part of the loop extends beyond the abdominal cavity into the umbilical cord, but by the end of the third month it is withdrawn within the cavity. With the lengthening of the tube, the mesoderm, which attaches it to the future vertebral column and carries the bloodvessels for the supply of the gut, is thinned and drawn out to form the posterior common mesentery. The portion of this mesentery attached to the greater curvature of the stomach is named the dorsal mesogastrium, and the part which suspends the